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(gcc-300.info)C++ Dialect Options
Options Controlling C++ Dialect
===============================
This section describes the command-line options that are only
meaningful for C++ programs; but 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 `firstClass.C' like this:
g++ -g -frepo -O -c firstClass.C
In this example, only `-frepo' is an option meant only for C++
programs; you can use the other options with any language supported by
GCC.
Here is a list of options that are _only_ for compiling C++ programs:
`-fno-access-control'
Turn off all access checking. This switch is mainly useful for
working around bugs in the access control code.
`-fcheck-new'
Check that the pointer returned by `operator new' is non-null
before attempting to modify the storage allocated. The current
Working Paper requires that `operator new' never return a null
pointer, so this check is normally unnecessary.
An alternative to using this option is to specify that your
`operator new' does not throw any exceptions; if you declare it
`throw()', G++ will check the return value. See also `new
(nothrow)'.
`-fconserve-space'
Put uninitialized or runtime-initialized global variables into the
common segment, as C does. This saves space in the executable at
the cost of not diagnosing duplicate definitions. If you compile
with this flag and your program mysteriously crashes after
`main()' has completed, you may have an object that is being
destroyed twice because two definitions were merged.
This option is no longer useful on most targets, now that support
has been added for putting variables into BSS without making them
common.
`-fno-const-strings'
Give string constants type `char *' instead of type `const char
*'. By default, G++ uses type `const char *' as required by the
standard. Even if you use `-fno-const-strings', you cannot
actually modify the value of a string constant, unless you also use
`-fwritable-strings'.
This option might be removed in a future release of G++. For
maximum portability, you should structure your code so that it
works with string constants that have type `const char *'.
`-fdollars-in-identifiers'
Accept `$' in identifiers. You can also explicitly prohibit use of
`$' with the option `-fno-dollars-in-identifiers'. (GNU C allows
`$' by default on most target systems, but there are a few
exceptions.) Traditional C allowed the character `$' to form part
of identifiers. However, ISO C and C++ forbid `$' in identifiers.
`-fno-elide-constructors'
The C++ standard allows an implementation to omit creating a
temporary which 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.
`-fno-enforce-eh-specs'
Don't check for violation of exception specifications at runtime.
This option violates the C++ standard, but may be useful for
reducing code size in production builds, much like defining
`NDEBUG'. The compiler will still optimize based on the exception
specifications.
`-fexternal-templates'
Cause template instantiations to obey `#pragma interface' and
`implementation'; template instances are emitted or not according
to the location of the template definition. Note: Template
Instantiation, for more information.
This option is deprecated.
`-falt-external-templates'
Similar to `-fexternal-templates', but template instances are
emitted or not according to the place where they are first
instantiated. Note: Template Instantiation, for more
information.
This option is deprecated.
`-ffor-scope'
`-fno-for-scope'
If `-ffor-scope' is specified, the scope of variables declared in
a for-init-statement is limited to the `for' loop itself, as
specified by the C++ standard. If `-fno-for-scope' is specified,
the scope of variables declared in a for-init-statement extends to
the end of the enclosing scope, as was the case in old versions of
G++, and other (traditional) implementations of C++.
The default if neither flag is given to follow the standard, but
to allow and give a warning for old-style code that would
otherwise be invalid, or have different behavior.
`-fno-gnu-keywords'
Do not recognize `typeof' as a keyword, so that code can use this
word as an identifier. You can use the keyword `__typeof__'
instead. `-ansi' implies `-fno-gnu-keywords'.
`-fno-implicit-templates'
Never emit code for non-inline templates which are instantiated
implicitly (i.e. by use); only emit code for explicit
instantiations. Note: Template Instantiation, for more
information.
`-fno-implicit-inline-templates'
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 will need the same set of
explicit instantiations.
`-fno-implement-inlines'
To save space, do not emit out-of-line copies of inline functions
controlled by `#pragma implementation'. This will cause linker
errors if these functions are not inlined everywhere they are
called.
`-fms-extensions'
Disable pedantic warnings about constructs used in MFC, such as
implicit int and getting a pointer to member function via
non-standard syntax.
`-fno-nonansi-builtins'
Disable built-in declarations of functions that are not mandated by
ANSI/ISO C. These include `ffs', `alloca', `_exit', `index',
`bzero', `conjf', and other related functions.
`-fno-operator-names'
Do not treat the operator name keywords `and', `bitand', `bitor',
`compl', `not', `or' and `xor' as synonyms as keywords.
`-fno-optional-diags'
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.
`-fpermissive'
Downgrade messages about nonconformant code from errors to
warnings. By default, G++ effectively sets `-pedantic-errors'
without `-pedantic'; this option reverses that. This behavior and
this option are superseded by `-pedantic', which works as it does
for GNU C.
`-frepo'
Enable automatic template instantiation. This option also implies
`-fno-implicit-templates'. Note: Template Instantiation, for
more information.
`-fno-rtti'
Disable generation of information about every class with virtual
functions for use by the C++ runtime type identification features
(`dynamic_cast' and `typeid'). 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 it will
generate it as needed.
`-fstats'
Emit statistics about front-end processing at the end of the
compilation. This information is generally only useful to the G++
development team.
`-ftemplate-depth-N'
Set the maximum instantiation depth for template classes to N. 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.
`-fuse-cxa-atexit'
Register destructors for objects with static storage duration with
the `__cxa_atexit' function rather than the `atexit' function.
This option is required for fully standards-compliant handling of
static destructors, but will only work if your C library supports
`__cxa_atexit'.
`-fno-weak'
Do not use weak symbol support, even if it is provided by the
linker. By default, G++ will use weak symbols if they are
available. This option exists only for testing, and should not be
used by end-users; it will result in inferior code and has no
benefits. This option may be removed in a future release of G++.
`-nostdinc++'
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.)
In addition, these optimization, warning, and code generation options
have meanings only for C++ programs:
`-fno-default-inline'
Do not assume `inline' for functions defined inside a class scope.
Note: Options That Control Optimization. Note
that these functions will have linkage like inline functions; they
just won't be inlined by default.
`-Wctor-dtor-privacy (C++ only)'
Warn when a class seems unusable, because all the constructors or
destructors in a class are private and the class has no friends or
public static member functions.
`-Wnon-virtual-dtor (C++ only)'
Warn when a class declares a non-virtual destructor that should
probably be virtual, because it looks like the class will be used
polymorphically.
`-Wreorder (C++ only)'
Warn when the order of member initializers given in the code does
not match the order in which they must be executed. For instance:
struct A {
int i;
int j;
A(): j (0), i (1) { }
};
Here the compiler will warn that the member initializers for `i'
and `j' will be rearranged to match the declaration order of the
members.
The following `-W...' options are not affected by `-Wall'.
`-Weffc++ (C++ only)'
Warn about violations of various style guidelines from Scott
Meyers' `Effective C++' books. If you use this option, you should
be aware that the standard library headers do not obey all of
these guidelines; you can use `grep -v' to filter out those
warnings.
`-Wno-deprecated (C++ only)'
Do not warn about usage of deprecated features. Note: Deprecated
Features.
`-Wno-non-template-friend (C++ only)'
Disable warnings when non-templatized friend functions are declared
within a template. With the advent of explicit template
specification support in G++, if the name of the friend is an
unqualified-id (i.e., `friend foo(int)'), 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++, `-Wnon-template-friend' 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 `-Wno-non-template-friend' which keeps the conformant
compiler code but disables the helpful warning.
`-Wold-style-cast (C++ only)'
Warn if an old-style (C-style) cast is used within a C++ program.
The new-style casts (`static_cast', `reinterpret_cast', and
`const_cast') are less vulnerable to unintended effects, and much
easier to grep for.
`-Woverloaded-virtual (C++ only)'
Warn when a derived class function declaration may be an error in
defining a virtual function. In a derived class, the definitions
of virtual functions must match the type signature of a virtual
function declared in the base class. With this option, the
compiler warns when you define a function with the same name as a
virtual function, but with a type signature that does not match any
declarations from the base class.
`-Wno-pmf-conversions (C++ only)'
Disable the diagnostic for converting a bound pointer to member
function to a plain pointer.
`-Wsign-promo (C++ only)'
Warn when overload resolution chooses a promotion from unsigned or
enumeral type to a signed type over a conversion to an unsigned
type of the same size. Previous versions of G++ would try to
preserve unsignedness, but the standard mandates the current
behavior.
`-Wsynth (C++ only)'
Warn when G++'s synthesis behavior does not match that of cfront.
For instance:
struct A {
operator int ();
A& operator = (int);
};
main ()
{
A a,b;
a = b;
}
In this example, G++ will synthesize a default `A& operator =
(const A&);', while cfront will use the user-defined `operator ='.
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