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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=nThe default is version 2.
Version 3 corrects an error in mangling a constant address as a template argument.
Version 4, which first appeared in G++ 4.5, implements a standard mangling for vector types.
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.
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.
See also -Wabi.
-fno-access-control-fcheck-newoperator 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-spacemain() 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.
-fconstexpr-depth=n-fdeduce-init-list template <class T> auto forward(T t) -> decltype (realfn (t))
{
return realfn (t);
}
void f()
{
forward({1,2}); // call forward<std::initializer_list<int>>
}
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++.
-ffriend-injectionThis option is for compatibility, and may be removed in a future
release of G++.
-fno-elide-constructors-fno-enforce-eh-specs-ffor-scope-fno-for-scopeThe 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-keywordstypeof 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-fno-implicit-inline-templates-fno-implement-inlines-fms-extensions-fno-nonansi-builtinsffs, alloca, _exit,
index, bzero, conjf, and other related functions.
-fnothrow-optthrow() exception specification as though it were a
noexcept 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 will actually make 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 will result in a call
to terminate rather than unexpected.
-fno-operator-namesand, bitand,
bitor, compl, not, or and xor as
synonyms as keywords.
-fno-optional-diags-fpermissive-fno-pretty-templatesvoid f(T) [with T = int]
rather than void f(int)) so that it's clear which template is
involved. When an error message refers to a specialization of a class
template, the compiler will omit 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, using -fno-pretty-templates will disable them.
-frepo-fno-rttivoid * or to
unambiguous base classes.
-fstats-fstrict-enums-ftemplate-depth=n-fno-threadsafe-statics-fuse-cxa-atexit__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__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-hiddenThe 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-compatThe 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 classes that 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-nostdinc++In addition, these optimization, warning, and code generation options have meanings only for C++ programs:
-fno-default-inline-Wabi (C, Objective-C, C++ and Objective-C++ only)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 in -fabi-version=2 (the default) include:
extern int N;
template <int &> struct S {};
void n (S<N>) {2}
This is fixed in -fabi-version=3.
__attribute ((vector_size)) are
mangled in a non-standard way that does not allow for overloading of
functions taking vectors of different sizes.
The mangling is changed in -fabi-version=4.
The known incompatibilities in -fabi-version=1 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 classes); 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.
It also warns psABI related changes. The known psABI changes at this point include:
union U {
long double ld;
int i;
};
union U will always be passed in memory.
-Wctor-dtor-privacy (C++ and Objective-C++ only)-Wdelete-non-virtual-dtor (C++ and Objective-C++ only)-Wnarrowing (C++ and Objective-C++ only) int i = { 2.2 }; // error: narrowing from double to int
This flag is included in -Wall and -Wc++11-compat.
With -std=c++11, -Wno-narrowing 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.
-Wnoexcept (C++ and Objective-C++ only)-Wnon-virtual-dtor (C++ and Objective-C++ only)-Wreorder (C++ and Objective-C++ only) 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)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.
-Wstrict-null-sentinel (C++ and Objective-C++ only)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 be 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)-Wold-style-cast (C++ and Objective-C++ only)-Woverloaded-virtual (C++ and Objective-C++ only) 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)-Wsign-promo (C++ and Objective-C++ only) 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 ='.