GNU Info
(gcc-295.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.
`-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, ANSI 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.
`-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 draft 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 gcc, 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 `classof', `headof', `signature', `sigof' or
`typeof' as a keyword, so that code can use these words as
identifiers. You can use the keywords `__classof__',
`__headof__', `__signature__', `__sigof__', and `__typeof__'
instead. `-ansi' implies `-fno-gnu-keywords'.
`-fguiding-decls'
Treat a function declaration with the same type as a potential
function template instantiation as though it declares that
instantiation, not a normal function. If a definition is given
for the function later in the translation unit (or another
translation unit if the target supports weak symbols), that
definition will be used; otherwise the template will be
instantiated. This behavior reflects the C++ language prior to
September 1996, when guiding declarations were removed.
This option implies `-fname-mangling-version-0', and will not work
with other name mangling versions. Like all options that change
the ABI, all C++ code, *including libgcc.a* must be built with the
same setting of this option.
`-fhandle-signatures'
Recognize the `signature' and `sigof' keywords for specifying
abstract types. The default (`-fno-handle-signatures') is not to
recognize them. Note: Type Abstraction using Signatures.
`-fhonor-std'
Treat the `namespace std' as a namespace, instead of ignoring it.
For compatibility with earlier versions of g++, the compiler will,
by default, ignore `namespace-declarations', `using-declarations',
`using-directives', and `namespace-names', if they involve `std'.
`-fhuge-objects'
Support virtual function calls for objects that exceed the size
representable by a `short int'. Users should not use this flag by
default; if you need to use it, the compiler will tell you so.
This flag is not useful when compiling with -fvtable-thunks.
Like all options that change the ABI, all C++ code, *including
libgcc* must be built with the same setting of this option.
`-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.
`-finit-priority'
Support `__attribute__ ((init_priority (n)))' for controlling the
order of initialization of file-scope objects. On ELF targets,
this requires GNU ld 2.10 or later.
`-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.
`-fname-mangling-version-N'
Control the way in which names are mangled. Version 0 is
compatible with versions of g++ before 2.8. Version 1 is the
default. Version 1 will allow correct mangling of function
templates. For example, version 0 mangling does not mangle
foo<int, double> and foo<int, char> given this declaration:
template <class T, class U> void foo(T t);
Like all options that change the ABI, all C++ code, *including
libgcc* must be built with the same setting of this option.
`-foperator-names'
Recognize the operator name keywords `and', `bitand', `bitor',
`compl', `not', `or' and `xor' as synonyms for the symbols they
refer to. `-ansi' implies `-foperator-names'.
`-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 superceded 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 the information used by C++ runtime type
identification features (`dynamic_cast' and `typeid'). If you
don't use those parts of the language (or exception handling,
which uses `dynamic_cast' internally), you can save some space by
using this flag.
`-fstrict-prototype'
Within an `extern "C"' linkage specification, treat a function
declaration with no arguments, such as `int foo ();', as declaring
the function to take no arguments. Normally, such a declaration
means that the function `foo' can take any combination of
arguments, as in C. `-pedantic' implies `-fstrict-prototype'
unless overridden with `-fno-strict-prototype'.
Specifying this option will also suppress implicit declarations of
functions.
This flag no longer affects declarations with C++ linkage.
`-fsquangle'
`-fno-squangle'
`-fsquangle' will enable a compressed form of name mangling for
identifiers. In particular, it helps to shorten very long names by
recognizing types and class names which occur more than once,
replacing them with special short ID codes. This option also
requires any C++ libraries being used to be compiled with this
option as well. The compiler has this disabled (the equivalent of
`-fno-squangle') by default.
Like all options that change the ABI, all C++ code, *including
libgcc.a* must be built with the same setting of this option.
`-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.
`-fthis-is-variable'
Permit assignment to `this'. The incorporation of user-defined
free store management into C++ has made assignment to `this' an
anachronism. Therefore, by default it is invalid to assign to
`this' within a class member function; that is, GNU C++ treats
`this' in a member function of class `X' as a non-lvalue of type
`X *'. However, for backwards compatibility, you can make it
valid with `-fthis-is-variable'.
`-fvtable-thunks=THUNKS-VERSION'
Use `thunks' to implement the virtual function dispatch table
(`vtable'). The traditional (cfront-style) approach to
implementing vtables was to store a pointer to the function and two
offsets for adjusting the `this' pointer at the call site. Newer
implementations store a single pointer to a `thunk' function which
does any necessary adjustment and then calls the target function.
The original implementation of thunks (version 1) had a bug
regarding virtual base classes; this bug is fixed with version 2
of the thunks implementation. With setting the version to 2,
compatibility to the version 1 thunks is provided, at the cost of
extra machine code. Version 3 does not include this compatibility.
This option also enables a heuristic for controlling emission of
vtables; if a class has any non-inline virtual functions, the
vtable will be emitted in the translation unit containing the
first one of those.
Like all options that change the ABI, all C++ code, *including
libgcc.a* must be built with the same setting of this option. Since
version 1 and version 2 are also incompatible (for classes with
virtual bases defining virtual functions), all code must also be
compiled with the same version.
In this version of gcc, there are no targets for which version 2
thunks are the default. On all targets, not giving the option
will use the traditional implementation, and -fvtable-thunks will
produce version 2 thunks.
`-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 (ie, `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 also be turned
off with the flag `-fguiding-decls', which activates the older,
non-specification compiler code, or 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.
`-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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