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<title>Using the GNU Compiler Collection (GCC): C++ Dialect Options</title>
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<div class="header">
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<p>
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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> [<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>
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</div>
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<hr>
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<a name="Options-Controlling-C_002b_002b-Dialect"></a>
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<h3 class="section">3.5 Options Controlling C++ Dialect</h3>
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<a name="index-compiler-options_002c-C_002b_002b"></a>
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<a name="index-C_002b_002b-options_002c-command_002dline"></a>
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<a name="index-options_002c-C_002b_002b"></a>
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<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 –, 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. ‘<samp>stdcall</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>-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
|
‘<samp>new (nothrow)</samp>’.
|
</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 <class T> concept bool Addable = requires (T t) { t + t; };
|
template <Addable T> 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>-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 <class T> auto forward(T t) -> decltype (realfn (t))
|
{
|
return realfn (t);
|
}
|
|
void f()
|
{
|
forward({1,2}); // call forward<std::initializer_list<int>>
|
}
|
</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.
|
</p>
|
</dd>
|
<dt><code>-fno-enforce-eh-specs</code></dt>
|
<dd><a name="index-fno_002denforce_002deh_002dspecs"></a>
|
<p>Don’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’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’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>-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’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’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>-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’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’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 ("hidden")))</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’s C++
|
linkage model compatible with that of Microsoft Visual Studio.
|
</p>
|
<p>The flag makes these changes to GCC’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 <int &> struct S {};
|
void n (S<N>) {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’s at least one private member function that isn’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><inttypes.h></code>.
|
For example:
|
</p>
|
<div class="smallexample">
|
<pre class="smallexample">#define __STDC_FORMAT_MACROS
|
#include <inttypes.h>
|
#include <stdio.h>
|
|
int main() {
|
int64_t i64 = 123;
|
printf("My int64: %" PRId64"\n", i64);
|
}
|
</pre></div>
|
|
<p>In this case, <code>PRId64</code> is treated as a separate preprocessing token.
|
</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>-Wnarrowing <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
|
<dd><a name="index-Wnarrowing"></a>
|
<a name="index-Wno_002dnarrowing"></a>
|
<p>With <samp>-std=gnu++98</samp> or <samp>-std=c++98</samp>, warn when a narrowing
|
conversion prohibited by C++11 occurs within
|
‘<samp>{ }</samp>’, 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>
|
<p>When a later standard is in effect, e.g. when using <samp>-std=c++11</samp>,
|
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>
|
</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>-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>-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…</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’
|
<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’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>&&</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 ‘<samp>grep -v</samp>’
|
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-templatized friend functions are declared
|
within a template. Since the advent of explicit template specification
|
support in G++, if the name of the friend is an unqualified-id (i.e.,
|
‘<samp>friend foo(int)</samp>’), the C++ language specification demands that the
|
friend declare or define an ordinary, nontemplate function. (Section
|
14.5.3). Before G++ implemented explicit specification, unqualified-ids
|
could be interpreted as a particular specialization of a templatized
|
function. Because this non-conforming behavior is no longer the default
|
behavior for G++, <samp>-Wnon-template-friend</samp> allows the compiler to
|
check existing code for potential trouble spots and is on by default.
|
This new compiler behavior can be turned off with
|
<samp>-Wno-non-template-friend</samp>, which keeps the conformant compiler code
|
but disables the helpful warning.
|
</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->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 classe. 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>
|
|
<hr>
|
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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> [<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>
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