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<a name="C_002b_002b-Dialect-Options"></a>
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<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>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 8 (GCC 5
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>
<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>
</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>
<dd><a name="index-fconcepts"></a>
<p>Enable support for the C++ Extensions for Concepts Technical
Specification, ISO 19217 (2015), which allows code like
</p>
<div class="smallexample">
<pre class="smallexample">template &lt;class T&gt; concept bool Addable = requires (T t) { t + t; };
template &lt;Addable T&gt; T add (T a, T b) { return a + b; }
</pre></div>

</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-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>-fdeduce-init-list</code></dt>
<dd><a name="index-fdeduce_002dinit_002dlist"></a>
<p>Enable deduction of a template type parameter as
<code>std::initializer_list</code> from a brace-enclosed initializer list, i.e.
</p>
<div class="smallexample">
<pre class="smallexample">template &lt;class T&gt; auto forward(T t) -&gt; decltype (realfn (t))
{
  return realfn (t);
}

void f()
{
  forward({1,2}); // call forward&lt;std::initializer_list&lt;int&gt;&gt;
}
</pre></div>

<p>This deduction was implemented as a possible extension to the
originally proposed semantics for the C++11 standard, but was not part
of the final standard, so it is disabled by default.  This option is
deprecated, and may be removed in a future version of G++.
</p>
</dd>
<dt><code>-ffriend-injection</code></dt>
<dd><a name="index-ffriend_002dinjection"></a>
<p>Inject friend functions into the enclosing namespace, so that they are
visible outside the scope of the class in which they are declared.
Friend functions were documented to work this way in the old Annotated
C++ Reference Manual.  
However, in ISO C++ a friend function that is not declared
in an enclosing scope can only be found using argument dependent
lookup.  GCC defaults to the standard behavior.
</p>
<p>This option is for compatibility, and may be removed in a future
release of G++.
</p>
</dd>
<dt><code>-fno-elide-constructors</code></dt>
<dd><a name="index-fno_002delide_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>
<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>-ffor-scope</code></dt>
<dt><code>-fno-for-scope</code></dt>
<dd><a name="index-ffor_002dscope"></a>
<a name="index-fno_002dfor_002dscope"></a>
<p>If <samp>-ffor-scope</samp> is specified, the scope of variables declared in
a <i>for-init-statement</i> is limited to the <code>for</code> loop itself,
as specified by the C++ standard.
If <samp>-fno-for-scope</samp> is specified, the scope of variables declared in
a <i>for-init-statement</i> extends to the end of the enclosing scope,
as was the case in old versions of G++, and other (traditional)
implementations of C++.
</p>
<p>If neither flag is given, the default is to follow the standard,
but to allow and give a warning for old-style code that would
otherwise be invalid, or have different behavior.
</p>
</dd>
<dt><code>-fno-gnu-keywords</code></dt>
<dd><a name="index-fno_002dgnu_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>
<p>Never emit code for non-inline templates that are instantiated
implicitly (i.e. by use); only emit code for explicit instantiations.
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>
<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>
<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++1z</samp>.
</p>
</dd>
<dt><code>-fno-nonansi-builtins</code></dt>
<dd><a name="index-fno_002dnonansi_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>
<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>
<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>
<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>-frepo</code></dt>
<dd><a name="index-frepo"></a>
<p>Enable automatic template instantiation at link time.  This option also
implies <samp>-fno-implicit-templates</samp>.  See <a href="Template-Instantiation.html#Template-Instantiation">Template Instantiation</a>, for more information.
</p>
</dd>
<dt><code>-fno-rtti</code></dt>
<dd><a name="index-fno_002drtti"></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>
</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++1z</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++1z</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>
<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>
<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>
<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>-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 optimization, warning, and code generation options
have meanings only for C++ programs:
</p>
<dl compact="compact">
<dt><code>-Wabi <span class="roman">(C, Objective-C, C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wabi"></a>
<a name="index-Wno_002dabi"></a>
<p>Warn when G++ it generates code that is probably not compatible with
the vendor-neutral C++ ABI.  Since G++ now defaults to updating the
ABI with each major release, normally <samp>-Wabi</samp> will warn only if
there is a check added later in a release series for an ABI issue
discovered since the initial release.  <samp>-Wabi</samp> will warn about
more things if an older ABI version is selected (with
<samp>-fabi-version=<var>n</var></samp>).
</p>
<p><samp>-Wabi</samp> can also be used with an explicit version number to
warn about compatibility with a particular <samp>-fabi-version</samp>
level, e.g. <samp>-Wabi=2</samp> to warn about changes relative to
<samp>-fabi-version=2</samp>.
</p>
<p>If an explicit version number is provided and
<samp>-fabi-compat-version</samp> is not specified, the version number
from this option is used for compatibility aliases.  If no explicit
version number is provided with this option, but
<samp>-fabi-compat-version</samp> is specified, that version number is
used for ABI warnings.
</p>
<p>Although an effort has been made to warn about
all such cases, there are probably some cases that are not warned about,
even though G++ is generating incompatible code.  There may also be
cases where warnings are emitted even though the code that is generated
is compatible.
</p>
<p>You should rewrite your code to avoid these warnings if you are
concerned about the fact that code generated by G++ may not be binary
compatible with code generated by other compilers.
</p>
<p>Known incompatibilities in <samp>-fabi-version=2</samp> (which was the
default from GCC 3.4 to 4.9) include:
</p>
<ul>
<li> A template with a non-type template parameter of reference type was
mangled incorrectly:
<div class="smallexample">
<pre class="smallexample">extern int N;
template &lt;int &amp;&gt; struct S {};
void n (S&lt;N&gt;) {2}
</pre></div>

<p>This was fixed in <samp>-fabi-version=3</samp>.
</p>
</li><li> SIMD vector types declared using <code>__attribute ((vector_size))</code> were
mangled in a non-standard way that does not allow for overloading of
functions taking vectors of different sizes.

<p>The mangling was changed in <samp>-fabi-version=4</samp>.
</p>
</li><li> <code>__attribute ((const))</code> and <code>noreturn</code> were mangled as type
qualifiers, and <code>decltype</code> of a plain declaration was folded away.

<p>These mangling issues were fixed in <samp>-fabi-version=5</samp>.
</p>
</li><li> Scoped enumerators passed as arguments to a variadic function are
promoted like unscoped enumerators, causing <code>va_arg</code> to complain.
On most targets this does not actually affect the parameter passing
ABI, as there is no way to pass an argument smaller than <code>int</code>.

<p>Also, the ABI changed the mangling of template argument packs,
<code>const_cast</code>, <code>static_cast</code>, prefix increment/decrement, and
a class scope function used as a template argument.
</p>
<p>These issues were corrected in <samp>-fabi-version=6</samp>.
</p>
</li><li> Lambdas in default argument scope were mangled incorrectly, and the
ABI changed the mangling of <code>nullptr_t</code>.

<p>These issues were corrected in <samp>-fabi-version=7</samp>.
</p>
</li><li> When mangling a function type with function-cv-qualifiers, the
un-qualified function type was incorrectly treated as a substitution
candidate.

<p>This was fixed in <samp>-fabi-version=8</samp>, the default for GCC 5.1.
</p>
</li><li> <code>decltype(nullptr)</code> incorrectly had an alignment of 1, leading to
unaligned accesses.  Note that this did not affect the ABI of a
function with a <code>nullptr_t</code> parameter, as parameters have a
minimum alignment.

<p>This was fixed in <samp>-fabi-version=9</samp>, the default for GCC 5.2.
</p>
</li><li> Target-specific attributes that affect the identity of a type, such as
ia32 calling conventions on a function type (stdcall, regparm, etc.),
did not affect the mangled name, leading to name collisions when
function pointers were used as template arguments.

<p>This was fixed in <samp>-fabi-version=10</samp>, the default for GCC 6.1.
</p>
</li></ul>

<p>It also warns about psABI-related changes.  The known psABI changes at this
point include:
</p>
<ul>
<li> For SysV/x86-64, unions with <code>long double</code> members are 
passed in memory as specified in psABI.  For example:

<div class="smallexample">
<pre class="smallexample">union U {
  long double ld;
  int i;
};
</pre></div>

<p><code>union U</code> is always passed in memory.
</p>
</li></ul>

</dd>
<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-_002dWabi_002dtag"></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>-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>-Wliteral-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>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>Additionally, warn 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>This warning is enabled by default.
</p>
</dd>
<dt><code>-Wlto-type-mismatch</code></dt>
<dd><a name="index-Wlto_002dtype_002dmismatch"></a>
<a name="index-Wno_002dlto_002dtype_002dmismatch"></a>

<p>During the link-time optimization warn about type mismatches in
global declarations from different compilation units.
Requires <samp>-flto</samp> to be enabled.  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++1z 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++1z-compat</samp>.
</p>
<div class="smallexample">
<pre class="smallexample">template &lt;class T&gt; void f(T t) { t(); };
void g() noexcept;
void h() { f(g); } // in C++14 calls f&lt;void(*)()&gt;, in C++1z calls f&lt;void(*)()noexcept&gt;
</pre></div>


</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++1z</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>-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>
</dl>

<p>The following <samp>-W&hellip;</samp> options are not affected by <samp>-Wall</samp>.
</p>
<dl compact="compact">
<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>
<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>-Wmultiple-inheritance <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wmultiple_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>
<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>-Wnamespaces</code></dt>
<dd><a name="index-Wnamespaces"></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>
</dl>

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