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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 <code>firstClass.C</code> like this:
</p>
<div class="smallexample">
<pre class="smallexample">g++ -g -frepo -O -c firstClass.C
</pre></div>
<p>In this example, only <samp>-frepo</samp> is an option meant
only for C++ programs; you can use the other options with any
language supported by GCC.
</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 2.
</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.
</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>See also <samp>-Wabi</samp>.
</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
&lsquo;<samp>throw()</samp>&rsquo;, 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>-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 &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, and versions of G++ before 4.1 always worked
that way. However, in ISO C++ a friend function that is not declared
in an enclosing scope can only be found using argument dependent
lookup. This option causes friends to be injected as they were in
earlier releases.
</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&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
&lsquo;<samp>NDEBUG</samp>&rsquo;. 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 &lsquo;<samp>thread_local</samp>&rsquo; and
&lsquo;<samp>threadprivate</samp>&rsquo; 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 &lsquo;<samp>for</samp>&rsquo; 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.
<samp>-ansi</samp> implies <samp>-fno-gnu-keywords</samp>.
</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 &lsquo;<samp>#pragma implementation</samp>&rsquo;. 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&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
(&lsquo;<samp>dynamic_cast</samp>&rsquo; and &lsquo;<samp>typeid</samp>&rsquo;). 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 &lsquo;<samp>dynamic_cast</samp>&rsquo; 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>-fstats</code></dt>
<dd><a name="index-fstats"></a>
<p>Emit statistics about front-end processing at the end of the compilation.
This information is generally only useful to the G++ development team.
</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&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>-fvtable-verify=<var>std|preinit|none</var></code></dt>
<dd><a name="index-fvtable_002dverify"></a>
<p>Turn on (or off, if using <samp>-fvtable-verify=none</samp>) the security
feature that verifies at runtime, for every virtual call that is made, that
the vtable pointer through which the call is made is valid for the type of
the object, and has not been corrupted or overwritten. If an invalid vtable
pointer is detected (at runtime), an error is reported and execution of the
program is immediately halted.
</p>
<p>This option causes runtime data structures to be built, at program start up,
for verifying the vtable pointers. The options <code>std</code> and <code>preinit</code>
control the timing of when these data structures are built. In both cases the
data structures are built before execution reaches &rsquo;main&rsquo;. The
<samp>-fvtable-verify=std</samp> causes these data structure to be built after the
shared libraries have been loaded and initialized.
<samp>-fvtable-verify=preinit</samp> causes them to be built before the shared
libraries have been loaded and initialized.
</p>
<p>If this option appears multiple times in the compiler line, with different
values specified, &rsquo;none&rsquo; will take highest priority over both &rsquo;std&rsquo; and
&rsquo;preinit&rsquo;; &rsquo;preinit&rsquo; will take priority over &rsquo;std&rsquo;.
</p>
</dd>
<dt><code>-fvtv-debug</code></dt>
<dd><a name="index-_0028fvtv_002ddebug_0029"></a>
<p>Causes debug versions of the runtime functions for the vtable verification
feature to be called. This assumes the <samp>-fvtable-verify=std</samp> or
<samp>-fvtable-verify=preinit</samp> has been used. This flag will also cause the
compiler to keep track of which vtable pointers it found for each class, and
record that information in the file &ldquo;vtv_set_ptr_data.log&rdquo;, in the dump
file directory on the user&rsquo;s machine.
</p>
<p>Note: This feature APPENDS data to the log file. If you want a fresh log
file, be sure to delete any existing one.
</p>
</dd>
<dt><code>-fvtv-counts</code></dt>
<dd><a name="index-fvtv_002dcounts"></a>
<p>This is a debugging flag. When used in conjunction with
<samp>-fvtable-verify=std</samp> or <samp>-fvtable-verify=preinit</samp>, this
causes the compiler to keep track of the total number of virtual calls
it encountered and the number of verifications it inserted. It also
counts the number of calls to certain runtime library functions
that it inserts. This information, for each compilation unit, is written
to a file named &ldquo;vtv_count_data.log&rdquo;, in the dump_file directory on
the user&rsquo;s machine. It also counts the size of the vtable pointer sets
for each class, and writes this information to &ldquo;vtv_class_set_sizes.log&rdquo;
in the same directory.
</p>
<p>Note: This feature APPENDS data to the log files. To get a fresh log
files, be sure to delete any existing ones.
</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++ generates code that is probably not compatible with the
vendor-neutral C++ ABI. 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>The known incompatibilities in <samp>-fabi-version=2</samp> (the default) include:
</p>
<ul>
<li> A template with a non-type template parameter of reference type is
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 is fixed in <samp>-fabi-version=3</samp>.
</p>
</li><li> SIMD vector types declared using <code>__attribute ((vector_size))</code> are
mangled in a non-standard way that does not allow for overloading of
functions taking vectors of different sizes.
<p>The mangling is changed in <samp>-fabi-version=4</samp>.
</p></li></ul>
<p>The known incompatibilities in <samp>-fabi-version=1</samp> include:
</p>
<ul>
<li> Incorrect handling of tail-padding for bit-fields. G++ may attempt to
pack data into the same byte as a base class. For example:
<div class="smallexample">
<pre class="smallexample">struct A { virtual void f(); int f1 : 1; };
struct B : public A { int f2 : 1; };
</pre></div>
<p>In this case, G++ places <code>B::f2</code> into the same byte
as <code>A::f1</code>; other compilers do not. You can avoid this problem
by explicitly padding <code>A</code> so that its size is a multiple of the
byte size on your platform; that causes G++ and other compilers to
lay out <code>B</code> identically.
</p>
</li><li> Incorrect handling of tail-padding for virtual bases. G++ does not use
tail padding when laying out virtual bases. For example:
<div class="smallexample">
<pre class="smallexample">struct A { virtual void f(); char c1; };
struct B { B(); char c2; };
struct C : public A, public virtual B {};
</pre></div>
<p>In this case, G++ does not place <code>B</code> into the tail-padding for
<code>A</code>; other compilers do. You can avoid this problem by
explicitly padding <code>A</code> so that its size is a multiple of its
alignment (ignoring virtual base classes); that causes G++ and other
compilers to lay out <code>C</code> identically.
</p>
</li><li> Incorrect handling of bit-fields with declared widths greater than that
of their underlying types, when the bit-fields appear in a union. For
example:
<div class="smallexample">
<pre class="smallexample">union U { int i : 4096; };
</pre></div>
<p>Assuming that an <code>int</code> does not have 4096 bits, G++ makes the
union too small by the number of bits in an <code>int</code>.
</p>
</li><li> Empty classes can be placed at incorrect offsets. For example:
<div class="smallexample">
<pre class="smallexample">struct A {};
struct B {
A a;
virtual void f ();
};
struct C : public B, public A {};
</pre></div>
<p>G++ places the <code>A</code> base class of <code>C</code> at a nonzero offset;
it should be placed at offset zero. G++ mistakenly believes that the
<code>A</code> data member of <code>B</code> is already at offset zero.
</p>
</li><li> Names of template functions whose types involve <code>typename</code> or
template template parameters can be mangled incorrectly.
<div class="smallexample">
<pre class="smallexample">template &lt;typename Q&gt;
void f(typename Q::X) {}
template &lt;template &lt;typename&gt; class Q&gt;
void f(typename Q&lt;int&gt;::X) {}
</pre></div>
<p>Instantiations of these templates may be mangled incorrectly.
</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>-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 &lsquo;<samp>delete</samp>&rsquo; 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>This warning is 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>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>
<p>With <samp>-std=c++11</samp>, <samp>-Wno-narrowing</samp> suppresses the diagnostic
required by the standard. Note that this does not affect the meaning
of well-formed code; narrowing conversions are still considered
ill-formed in SFINAE context.
</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. &lsquo;<samp>throw()</samp>&rsquo; or &lsquo;<samp>noexcept</samp>&rsquo;) 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 &lsquo;<samp>i</samp>&rsquo;
and &lsquo;<samp>j</samp>&rsquo; 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++1y</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-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.,
&lsquo;<samp>friend foo(int)</samp>&rsquo;), 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 (&lsquo;<samp>dynamic_cast</samp>&rsquo;,
&lsquo;<samp>static_cast</samp>&rsquo;, &lsquo;<samp>reinterpret_cast</samp>&rsquo;, and &lsquo;<samp>const_cast</samp>&rsquo;) 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>
</dl>
<hr>
<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>
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