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<a name="C_002b_002b-Dialect-Options"></a>
<div class="header">
<p>
Next: <a href="Objective_002dC-and-Objective_002dC_002b_002b-Dialect-Options.html#Objective_002dC-and-Objective_002dC_002b_002b-Dialect-Options" accesskey="n" rel="next">Objective-C and Objective-C++ Dialect Options</a>, Previous: <a href="C-Dialect-Options.html#C-Dialect-Options" accesskey="p" rel="prev">C Dialect Options</a>, Up: <a href="Invoking-GCC.html#Invoking-GCC" accesskey="u" rel="up">Invoking GCC</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="Options-Controlling-C_002b_002b-Dialect"></a>
<h3 class="section">3.5 Options Controlling C++ Dialect</h3>

<a name="index-compiler-options_002c-C_002b_002b"></a>
<a name="index-C_002b_002b-options_002c-command_002dline"></a>
<a name="index-options_002c-C_002b_002b"></a>
<p>This section describes the command-line options that are only meaningful
for C++ programs.  You can also use most of the GNU compiler options
regardless of what language your program is in.  For example, you
might compile a file <samp>firstClass.C</samp> like this:
</p>
<div class="smallexample">
<pre class="smallexample">g++ -g -fstrict-enums -O -c firstClass.C
</pre></div>

<p>In this example, only <samp>-fstrict-enums</samp> is an option meant
only for C++ programs; you can use the other options with any
language supported by GCC.
</p>
<p>Some options for compiling C programs, such as <samp>-std</samp>, are also
relevant for C++ programs.
See <a href="C-Dialect-Options.html#C-Dialect-Options">Options Controlling C Dialect</a>.
</p>
<p>Here is a list of options that are <em>only</em> for compiling C++ programs:
</p>
<dl compact="compact">
<dt><code>-fabi-version=<var>n</var></code></dt>
<dd><a name="index-fabi_002dversion"></a>
<p>Use version <var>n</var> of the C++ ABI.  The default is version 0.
</p>
<p>Version 0 refers to the version conforming most closely to
the C++ ABI specification.  Therefore, the ABI obtained using version 0
will change in different versions of G++ as ABI bugs are fixed.
</p>
<p>Version 1 is the version of the C++ ABI that first appeared in G++ 3.2.
</p>
<p>Version 2 is the version of the C++ ABI that first appeared in G++
3.4, and was the default through G++ 4.9.
</p>
<p>Version 3 corrects an error in mangling a constant address as a
template argument.
</p>
<p>Version 4, which first appeared in G++ 4.5, implements a standard
mangling for vector types.
</p>
<p>Version 5, which first appeared in G++ 4.6, corrects the mangling of
attribute const/volatile on function pointer types, decltype of a
plain decl, and use of a function parameter in the declaration of
another parameter.
</p>
<p>Version 6, which first appeared in G++ 4.7, corrects the promotion
behavior of C++11 scoped enums and the mangling of template argument
packs, const/static_cast, prefix ++ and &ndash;, and a class scope function
used as a template argument.
</p>
<p>Version 7, which first appeared in G++ 4.8, that treats nullptr_t as a
builtin type and corrects the mangling of lambdas in default argument
scope.
</p>
<p>Version 8, which first appeared in G++ 4.9, corrects the substitution
behavior of function types with function-cv-qualifiers.
</p>
<p>Version 9, which first appeared in G++ 5.2, corrects the alignment of
<code>nullptr_t</code>.
</p>
<p>Version 10, which first appeared in G++ 6.1, adds mangling of
attributes that affect type identity, such as ia32 calling convention
attributes (e.g. &lsquo;<samp>stdcall</samp>&rsquo;).
</p>
<p>Version 11, which first appeared in G++ 7, corrects the mangling of
sizeof... expressions and operator names.  For multiple entities with
the same name within a function, that are declared in different scopes,
the mangling now changes starting with the twelfth occurrence.  It also
implies <samp>-fnew-inheriting-ctors</samp>.
</p>
<p>Version 12, which first appeared in G++ 8, corrects the calling
conventions for empty classes on the x86_64 target and for classes
with only deleted copy/move constructors.  It accidentally changes the
calling convention for classes with a deleted copy constructor and a
trivial move constructor.
</p>
<p>Version 13, which first appeared in G++ 8.2, fixes the accidental
change in version 12.
</p>
<p>Version 14, which first appeared in G++ 10, corrects the mangling of
the nullptr expression.
</p>
<p>See also <samp>-Wabi</samp>.
</p>
</dd>
<dt><code>-fabi-compat-version=<var>n</var></code></dt>
<dd><a name="index-fabi_002dcompat_002dversion"></a>
<p>On targets that support strong aliases, G++
works around mangling changes by creating an alias with the correct
mangled name when defining a symbol with an incorrect mangled name.
This switch specifies which ABI version to use for the alias.
</p>
<p>With <samp>-fabi-version=0</samp> (the default), this defaults to 11 (GCC 7
compatibility).  If another ABI version is explicitly selected, this
defaults to 0.  For compatibility with GCC versions 3.2 through 4.9,
use <samp>-fabi-compat-version=2</samp>.
</p>
<p>If this option is not provided but <samp>-Wabi=<var>n</var></samp> is, that
version is used for compatibility aliases.  If this option is provided
along with <samp>-Wabi</samp> (without the version), the version from this
option is used for the warning.
</p>
</dd>
<dt><code>-fno-access-control</code></dt>
<dd><a name="index-fno_002daccess_002dcontrol"></a>
<a name="index-faccess_002dcontrol"></a>
<p>Turn off all access checking.  This switch is mainly useful for working
around bugs in the access control code.
</p>
</dd>
<dt><code>-faligned-new</code></dt>
<dd><a name="index-faligned_002dnew"></a>
<p>Enable support for C++17 <code>new</code> of types that require more
alignment than <code>void* ::operator new(std::size_t)</code> provides.  A
numeric argument such as <code>-faligned-new=32</code> can be used to
specify how much alignment (in bytes) is provided by that function,
but few users will need to override the default of
<code>alignof(std::max_align_t)</code>.
</p>
<p>This flag is enabled by default for <samp>-std=c++17</samp>.
</p>
</dd>
<dt><code>-fchar8_t</code></dt>
<dt><code>-fno-char8_t</code></dt>
<dd><a name="index-fchar8_005ft"></a>
<a name="index-fno_002dchar8_005ft"></a>
<p>Enable support for <code>char8_t</code> as adopted for C++2a.  This includes
the addition of a new <code>char8_t</code> fundamental type, changes to the
types of UTF-8 string and character literals, new signatures for
user-defined literals, associated standard library updates, and new
<code>__cpp_char8_t</code> and <code>__cpp_lib_char8_t</code> feature test macros.
</p>
<p>This option enables functions to be overloaded for ordinary and UTF-8
strings:
</p>
<div class="smallexample">
<pre class="smallexample">int f(const char *);    // #1
int f(const char8_t *); // #2
int v1 = f(&quot;text&quot;);     // Calls #1
int v2 = f(u8&quot;text&quot;);   // Calls #2
</pre></div>

<p>and introduces new signatures for user-defined literals:
</p>
<div class="smallexample">
<pre class="smallexample">int operator&quot;&quot;_udl1(char8_t);
int v3 = u8'x'_udl1;
int operator&quot;&quot;_udl2(const char8_t*, std::size_t);
int v4 = u8&quot;text&quot;_udl2;
template&lt;typename T, T...&gt; int operator&quot;&quot;_udl3();
int v5 = u8&quot;text&quot;_udl3;
</pre></div>

<p>The change to the types of UTF-8 string and character literals introduces
incompatibilities with ISO C++11 and later standards.  For example, the
following code is well-formed under ISO C++11, but is ill-formed when
<samp>-fchar8_t</samp> is specified.
</p>
<div class="smallexample">
<pre class="smallexample">char ca[] = u8&quot;xx&quot;;     // error: char-array initialized from wide
                        //        string
const char *cp = u8&quot;xx&quot;;// error: invalid conversion from
                        //        `const char8_t*' to `const char*'
int f(const char*);
auto v = f(u8&quot;xx&quot;);     // error: invalid conversion from
                        //        `const char8_t*' to `const char*'
std::string s{u8&quot;xx&quot;};  // error: no matching function for call to
                        //        `std::basic_string&lt;char&gt;::basic_string()'
using namespace std::literals;
s = u8&quot;xx&quot;s;            // error: conversion from
                        //        `basic_string&lt;char8_t&gt;' to non-scalar
                        //        type `basic_string&lt;char&gt;' requested
</pre></div>

</dd>
<dt><code>-fcheck-new</code></dt>
<dd><a name="index-fcheck_002dnew"></a>
<p>Check that the pointer returned by <code>operator new</code> is non-null
before attempting to modify the storage allocated.  This check is
normally unnecessary because the C++ standard specifies that
<code>operator new</code> only returns <code>0</code> if it is declared
<code>throw()</code>, in which case the compiler always checks the
return value even without this option.  In all other cases, when
<code>operator new</code> has a non-empty exception specification, memory
exhaustion is signalled by throwing <code>std::bad_alloc</code>.  See also
&lsquo;<samp>new (nothrow)</samp>&rsquo;.
</p>
</dd>
<dt><code>-fconcepts</code></dt>
<dt><code>-fconcepts-ts</code></dt>
<dd><a name="index-fconcepts"></a>
<a name="index-fconcepts_002dts"></a>
<p>Below <samp>-std=c++2a</samp>, <samp>-fconcepts</samp> enables support for the
C++ Extensions for Concepts Technical Specification, ISO 19217 (2015).
</p>
<p>With <samp>-std=c++2a</samp> and above, Concepts are part of the language
standard, so <samp>-fconcepts</samp> defaults to on.  But the standard
specification of Concepts differs significantly from the TS, so some
constructs that were allowed in the TS but didn&rsquo;t make it into the
standard can still be enabled by <samp>-fconcepts-ts</samp>.
</p>
</dd>
<dt><code>-fconstexpr-depth=<var>n</var></code></dt>
<dd><a name="index-fconstexpr_002ddepth"></a>
<p>Set the maximum nested evaluation depth for C++11 constexpr functions
to <var>n</var>.  A limit is needed to detect endless recursion during
constant expression evaluation.  The minimum specified by the standard
is 512.
</p>
</dd>
<dt><code>-fconstexpr-cache-depth=<var>n</var></code></dt>
<dd><a name="index-fconstexpr_002dcache_002ddepth"></a>
<p>Set the maximum level of nested evaluation depth for C++11 constexpr
functions that will be cached to <var>n</var>.  This is a heuristic that
trades off compilation speed (when the cache avoids repeated
calculations) against memory consumption (when the cache grows very
large from highly recursive evaluations).  The default is 8.  Very few
users are likely to want to adjust it, but if your code does heavy
constexpr calculations you might want to experiment to find which
value works best for you.
</p>
</dd>
<dt><code>-fconstexpr-loop-limit=<var>n</var></code></dt>
<dd><a name="index-fconstexpr_002dloop_002dlimit"></a>
<p>Set the maximum number of iterations for a loop in C++14 constexpr functions
to <var>n</var>.  A limit is needed to detect infinite loops during
constant expression evaluation.  The default is 262144 (1&lt;&lt;18).
</p>
</dd>
<dt><code>-fconstexpr-ops-limit=<var>n</var></code></dt>
<dd><a name="index-fconstexpr_002dops_002dlimit"></a>
<p>Set the maximum number of operations during a single constexpr evaluation.
Even when number of iterations of a single loop is limited with the above limit,
if there are several nested loops and each of them has many iterations but still
smaller than the above limit, or if in a body of some loop or even outside
of a loop too many expressions need to be evaluated, the resulting constexpr
evaluation might take too long.
The default is 33554432 (1&lt;&lt;25).
</p>
</dd>
<dt><code>-fcoroutines</code></dt>
<dd><a name="index-fcoroutines"></a>
<p>Enable support for the C++ coroutines extension (experimental).
</p>
</dd>
<dt><code>-fno-elide-constructors</code></dt>
<dd><a name="index-fno_002delide_002dconstructors"></a>
<a name="index-felide_002dconstructors"></a>
<p>The C++ standard allows an implementation to omit creating a temporary
that is only used to initialize another object of the same type.
Specifying this option disables that optimization, and forces G++ to
call the copy constructor in all cases.  This option also causes G++
to call trivial member functions which otherwise would be expanded inline.
</p>
<p>In C++17, the compiler is required to omit these temporaries, but this
option still affects trivial member functions.
</p>
</dd>
<dt><code>-fno-enforce-eh-specs</code></dt>
<dd><a name="index-fno_002denforce_002deh_002dspecs"></a>
<a name="index-fenforce_002deh_002dspecs"></a>
<p>Don&rsquo;t generate code to check for violation of exception specifications
at run time.  This option violates the C++ standard, but may be useful
for reducing code size in production builds, much like defining
<code>NDEBUG</code>.  This does not give user code permission to throw
exceptions in violation of the exception specifications; the compiler
still optimizes based on the specifications, so throwing an
unexpected exception results in undefined behavior at run time.
</p>
</dd>
<dt><code>-fextern-tls-init</code></dt>
<dt><code>-fno-extern-tls-init</code></dt>
<dd><a name="index-fextern_002dtls_002dinit"></a>
<a name="index-fno_002dextern_002dtls_002dinit"></a>
<p>The C++11 and OpenMP standards allow <code>thread_local</code> and
<code>threadprivate</code> variables to have dynamic (runtime)
initialization.  To support this, any use of such a variable goes
through a wrapper function that performs any necessary initialization.
When the use and definition of the variable are in the same
translation unit, this overhead can be optimized away, but when the
use is in a different translation unit there is significant overhead
even if the variable doesn&rsquo;t actually need dynamic initialization.  If
the programmer can be sure that no use of the variable in a
non-defining TU needs to trigger dynamic initialization (either
because the variable is statically initialized, or a use of the
variable in the defining TU will be executed before any uses in
another TU), they can avoid this overhead with the
<samp>-fno-extern-tls-init</samp> option.
</p>
<p>On targets that support symbol aliases, the default is
<samp>-fextern-tls-init</samp>.  On targets that do not support symbol
aliases, the default is <samp>-fno-extern-tls-init</samp>.
</p>
</dd>
<dt><code>-fno-gnu-keywords</code></dt>
<dd><a name="index-fno_002dgnu_002dkeywords"></a>
<a name="index-fgnu_002dkeywords"></a>
<p>Do not recognize <code>typeof</code> as a keyword, so that code can use this
word as an identifier.  You can use the keyword <code>__typeof__</code> instead.
This option is implied by the strict ISO C++ dialects: <samp>-ansi</samp>,
<samp>-std=c++98</samp>, <samp>-std=c++11</samp>, etc.
</p>
</dd>
<dt><code>-fno-implicit-templates</code></dt>
<dd><a name="index-fno_002dimplicit_002dtemplates"></a>
<a name="index-fimplicit_002dtemplates"></a>
<p>Never emit code for non-inline templates that are instantiated
implicitly (i.e. by use); only emit code for explicit instantiations.
If you use this option, you must take care to structure your code to
include all the necessary explicit instantiations to avoid getting
undefined symbols at link time.
See <a href="Template-Instantiation.html#Template-Instantiation">Template Instantiation</a>, for more information.
</p>
</dd>
<dt><code>-fno-implicit-inline-templates</code></dt>
<dd><a name="index-fno_002dimplicit_002dinline_002dtemplates"></a>
<a name="index-fimplicit_002dinline_002dtemplates"></a>
<p>Don&rsquo;t emit code for implicit instantiations of inline templates, either.
The default is to handle inlines differently so that compiles with and
without optimization need the same set of explicit instantiations.
</p>
</dd>
<dt><code>-fno-implement-inlines</code></dt>
<dd><a name="index-fno_002dimplement_002dinlines"></a>
<a name="index-fimplement_002dinlines"></a>
<p>To save space, do not emit out-of-line copies of inline functions
controlled by <code>#pragma implementation</code>.  This causes linker
errors if these functions are not inlined everywhere they are called.
</p>
</dd>
<dt><code>-fms-extensions</code></dt>
<dd><a name="index-fms_002dextensions-1"></a>
<p>Disable Wpedantic warnings about constructs used in MFC, such as implicit
int and getting a pointer to member function via non-standard syntax.
</p>
</dd>
<dt><code>-fnew-inheriting-ctors</code></dt>
<dd><a name="index-fnew_002dinheriting_002dctors"></a>
<p>Enable the P0136 adjustment to the semantics of C++11 constructor
inheritance.  This is part of C++17 but also considered to be a Defect
Report against C++11 and C++14.  This flag is enabled by default
unless <samp>-fabi-version=10</samp> or lower is specified.
</p>
</dd>
<dt><code>-fnew-ttp-matching</code></dt>
<dd><a name="index-fnew_002dttp_002dmatching"></a>
<p>Enable the P0522 resolution to Core issue 150, template template
parameters and default arguments: this allows a template with default
template arguments as an argument for a template template parameter
with fewer template parameters.  This flag is enabled by default for
<samp>-std=c++17</samp>.
</p>
</dd>
<dt><code>-fno-nonansi-builtins</code></dt>
<dd><a name="index-fno_002dnonansi_002dbuiltins"></a>
<a name="index-fnonansi_002dbuiltins"></a>
<p>Disable built-in declarations of functions that are not mandated by
ANSI/ISO C.  These include <code>ffs</code>, <code>alloca</code>, <code>_exit</code>,
<code>index</code>, <code>bzero</code>, <code>conjf</code>, and other related functions.
</p>
</dd>
<dt><code>-fnothrow-opt</code></dt>
<dd><a name="index-fnothrow_002dopt"></a>
<p>Treat a <code>throw()</code> exception specification as if it were a
<code>noexcept</code> specification to reduce or eliminate the text size
overhead relative to a function with no exception specification.  If
the function has local variables of types with non-trivial
destructors, the exception specification actually makes the
function smaller because the EH cleanups for those variables can be
optimized away.  The semantic effect is that an exception thrown out of
a function with such an exception specification results in a call
to <code>terminate</code> rather than <code>unexpected</code>.
</p>
</dd>
<dt><code>-fno-operator-names</code></dt>
<dd><a name="index-fno_002doperator_002dnames"></a>
<a name="index-foperator_002dnames"></a>
<p>Do not treat the operator name keywords <code>and</code>, <code>bitand</code>,
<code>bitor</code>, <code>compl</code>, <code>not</code>, <code>or</code> and <code>xor</code> as
synonyms as keywords.
</p>
</dd>
<dt><code>-fno-optional-diags</code></dt>
<dd><a name="index-fno_002doptional_002ddiags"></a>
<a name="index-foptional_002ddiags"></a>
<p>Disable diagnostics that the standard says a compiler does not need to
issue.  Currently, the only such diagnostic issued by G++ is the one for
a name having multiple meanings within a class.
</p>
</dd>
<dt><code>-fpermissive</code></dt>
<dd><a name="index-fpermissive"></a>
<p>Downgrade some diagnostics about nonconformant code from errors to
warnings.  Thus, using <samp>-fpermissive</samp> allows some
nonconforming code to compile.
</p>
</dd>
<dt><code>-fno-pretty-templates</code></dt>
<dd><a name="index-fno_002dpretty_002dtemplates"></a>
<a name="index-fpretty_002dtemplates"></a>
<p>When an error message refers to a specialization of a function
template, the compiler normally prints the signature of the
template followed by the template arguments and any typedefs or
typenames in the signature (e.g. <code>void f(T) [with T = int]</code>
rather than <code>void f(int)</code>) so that it&rsquo;s clear which template is
involved.  When an error message refers to a specialization of a class
template, the compiler omits any template arguments that match
the default template arguments for that template.  If either of these
behaviors make it harder to understand the error message rather than
easier, you can use <samp>-fno-pretty-templates</samp> to disable them.
</p>
</dd>
<dt><code>-fno-rtti</code></dt>
<dd><a name="index-fno_002drtti"></a>
<a name="index-frtti"></a>
<p>Disable generation of information about every class with virtual
functions for use by the C++ run-time type identification features
(<code>dynamic_cast</code> and <code>typeid</code>).  If you don&rsquo;t use those parts
of the language, you can save some space by using this flag.  Note that
exception handling uses the same information, but G++ generates it as
needed. The <code>dynamic_cast</code> operator can still be used for casts that
do not require run-time type information, i.e. casts to <code>void *</code> or to
unambiguous base classes.
</p>
<p>Mixing code compiled with <samp>-frtti</samp> with that compiled with
<samp>-fno-rtti</samp> may not work.  For example, programs may
fail to link if a class compiled with <samp>-fno-rtti</samp> is used as a base 
for a class compiled with <samp>-frtti</samp>.  
</p>
</dd>
<dt><code>-fsized-deallocation</code></dt>
<dd><a name="index-fsized_002ddeallocation"></a>
<p>Enable the built-in global declarations
</p><div class="smallexample">
<pre class="smallexample">void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;
</pre></div>
<p>as introduced in C++14.  This is useful for user-defined replacement
deallocation functions that, for example, use the size of the object
to make deallocation faster.  Enabled by default under
<samp>-std=c++14</samp> and above.  The flag <samp>-Wsized-deallocation</samp>
warns about places that might want to add a definition.
</p>
</dd>
<dt><code>-fstrict-enums</code></dt>
<dd><a name="index-fstrict_002denums"></a>
<p>Allow the compiler to optimize using the assumption that a value of
enumerated type can only be one of the values of the enumeration (as
defined in the C++ standard; basically, a value that can be
represented in the minimum number of bits needed to represent all the
enumerators).  This assumption may not be valid if the program uses a
cast to convert an arbitrary integer value to the enumerated type.
</p>
</dd>
<dt><code>-fstrong-eval-order</code></dt>
<dd><a name="index-fstrong_002deval_002dorder"></a>
<p>Evaluate member access, array subscripting, and shift expressions in
left-to-right order, and evaluate assignment in right-to-left order,
as adopted for C++17.  Enabled by default with <samp>-std=c++17</samp>.
<samp>-fstrong-eval-order=some</samp> enables just the ordering of member
access and shift expressions, and is the default without
<samp>-std=c++17</samp>.
</p>
</dd>
<dt><code>-ftemplate-backtrace-limit=<var>n</var></code></dt>
<dd><a name="index-ftemplate_002dbacktrace_002dlimit"></a>
<p>Set the maximum number of template instantiation notes for a single
warning or error to <var>n</var>.  The default value is 10.
</p>
</dd>
<dt><code>-ftemplate-depth=<var>n</var></code></dt>
<dd><a name="index-ftemplate_002ddepth"></a>
<p>Set the maximum instantiation depth for template classes to <var>n</var>.
A limit on the template instantiation depth is needed to detect
endless recursions during template class instantiation.  ANSI/ISO C++
conforming programs must not rely on a maximum depth greater than 17
(changed to 1024 in C++11).  The default value is 900, as the compiler
can run out of stack space before hitting 1024 in some situations.
</p>
</dd>
<dt><code>-fno-threadsafe-statics</code></dt>
<dd><a name="index-fno_002dthreadsafe_002dstatics"></a>
<a name="index-fthreadsafe_002dstatics"></a>
<p>Do not emit the extra code to use the routines specified in the C++
ABI for thread-safe initialization of local statics.  You can use this
option to reduce code size slightly in code that doesn&rsquo;t need to be
thread-safe.
</p>
</dd>
<dt><code>-fuse-cxa-atexit</code></dt>
<dd><a name="index-fuse_002dcxa_002datexit"></a>
<p>Register destructors for objects with static storage duration with the
<code>__cxa_atexit</code> function rather than the <code>atexit</code> function.
This option is required for fully standards-compliant handling of static
destructors, but only works if your C library supports
<code>__cxa_atexit</code>.
</p>
</dd>
<dt><code>-fno-use-cxa-get-exception-ptr</code></dt>
<dd><a name="index-fno_002duse_002dcxa_002dget_002dexception_002dptr"></a>
<a name="index-fuse_002dcxa_002dget_002dexception_002dptr"></a>
<p>Don&rsquo;t use the <code>__cxa_get_exception_ptr</code> runtime routine.  This
causes <code>std::uncaught_exception</code> to be incorrect, but is necessary
if the runtime routine is not available.
</p>
</dd>
<dt><code>-fvisibility-inlines-hidden</code></dt>
<dd><a name="index-fvisibility_002dinlines_002dhidden"></a>
<p>This switch declares that the user does not attempt to compare
pointers to inline functions or methods where the addresses of the two functions
are taken in different shared objects.
</p>
<p>The effect of this is that GCC may, effectively, mark inline methods with
<code>__attribute__ ((visibility (&quot;hidden&quot;)))</code> so that they do not
appear in the export table of a DSO and do not require a PLT indirection
when used within the DSO.  Enabling this option can have a dramatic effect
on load and link times of a DSO as it massively reduces the size of the
dynamic export table when the library makes heavy use of templates.
</p>
<p>The behavior of this switch is not quite the same as marking the
methods as hidden directly, because it does not affect static variables
local to the function or cause the compiler to deduce that
the function is defined in only one shared object.
</p>
<p>You may mark a method as having a visibility explicitly to negate the
effect of the switch for that method.  For example, if you do want to
compare pointers to a particular inline method, you might mark it as
having default visibility.  Marking the enclosing class with explicit
visibility has no effect.
</p>
<p>Explicitly instantiated inline methods are unaffected by this option
as their linkage might otherwise cross a shared library boundary.
See <a href="Template-Instantiation.html#Template-Instantiation">Template Instantiation</a>.
</p>
</dd>
<dt><code>-fvisibility-ms-compat</code></dt>
<dd><a name="index-fvisibility_002dms_002dcompat"></a>
<p>This flag attempts to use visibility settings to make GCC&rsquo;s C++
linkage model compatible with that of Microsoft Visual Studio.
</p>
<p>The flag makes these changes to GCC&rsquo;s linkage model:
</p>
<ol>
<li> It sets the default visibility to <code>hidden</code>, like
<samp>-fvisibility=hidden</samp>.

</li><li> Types, but not their members, are not hidden by default.

</li><li> The One Definition Rule is relaxed for types without explicit
visibility specifications that are defined in more than one
shared object: those declarations are permitted if they are
permitted when this option is not used.
</li></ol>

<p>In new code it is better to use <samp>-fvisibility=hidden</samp> and
export those classes that are intended to be externally visible.
Unfortunately it is possible for code to rely, perhaps accidentally,
on the Visual Studio behavior.
</p>
<p>Among the consequences of these changes are that static data members
of the same type with the same name but defined in different shared
objects are different, so changing one does not change the other;
and that pointers to function members defined in different shared
objects may not compare equal.  When this flag is given, it is a
violation of the ODR to define types with the same name differently.
</p>
</dd>
<dt><code>-fno-weak</code></dt>
<dd><a name="index-fno_002dweak"></a>
<a name="index-fweak"></a>
<p>Do not use weak symbol support, even if it is provided by the linker.
By default, G++ uses weak symbols if they are available.  This
option exists only for testing, and should not be used by end-users;
it results in inferior code and has no benefits.  This option may
be removed in a future release of G++.
</p>
</dd>
<dt><code>-fext-numeric-literals <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-fext_002dnumeric_002dliterals"></a>
<a name="index-fno_002dext_002dnumeric_002dliterals"></a>
<p>Accept imaginary, fixed-point, or machine-defined
literal number suffixes as GNU extensions.
When this option is turned off these suffixes are treated
as C++11 user-defined literal numeric suffixes.
This is on by default for all pre-C++11 dialects and all GNU dialects:
<samp>-std=c++98</samp>, <samp>-std=gnu++98</samp>, <samp>-std=gnu++11</samp>,
<samp>-std=gnu++14</samp>.
This option is off by default
for ISO C++11 onwards (<samp>-std=c++11</samp>, ...).
</p>
</dd>
<dt><code>-nostdinc++</code></dt>
<dd><a name="index-nostdinc_002b_002b"></a>
<p>Do not search for header files in the standard directories specific to
C++, but do still search the other standard directories.  (This option
is used when building the C++ library.)
</p></dd>
</dl>

<p>In addition, these warning options have meanings only for C++ programs:
</p>
<dl compact="compact">
<dt><code>-Wabi-tag <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wabi_002dtag"></a>
<a name="index-Wabi_002dtag-1"></a>
<p>Warn when a type with an ABI tag is used in a context that does not
have that ABI tag.  See <a href="C_002b_002b-Attributes.html#C_002b_002b-Attributes">C++ Attributes</a> for more information
about ABI tags.
</p>
</dd>
<dt><code>-Wcomma-subscript <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wcomma_002dsubscript"></a>
<a name="index-Wno_002dcomma_002dsubscript"></a>
<p>Warn about uses of a comma expression within a subscripting expression.
This usage was deprecated in C++2a.  However, a comma expression wrapped
in <code>( )</code> is not deprecated.  Example:
</p>
<div class="smallexample">
<pre class="smallexample">void f(int *a, int b, int c) {
    a[b,c];     // deprecated
    a[(b,c)];   // OK
}
</pre></div>

<p>Enabled by default with <samp>-std=c++2a</samp>.
</p>
</dd>
<dt><code>-Wctor-dtor-privacy <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wctor_002ddtor_002dprivacy"></a>
<a name="index-Wno_002dctor_002ddtor_002dprivacy"></a>
<p>Warn when a class seems unusable because all the constructors or
destructors in that class are private, and it has neither friends nor
public static member functions.  Also warn if there are no non-private
methods, and there&rsquo;s at least one private member function that isn&rsquo;t
a constructor or destructor.
</p>
</dd>
<dt><code>-Wdelete-non-virtual-dtor <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wdelete_002dnon_002dvirtual_002ddtor"></a>
<a name="index-Wno_002ddelete_002dnon_002dvirtual_002ddtor"></a>
<p>Warn when <code>delete</code> is used to destroy an instance of a class that
has virtual functions and non-virtual destructor. It is unsafe to delete
an instance of a derived class through a pointer to a base class if the
base class does not have a virtual destructor.  This warning is enabled
by <samp>-Wall</samp>.
</p>
</dd>
<dt><code>-Wdeprecated-copy <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wdeprecated_002dcopy"></a>
<a name="index-Wno_002ddeprecated_002dcopy"></a>
<p>Warn that the implicit declaration of a copy constructor or copy
assignment operator is deprecated if the class has a user-provided
copy constructor or copy assignment operator, in C++11 and up.  This
warning is enabled by <samp>-Wextra</samp>.  With
<samp>-Wdeprecated-copy-dtor</samp>, also deprecate if the class has a
user-provided destructor.
</p>
</dd>
<dt><code>-Wno-init-list-lifetime <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Winit_002dlist_002dlifetime"></a>
<a name="index-Wno_002dinit_002dlist_002dlifetime"></a>
<p>Do not warn about uses of <code>std::initializer_list</code> that are likely
to result in dangling pointers.  Since the underlying array for an
<code>initializer_list</code> is handled like a normal C++ temporary object,
it is easy to inadvertently keep a pointer to the array past the end
of the array&rsquo;s lifetime.  For example:
</p>
<ul>
<li> If a function returns a temporary <code>initializer_list</code>, or a local
<code>initializer_list</code> variable, the array&rsquo;s lifetime ends at the end
of the return statement, so the value returned has a dangling pointer.

</li><li> If a new-expression creates an <code>initializer_list</code>, the array only
lives until the end of the enclosing full-expression, so the
<code>initializer_list</code> in the heap has a dangling pointer.

</li><li> When an <code>initializer_list</code> variable is assigned from a
brace-enclosed initializer list, the temporary array created for the
right side of the assignment only lives until the end of the
full-expression, so at the next statement the <code>initializer_list</code>
variable has a dangling pointer.

<div class="smallexample">
<pre class="smallexample">// li's initial underlying array lives as long as li
std::initializer_list&lt;int&gt; li = { 1,2,3 };
// assignment changes li to point to a temporary array
li = { 4, 5 };
// now the temporary is gone and li has a dangling pointer
int i = li.begin()[0] // undefined behavior
</pre></div>

</li><li> When a list constructor stores the <code>begin</code> pointer from the
<code>initializer_list</code> argument, this doesn&rsquo;t extend the lifetime of
the array, so if a class variable is constructed from a temporary
<code>initializer_list</code>, the pointer is left dangling by the end of
the variable declaration statement.

</li></ul>

</dd>
<dt><code>-Wno-literal-suffix <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wliteral_002dsuffix"></a>
<a name="index-Wno_002dliteral_002dsuffix"></a>
<p>Do not warn when a string or character literal is followed by a
ud-suffix which does not begin with an underscore.  As a conforming
extension, GCC treats such suffixes as separate preprocessing tokens
in order to maintain backwards compatibility with code that uses
formatting macros from <code>&lt;inttypes.h&gt;</code>.  For example:
</p>
<div class="smallexample">
<pre class="smallexample">#define __STDC_FORMAT_MACROS
#include &lt;inttypes.h&gt;
#include &lt;stdio.h&gt;

int main() {
  int64_t i64 = 123;
  printf(&quot;My int64: %&quot; PRId64&quot;\n&quot;, i64);
}
</pre></div>

<p>In this case, <code>PRId64</code> is treated as a separate preprocessing token.
</p>
<p>This option also controls warnings when a user-defined literal
operator is declared with a literal suffix identifier that doesn&rsquo;t
begin with an underscore. Literal suffix identifiers that don&rsquo;t begin
with an underscore are reserved for future standardization.
</p>
<p>These warnings are enabled by default.
</p>
</dd>
<dt><code>-Wno-narrowing <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wnarrowing"></a>
<a name="index-Wno_002dnarrowing"></a>
<p>For C++11 and later standards, narrowing conversions are diagnosed by default,
as required by the standard.  A narrowing conversion from a constant produces
an error, and a narrowing conversion from a non-constant produces a warning,
but <samp>-Wno-narrowing</samp> suppresses the diagnostic.
Note that this does not affect the meaning of well-formed code;
narrowing conversions are still considered ill-formed in SFINAE contexts.
</p>
<p>With <samp>-Wnarrowing</samp> in C++98, warn when a narrowing
conversion prohibited by C++11 occurs within
&lsquo;<samp>{ }</samp>&rsquo;, e.g.
</p>
<div class="smallexample">
<pre class="smallexample">int i = { 2.2 }; // error: narrowing from double to int
</pre></div>

<p>This flag is included in <samp>-Wall</samp> and <samp>-Wc++11-compat</samp>.
</p>
</dd>
<dt><code>-Wnoexcept <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wnoexcept"></a>
<a name="index-Wno_002dnoexcept"></a>
<p>Warn when a noexcept-expression evaluates to false because of a call
to a function that does not have a non-throwing exception
specification (i.e. <code>throw()</code> or <code>noexcept</code>) but is known by
the compiler to never throw an exception.
</p>
</dd>
<dt><code>-Wnoexcept-type <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wnoexcept_002dtype"></a>
<a name="index-Wno_002dnoexcept_002dtype"></a>
<p>Warn if the C++17 feature making <code>noexcept</code> part of a function
type changes the mangled name of a symbol relative to C++14.  Enabled
by <samp>-Wabi</samp> and <samp>-Wc++17-compat</samp>.
</p>
<p>As an example:
</p>
<div class="smallexample">
<pre class="smallexample">template &lt;class T&gt; void f(T t) { t(); };
void g() noexcept;
void h() { f(g); } 
</pre></div>

<p>In C++14, <code>f</code> calls <code>f&lt;void(*)()&gt;</code>, but in
C++17 it calls <code>f&lt;void(*)()noexcept&gt;</code>.
</p>
</dd>
<dt><code>-Wclass-memaccess <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wclass_002dmemaccess"></a>
<a name="index-Wno_002dclass_002dmemaccess"></a>
<p>Warn when the destination of a call to a raw memory function such as
<code>memset</code> or <code>memcpy</code> is an object of class type, and when writing
into such an object might bypass the class non-trivial or deleted constructor
or copy assignment, violate const-correctness or encapsulation, or corrupt
virtual table pointers.  Modifying the representation of such objects may
violate invariants maintained by member functions of the class.  For example,
the call to <code>memset</code> below is undefined because it modifies a non-trivial
class object and is, therefore, diagnosed.  The safe way to either initialize
or clear the storage of objects of such types is by using the appropriate
constructor or assignment operator, if one is available.
</p><div class="smallexample">
<pre class="smallexample">std::string str = &quot;abc&quot;;
memset (&amp;str, 0, sizeof str);
</pre></div>
<p>The <samp>-Wclass-memaccess</samp> option is enabled by <samp>-Wall</samp>.
Explicitly casting the pointer to the class object to <code>void *</code> or
to a type that can be safely accessed by the raw memory function suppresses
the warning.
</p>
</dd>
<dt><code>-Wnon-virtual-dtor <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wnon_002dvirtual_002ddtor"></a>
<a name="index-Wno_002dnon_002dvirtual_002ddtor"></a>
<p>Warn when a class has virtual functions and an accessible non-virtual
destructor itself or in an accessible polymorphic base class, in which
case it is possible but unsafe to delete an instance of a derived
class through a pointer to the class itself or base class.  This
warning is automatically enabled if <samp>-Weffc++</samp> is specified.
</p>
</dd>
<dt><code>-Wregister <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wregister"></a>
<a name="index-Wno_002dregister"></a>
<p>Warn on uses of the <code>register</code> storage class specifier, except
when it is part of the GNU <a href="Explicit-Register-Variables.html#Explicit-Register-Variables">Explicit Register Variables</a> extension.
The use of the <code>register</code> keyword as storage class specifier has
been deprecated in C++11 and removed in C++17.
Enabled by default with <samp>-std=c++17</samp>.
</p>
</dd>
<dt><code>-Wreorder <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wreorder"></a>
<a name="index-Wno_002dreorder"></a>
<a name="index-reordering_002c-warning"></a>
<a name="index-warning-for-reordering-of-member-initializers"></a>
<p>Warn when the order of member initializers given in the code does not
match the order in which they must be executed.  For instance:
</p>
<div class="smallexample">
<pre class="smallexample">struct A {
  int i;
  int j;
  A(): j (0), i (1) { }
};
</pre></div>

<p>The compiler rearranges the member initializers for <code>i</code>
and <code>j</code> to match the declaration order of the members, emitting
a warning to that effect.  This warning is enabled by <samp>-Wall</samp>.
</p>
</dd>
<dt><code>-Wno-pessimizing-move <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wpessimizing_002dmove"></a>
<a name="index-Wno_002dpessimizing_002dmove"></a>
<p>This warning warns when a call to <code>std::move</code> prevents copy
elision.  A typical scenario when copy elision can occur is when returning in
a function with a class return type, when the expression being returned is the
name of a non-volatile automatic object, and is not a function parameter, and
has the same type as the function return type.
</p>
<div class="smallexample">
<pre class="smallexample">struct T {
&hellip;
};
T fn()
{
  T t;
  &hellip;
  return std::move (t);
}
</pre></div>

<p>But in this example, the <code>std::move</code> call prevents copy elision.
</p>
<p>This warning is enabled by <samp>-Wall</samp>.
</p>
</dd>
<dt><code>-Wno-redundant-move <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wredundant_002dmove"></a>
<a name="index-Wno_002dredundant_002dmove"></a>
<p>This warning warns about redundant calls to <code>std::move</code>; that is, when
a move operation would have been performed even without the <code>std::move</code>
call.  This happens because the compiler is forced to treat the object as if
it were an rvalue in certain situations such as returning a local variable,
where copy elision isn&rsquo;t applicable.  Consider:
</p>
<div class="smallexample">
<pre class="smallexample">struct T {
&hellip;
};
T fn(T t)
{
  &hellip;
  return std::move (t);
}
</pre></div>

<p>Here, the <code>std::move</code> call is redundant.  Because G++ implements Core
Issue 1579, another example is:
</p>
<div class="smallexample">
<pre class="smallexample">struct T { // convertible to U
&hellip;
};
struct U {
&hellip;
};
U fn()
{
  T t;
  &hellip;
  return std::move (t);
}
</pre></div>
<p>In this example, copy elision isn&rsquo;t applicable because the type of the
expression being returned and the function return type differ, yet G++
treats the return value as if it were designated by an rvalue.
</p>
<p>This warning is enabled by <samp>-Wextra</samp>.
</p>
</dd>
<dt><code>-Wredundant-tags <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wredundant_002dtags"></a>
<a name="index-Wno_002dredundant_002dtags"></a>
<p>Warn about redundant class-key and enum-key in references to class types
and enumerated types in contexts where the key can be eliminated without
causing an ambiguity.  For example:
</p>
<div class="smallexample">
<pre class="smallexample">struct foo;
struct foo *p;   // warn that keyword struct can be eliminated
</pre></div>

<p>On the other hand, in this example there is no warning:
</p>
<div class="smallexample">
<pre class="smallexample">struct foo;
void foo ();   // &quot;hides&quot; struct foo
void bar (struct foo&amp;);  // no warning, keyword struct is necessary
</pre></div>

</dd>
<dt><code>-Wno-subobject-linkage <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wsubobject_002dlinkage"></a>
<a name="index-Wno_002dsubobject_002dlinkage"></a>
<p>Do not warn
if a class type has a base or a field whose type uses the anonymous
namespace or depends on a type with no linkage.  If a type A depends on
a type B with no or internal linkage, defining it in multiple
translation units would be an ODR violation because the meaning of B
is different in each translation unit.  If A only appears in a single
translation unit, the best way to silence the warning is to give it
internal linkage by putting it in an anonymous namespace as well.  The
compiler doesn&rsquo;t give this warning for types defined in the main .C
file, as those are unlikely to have multiple definitions.
<samp>-Wsubobject-linkage</samp> is enabled by default.
</p>
</dd>
<dt><code>-Weffc++ <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Weffc_002b_002b"></a>
<a name="index-Wno_002deffc_002b_002b"></a>
<p>Warn about violations of the following style guidelines from Scott Meyers&rsquo;
<cite>Effective C++</cite> series of books:
</p>
<ul>
<li> Define a copy constructor and an assignment operator for classes
with dynamically-allocated memory.

</li><li> Prefer initialization to assignment in constructors.

</li><li> Have <code>operator=</code> return a reference to <code>*this</code>.

</li><li> Don&rsquo;t try to return a reference when you must return an object.

</li><li> Distinguish between prefix and postfix forms of increment and
decrement operators.

</li><li> Never overload <code>&amp;&amp;</code>, <code>||</code>, or <code>,</code>.

</li></ul>

<p>This option also enables <samp>-Wnon-virtual-dtor</samp>, which is also
one of the effective C++ recommendations.  However, the check is
extended to warn about the lack of virtual destructor in accessible
non-polymorphic bases classes too.
</p>
<p>When selecting this option, be aware that the standard library
headers do not obey all of these guidelines; use &lsquo;<samp>grep -v</samp>&rsquo;
to filter out those warnings.
</p>
</dd>
<dt><code>-Wstrict-null-sentinel <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wstrict_002dnull_002dsentinel"></a>
<a name="index-Wno_002dstrict_002dnull_002dsentinel"></a>
<p>Warn about the use of an uncasted <code>NULL</code> as sentinel.  When
compiling only with GCC this is a valid sentinel, as <code>NULL</code> is defined
to <code>__null</code>.  Although it is a null pointer constant rather than a
null pointer, it is guaranteed to be of the same size as a pointer.
But this use is not portable across different compilers.
</p>
</dd>
<dt><code>-Wno-non-template-friend <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wno_002dnon_002dtemplate_002dfriend"></a>
<a name="index-Wnon_002dtemplate_002dfriend"></a>
<p>Disable warnings when non-template friend functions are declared
within a template.  In very old versions of GCC that predate implementation
of the ISO standard, declarations such as 
&lsquo;<samp>friend int foo(int)</samp>&rsquo;, where the name of the friend is an unqualified-id,
could be interpreted as a particular specialization of a template
function; the warning exists to diagnose compatibility problems, 
and is enabled by default.
</p>
</dd>
<dt><code>-Wold-style-cast <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wold_002dstyle_002dcast"></a>
<a name="index-Wno_002dold_002dstyle_002dcast"></a>
<p>Warn if an old-style (C-style) cast to a non-void type is used within
a C++ program.  The new-style casts (<code>dynamic_cast</code>,
<code>static_cast</code>, <code>reinterpret_cast</code>, and <code>const_cast</code>) are
less vulnerable to unintended effects and much easier to search for.
</p>
</dd>
<dt><code>-Woverloaded-virtual <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Woverloaded_002dvirtual"></a>
<a name="index-Wno_002doverloaded_002dvirtual"></a>
<a name="index-overloaded-virtual-function_002c-warning"></a>
<a name="index-warning-for-overloaded-virtual-function"></a>
<p>Warn when a function declaration hides virtual functions from a
base class.  For example, in:
</p>
<div class="smallexample">
<pre class="smallexample">struct A {
  virtual void f();
};

struct B: public A {
  void f(int);
};
</pre></div>

<p>the <code>A</code> class version of <code>f</code> is hidden in <code>B</code>, and code
like:
</p>
<div class="smallexample">
<pre class="smallexample">B* b;
b-&gt;f();
</pre></div>

<p>fails to compile.
</p>
</dd>
<dt><code>-Wno-pmf-conversions <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wno_002dpmf_002dconversions"></a>
<a name="index-Wpmf_002dconversions"></a>
<p>Disable the diagnostic for converting a bound pointer to member function
to a plain pointer.
</p>
</dd>
<dt><code>-Wsign-promo <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wsign_002dpromo"></a>
<a name="index-Wno_002dsign_002dpromo"></a>
<p>Warn when overload resolution chooses a promotion from unsigned or
enumerated type to a signed type, over a conversion to an unsigned type of
the same size.  Previous versions of G++ tried to preserve
unsignedness, but the standard mandates the current behavior.
</p>
</dd>
<dt><code>-Wtemplates <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wtemplates"></a>
<a name="index-Wno_002dtemplates"></a>
<p>Warn when a primary template declaration is encountered.  Some coding
rules disallow templates, and this may be used to enforce that rule.
The warning is inactive inside a system header file, such as the STL, so
one can still use the STL.  One may also instantiate or specialize
templates.
</p>
</dd>
<dt><code>-Wmismatched-tags <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wmismatched_002dtags"></a>
<a name="index-Wno_002dmismatched_002dtags"></a>
<p>Warn for declarations of structs, classes, and class templates and their
specializations with a class-key that does not match either the definition
or the first declaration if no definition is provided.
</p>
<p>For example, the declaration of <code>struct Object</code> in the argument list
of <code>draw</code> triggers the warning.  To avoid it, either remove the redundant
class-key <code>struct</code> or replace it with <code>class</code> to match its definition.
</p><div class="smallexample">
<pre class="smallexample">class Object {
public:
  virtual ~Object () = 0;
};
void draw (struct Object*);
</pre></div>

<p>It is not wrong to declare a class with the class-key <code>struct</code> as
the example above shows.  The <samp>-Wmismatched-tags</samp> option is intended
to help achieve a consistent style of class declarations.  In code that is
intended to be portable to Windows-based compilers the warning helps prevent
unresolved references due to the difference in the mangling of symbols
declared with different class-keys.  The option can be used either on its
own or in conjunction with <samp>-Wredundant-tags</samp>.
</p>
</dd>
<dt><code>-Wmultiple-inheritance <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wmultiple_002dinheritance"></a>
<a name="index-Wno_002dmultiple_002dinheritance"></a>
<p>Warn when a class is defined with multiple direct base classes.  Some
coding rules disallow multiple inheritance, and this may be used to
enforce that rule.  The warning is inactive inside a system header file,
such as the STL, so one can still use the STL.  One may also define
classes that indirectly use multiple inheritance.
</p>
</dd>
<dt><code>-Wvirtual-inheritance</code></dt>
<dd><a name="index-Wvirtual_002dinheritance"></a>
<a name="index-Wno_002dvirtual_002dinheritance"></a>
<p>Warn when a class is defined with a virtual direct base class.  Some
coding rules disallow multiple inheritance, and this may be used to
enforce that rule.  The warning is inactive inside a system header file,
such as the STL, so one can still use the STL.  One may also define
classes that indirectly use virtual inheritance.
</p>
</dd>
<dt><code>-Wno-virtual-move-assign</code></dt>
<dd><a name="index-Wvirtual_002dmove_002dassign"></a>
<a name="index-Wno_002dvirtual_002dmove_002dassign"></a>
<p>Suppress warnings about inheriting from a virtual base with a
non-trivial C++11 move assignment operator.  This is dangerous because
if the virtual base is reachable along more than one path, it is
moved multiple times, which can mean both objects end up in the
moved-from state.  If the move assignment operator is written to avoid
moving from a moved-from object, this warning can be disabled.
</p>
</dd>
<dt><code>-Wnamespaces</code></dt>
<dd><a name="index-Wnamespaces"></a>
<a name="index-Wno_002dnamespaces"></a>
<p>Warn when a namespace definition is opened.  Some coding rules disallow
namespaces, and this may be used to enforce that rule.  The warning is
inactive inside a system header file, such as the STL, so one can still
use the STL.  One may also use using directives and qualified names.
</p>
</dd>
<dt><code>-Wno-terminate <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wterminate"></a>
<a name="index-Wno_002dterminate"></a>
<p>Disable the warning about a throw-expression that will immediately
result in a call to <code>terminate</code>.
</p>
</dd>
<dt><code>-Wno-class-conversion <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wno_002dclass_002dconversion"></a>
<a name="index-Wclass_002dconversion"></a>
<p>Do not warn when a conversion function converts an
object to the same type, to a base class of that type, or to void; such
a conversion function will never be called.
</p>
</dd>
<dt><code>-Wvolatile <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wvolatile"></a>
<a name="index-Wno_002dvolatile"></a>
<p>Warn about deprecated uses of the <code>volatile</code> qualifier.  This includes
postfix and prefix <code>++</code> and <code>--</code> expressions of
<code>volatile</code>-qualified types, using simple assignments where the left
operand is a <code>volatile</code>-qualified non-class type for their value,
compound assignments where the left operand is a <code>volatile</code>-qualified
non-class type, <code>volatile</code>-qualified function return type,
<code>volatile</code>-qualified parameter type, and structured bindings of a
<code>volatile</code>-qualified type.  This usage was deprecated in C++20.
</p>
<p>Enabled by default with <samp>-std=c++2a</samp>.
</p>
</dd>
<dt><code>-Wzero-as-null-pointer-constant <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wzero_002das_002dnull_002dpointer_002dconstant"></a>
<a name="index-Wno_002dzero_002das_002dnull_002dpointer_002dconstant"></a>
<p>Warn when a literal &lsquo;<samp>0</samp>&rsquo; is used as null pointer constant.  This can
be useful to facilitate the conversion to <code>nullptr</code> in C++11.
</p>
</dd>
<dt><code>-Waligned-new</code></dt>
<dd><a name="index-Waligned_002dnew"></a>
<a name="index-Wno_002daligned_002dnew"></a>
<p>Warn about a new-expression of a type that requires greater alignment
than the <code>alignof(std::max_align_t)</code> but uses an allocation
function without an explicit alignment parameter. This option is
enabled by <samp>-Wall</samp>.
</p>
<p>Normally this only warns about global allocation functions, but
<samp>-Waligned-new=all</samp> also warns about class member allocation
functions.
</p>
</dd>
<dt><code>-Wno-placement-new</code></dt>
<dt><code>-Wplacement-new=<var>n</var></code></dt>
<dd><a name="index-Wplacement_002dnew"></a>
<a name="index-Wno_002dplacement_002dnew"></a>
<p>Warn about placement new expressions with undefined behavior, such as
constructing an object in a buffer that is smaller than the type of
the object.  For example, the placement new expression below is diagnosed
because it attempts to construct an array of 64 integers in a buffer only
64 bytes large.
</p><div class="smallexample">
<pre class="smallexample">char buf [64];
new (buf) int[64];
</pre></div>
<p>This warning is enabled by default.
</p>
<dl compact="compact">
<dt><code>-Wplacement-new=1</code></dt>
<dd><p>This is the default warning level of <samp>-Wplacement-new</samp>.  At this
level the warning is not issued for some strictly undefined constructs that
GCC allows as extensions for compatibility with legacy code.  For example,
the following <code>new</code> expression is not diagnosed at this level even
though it has undefined behavior according to the C++ standard because
it writes past the end of the one-element array.
</p><div class="smallexample">
<pre class="smallexample">struct S { int n, a[1]; };
S *s = (S *)malloc (sizeof *s + 31 * sizeof s-&gt;a[0]);
new (s-&gt;a)int [32]();
</pre></div>

</dd>
<dt><code>-Wplacement-new=2</code></dt>
<dd><p>At this level, in addition to diagnosing all the same constructs as at level
1, a diagnostic is also issued for placement new expressions that construct
an object in the last member of structure whose type is an array of a single
element and whose size is less than the size of the object being constructed.
While the previous example would be diagnosed, the following construct makes
use of the flexible member array extension to avoid the warning at level 2.
</p><div class="smallexample">
<pre class="smallexample">struct S { int n, a[]; };
S *s = (S *)malloc (sizeof *s + 32 * sizeof s-&gt;a[0]);
new (s-&gt;a)int [32]();
</pre></div>

</dd>
</dl>

</dd>
<dt><code>-Wcatch-value</code></dt>
<dt><code>-Wcatch-value=<var>n</var> <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wcatch_002dvalue"></a>
<a name="index-Wno_002dcatch_002dvalue"></a>
<p>Warn about catch handlers that do not catch via reference.
With <samp>-Wcatch-value=1</samp> (or <samp>-Wcatch-value</samp> for short)
warn about polymorphic class types that are caught by value.
With <samp>-Wcatch-value=2</samp> warn about all class types that are caught
by value. With <samp>-Wcatch-value=3</samp> warn about all types that are
not caught by reference. <samp>-Wcatch-value</samp> is enabled by <samp>-Wall</samp>.
</p>
</dd>
<dt><code>-Wconditionally-supported <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wconditionally_002dsupported"></a>
<a name="index-Wno_002dconditionally_002dsupported"></a>
<p>Warn for conditionally-supported (C++11 [intro.defs]) constructs.
</p>
</dd>
<dt><code>-Wno-delete-incomplete <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wdelete_002dincomplete"></a>
<a name="index-Wno_002ddelete_002dincomplete"></a>
<p>Do not warn when deleting a pointer to incomplete type, which may cause
undefined behavior at runtime.  This warning is enabled by default.
</p>
</dd>
<dt><code>-Wextra-semi <span class="roman">(C++, Objective-C++ only)</span></code></dt>
<dd><a name="index-Wextra_002dsemi"></a>
<a name="index-Wno_002dextra_002dsemi"></a>
<p>Warn about redundant semicolons after in-class function definitions.
</p>
</dd>
<dt><code>-Wno-inaccessible-base <span class="roman">(C++, Objective-C++ only)</span></code></dt>
<dd><a name="index-Winaccessible_002dbase"></a>
<a name="index-Wno_002dinaccessible_002dbase"></a>
<p>This option controls warnings
when a base class is inaccessible in a class derived from it due to
ambiguity.  The warning is enabled by default.
Note that the warning for ambiguous virtual
bases is enabled by the <samp>-Wextra</samp> option.
</p><div class="smallexample">
<pre class="smallexample">struct A { int a; };

struct B : A { };

struct C : B, A { };
</pre></div>

</dd>
<dt><code>-Wno-inherited-variadic-ctor</code></dt>
<dd><a name="index-Winherited_002dvariadic_002dctor"></a>
<a name="index-Wno_002dinherited_002dvariadic_002dctor"></a>
<p>Suppress warnings about use of C++11 inheriting constructors when the
base class inherited from has a C variadic constructor; the warning is
on by default because the ellipsis is not inherited.
</p>
</dd>
<dt><code>-Wno-invalid-offsetof <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wno_002dinvalid_002doffsetof"></a>
<a name="index-Winvalid_002doffsetof"></a>
<p>Suppress warnings from applying the <code>offsetof</code> macro to a non-POD
type.  According to the 2014 ISO C++ standard, applying <code>offsetof</code>
to a non-standard-layout type is undefined.  In existing C++ implementations,
however, <code>offsetof</code> typically gives meaningful results.
This flag is for users who are aware that they are
writing nonportable code and who have deliberately chosen to ignore the
warning about it.
</p>
<p>The restrictions on <code>offsetof</code> may be relaxed in a future version
of the C++ standard.
</p>
</dd>
<dt><code>-Wsized-deallocation <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wsized_002ddeallocation"></a>
<a name="index-Wno_002dsized_002ddeallocation"></a>
<p>Warn about a definition of an unsized deallocation function
</p><div class="smallexample">
<pre class="smallexample">void operator delete (void *) noexcept;
void operator delete[] (void *) noexcept;
</pre></div>
<p>without a definition of the corresponding sized deallocation function
</p><div class="smallexample">
<pre class="smallexample">void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;
</pre></div>
<p>or vice versa.  Enabled by <samp>-Wextra</samp> along with
<samp>-fsized-deallocation</samp>.
</p>
</dd>
<dt><code>-Wsuggest-final-types</code></dt>
<dd><a name="index-Wno_002dsuggest_002dfinal_002dtypes"></a>
<a name="index-Wsuggest_002dfinal_002dtypes"></a>
<p>Warn about types with virtual methods where code quality would be improved
if the type were declared with the C++11 <code>final</code> specifier,
or, if possible,
declared in an anonymous namespace. This allows GCC to more aggressively
devirtualize the polymorphic calls. This warning is more effective with
link-time optimization,
where the information about the class hierarchy graph is
more complete.
</p>
</dd>
<dt><code>-Wsuggest-final-methods</code></dt>
<dd><a name="index-Wno_002dsuggest_002dfinal_002dmethods"></a>
<a name="index-Wsuggest_002dfinal_002dmethods"></a>
<p>Warn about virtual methods where code quality would be improved if the method
were declared with the C++11 <code>final</code> specifier,
or, if possible, its type were
declared in an anonymous namespace or with the <code>final</code> specifier.
This warning is
more effective with link-time optimization, where the information about the
class hierarchy graph is more complete. It is recommended to first consider
suggestions of <samp>-Wsuggest-final-types</samp> and then rebuild with new
annotations.
</p>
</dd>
<dt><code>-Wsuggest-override</code></dt>
<dd><a name="index-Wsuggest_002doverride"></a>
<a name="index-Wno_002dsuggest_002doverride"></a>
<p>Warn about overriding virtual functions that are not marked with the
<code>override</code> keyword.
</p>
</dd>
<dt><code>-Wuseless-cast <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wuseless_002dcast"></a>
<a name="index-Wno_002duseless_002dcast"></a>
<p>Warn when an expression is casted to its own type.
</p>
</dd>
<dt><code>-Wno-conversion-null <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wconversion_002dnull"></a>
<a name="index-Wno_002dconversion_002dnull"></a>
<p>Do not warn for conversions between <code>NULL</code> and non-pointer
types. <samp>-Wconversion-null</samp> is enabled by default.
</p>
</dd>
</dl>

<hr>
<div class="header">
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