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:
-fabi-version=
n
Use version n of the C++ ABI. Version 2 is the version of the C++ ABI that first appeared in G++ 3.4. Version 1 is the version of the C++ ABI that first appeared in G++ 3.2. Version 0 will always be the version that conforms most closely to the C++ ABI specification. Therefore, the ABI obtained using version 0 will change as ABI bugs are fixed.
The default is version 2.
-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. This check is
normally unnecessary because the C++ standard specifies that
operator new will only return 0 if it is declared
throw(), in which case the compiler will always check the
return value even without this option. In all other cases, when
operator new has a non-empty exception specification, memory
exhaustion is signalled by throwing std::bad_alloc. 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.
-ffriend-injection
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 which 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.
This option is for compatibility, and may be removed in a future
release of G++.
-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 generate code to 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. This does not give user code permission to throw
exceptions in violation of the exception specifications; the compiler
will still optimize based on the specifications, so throwing an
unexpected exception will result in undefined behavior.
-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.
See 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 some diagnostics about nonconformant code from errors to
warnings. Thus, using -fpermissive will allow some
nonconforming code to compile.
-frepo
Enable automatic template instantiation at link time. This option also
implies -fno-implicit-templates. See 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. The dynamic_cast operator can still be used for casts that
do not require runtime type information, i.e. casts to void * or to
unambiguous base subjects.
-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 subjects 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.
-fno-threadsafe-statics
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.
-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-use-cxa-get-exception-ptr
Don't use the __cxa_get_exception_ptr runtime routine. This
will cause std::uncaught_exception to be incorrect, but is necessary
if the runtime routine is not available.
-fvisibility-inlines-hidden
The effect of this is that GCC may, effectively, mark inline methods with
__attribute__ ((visibility ("hidden"))) 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.
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.
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 will have no effect.
Explicitly instantiated inline methods are unaffected by this option
as their linkage might otherwise cross a shared library boundary.
See Template Instantiation.
-fvisibility-ms-compat
This flag attempts to use visibility settings to make GCC's C++ linkage model compatible with that of Microsoft Visual Studio.
The flag makes these changes to GCC's linkage model:
hidden, like
-fvisibility=hidden.
In new code it is better to use -fvisibility=hidden and export those subjects which are intended to be externally visible. Unfortunately it is possible for code to rely, perhaps accidentally, on the Visual Studio behavior.
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 will be different, so changing one will 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.
-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.
See Options That Control Optimization. Note that these
functions will have linkage like inline functions; they just won't be
inlined by default.
-Wabi
(C++ and Objective-C++ only)
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 will be compatible.
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.
The known incompatibilities at this point include:
struct A { virtual void f(); int f1 : 1; };
struct B : public A { int f2 : 1; };
In this case, G++ will place B::f2 into the same byte
asA::f1; other compilers will not. You can avoid this problem
by explicitly padding A so that its size is a multiple of the
byte size on your platform; that will cause G++ and other compilers to
layout B identically.
struct A { virtual void f(); char c1; };
struct B { B(); char c2; };
struct C : public A, public virtual B {};
In this case, G++ will not place B into the tail-padding for
A; other compilers will. You can avoid this problem by
explicitly padding A so that its size is a multiple of its
alignment (ignoring virtual base subjects); that will cause G++ and other
compilers to layout C identically.
union U { int i : 4096; };
Assuming that an int does not have 4096 bits, G++ will make the
union too small by the number of bits in an int.
struct A {};
struct B {
A a;
virtual void f ();
};
struct C : public B, public A {};
G++ will place the A base class of C at a nonzero offset;
it should be placed at offset zero. G++ mistakenly believes that the
A data member of B is already at offset zero.
typename or
template template parameters can be mangled incorrectly.
template <typename Q>
void f(typename Q::X) {}
template <template <typename> class Q>
void f(typename Q<int>::X) {}
Instantiations of these templates may be mangled incorrectly.
-Wctor-dtor-privacy
(C++ and Objective-C++ only)
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.
-Wnon-virtual-dtor
(C++ and Objective-C++ only)
Warn when a class has virtual functions and accessible non-virtual
destructor, in which case it would be possible but unsafe to delete
an instance of a derived class through a pointer to the base class.
This warning is also enabled if -Weffc++ is specified.
-Wreorder
(C++ and Objective-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) { }
};
The compiler will rearrange the member initializers for i and j to match the declaration order of the members, emitting a warning to that effect. This warning is enabled by -Wall.
The following -W... options are not affected by -Wall.
-Weffc++
(C++ and Objective-C++ only)
Warn about violations of the following style guidelines from Scott Meyers' Effective C++ book:
operator= return a reference to *this.
Also warn about violations of the following style guidelines from Scott Meyers' More Effective C++ book:
&&, ||, or ,.
When selecting this option, be aware that the standard library
headers do not obey all of these guidelines; use grep -v
to filter out those warnings.
-Wno-deprecated
(C++ and Objective-C++ only)
Do not warn about usage of deprecated features. See Deprecated Features.
-Wstrict-null-sentinel
(C++ and Objective-C++ only)
Warn also about the use of an uncasted NULL as sentinel. When
compiling only with GCC this is a valid sentinel, as NULL is defined
to __null. Although it is a null pointer constant not a null pointer,
it is guaranteed to of the same size as a pointer. But this use is
not portable across different compilers.
-Wno-non-template-friend
(C++ and Objective-C++ only)
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.,
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++ and Objective-C++ only)
Warn if an old-style (C-style) cast to a non-void type is used within
a C++ program. The new-style casts (dynamic_cast,
static_cast, reinterpret_cast, and const_cast) are
less vulnerable to unintended effects and much easier to search for.
-Woverloaded-virtual
(C++ and Objective-C++ only)
Warn when a function declaration hides virtual functions from a base class. For example, in:
struct A {
virtual void f();
};
struct B: public A {
void f(int);
};
the A class version of f is hidden in B, and code
like:
B* b;
b->f();
will fail to compile.
-Wno-pmf-conversions
(C++ and Objective-C++ only)
Disable the diagnostic for converting a bound pointer to member function
to a plain pointer.
-Wsign-promo
(C++ and Objective-C++ only)
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++ would try to preserve unsignedness, but the standard mandates the current behavior.
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 =.