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 firstClass.C like this:
g++ -g -fstrict-enums -O -c firstClass.C
In this example, only -fstrict-enums is an option meant only for C++ programs; you can use the other options with any language supported by GCC.
Some options for compiling C programs, such as -std, are also relevant for C++ programs. See Options Controlling C Dialect.
Here is a list of options that are only for compiling C++ programs:
-fabi-version=n
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.
Version 1 is the version of the C++ ABI that first appeared in G++ 3.2.
Version 2 is the version of the C++ ABI that first appeared in G++ 3.4, and was the default through G++ 4.9.
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.
Version 7, which first appeared in G++ 4.8, that treats nullptr_t as a builtin type and corrects the mangling of lambdas in default argument scope.
Version 8, which first appeared in G++ 4.9, corrects the substitution behavior of function types with function-cv-qualifiers.
Version 9, which first appeared in G++ 5.2, corrects the alignment of nullptr_t.
Version 10, which first appeared in G++ 6.1, adds mangling of attributes that affect type identity, such as ia32 calling convention attributes (e.g. ‘stdcall’).
See also -Wabi.
-fabi-compat-version=n
With -fabi-version=0 (the default), this defaults to 8 (GCC 5 compatibility). If another ABI version is explicitly selected, this defaults to 0. For compatibility with GCC versions 3.2 through 4.9, use -fabi-compat-version=2.
If this option is not provided but -Wabi=n is, that version is used for compatibility aliases. If this option is provided along with -Wabi (without the version), the version from this option is used for the warning.
-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 only returns 0 if it is declared throw(), in which case the compiler always checks 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)’. -fconceptstemplate <class T> concept bool Addable = requires (T t) { t + t; };
template <Addable T> T add (T a, T b) { return a + b; } -fconstexpr-depth=n
-fdeduce-init-liststd::initializer_list from a brace-enclosed initializer list, i.e. 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-specsNDEBUG. 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. -fextern-tls-init-fno-extern-tls-initthread_local and threadprivate 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'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 -fno-extern-tls-init option. On targets that support symbol aliases, the default is -fextern-tls-init. On targets that do not support symbol aliases, the default is -fno-extern-tls-init.
-ffor-scope-fno-for-scope-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++. 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.
-fno-gnu-keywordstypeof as a keyword, so that code can use this word as an identifier. You can use the keyword __typeof__ instead. This option is implied by the strict ISO C++ dialects: -ansi, -std=c++98, -std=c++11, etc. -fno-implicit-templates-fno-implicit-inline-templates-fno-implement-inlines#pragma implementation. This causes linker errors if these functions are not inlined everywhere they are called. -fms-extensions-fno-nonansi-builtinsffs, alloca, _exit, index, bzero, conjf, and other related functions. -fnothrow-optthrow() exception specification as if 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 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 terminate rather than unexpected. -fno-operator-namesand, bitand, bitor, compl, not, or and xor as synonyms as keywords. -fno-optional-diags-fpermissive-fpermissive allows some nonconforming code to compile. -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 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 -fno-pretty-templates to disable them. -frepo-fno-implicit-templates. See Template Instantiation, for more information. -fno-rttidynamic_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 G++ generates it as needed. The dynamic_cast operator can still be used for casts that do not require run-time type information, i.e. casts to void * or to unambiguous base classes. -fsized-deallocationvoid operator delete (void *, std::size_t) noexcept; void operator delete[] (void *, std::size_t) noexcept;
as introduced in C++14. This is useful for user-defined replacement deallocation functions that, for example, use the size of the object to make deallocation faster. Enabled by default under -std=c++14 and above. The flag -Wsized-deallocation warns about places that might want to add a definition.
-fstrict-enums-ftemplate-backtrace-limit=n
-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 only works if your C library supports __cxa_atexit. -fno-use-cxa-get-exception-ptr__cxa_get_exception_ptr runtime routine. This causes 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 has 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 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.
-fno-weak-nostdinc++In addition, these optimization, warning, and code generation options have meanings only for C++ programs:
-Wabi (C, Objective-C, C++ and Objective-C++ only)
-Wabi will warn only if there is a check added later in a release series for an ABI issue discovered since the initial release. -Wabi will warn about more things if an older ABI version is selected (with -fabi-version=n). -Wabi can also be used with an explicit version number to warn about compatibility with a particular -fabi-version level, e.g. -Wabi=2 to warn about changes relative to -fabi-version=2.
If an explicit version number is provided and -fabi-compat-version is not specified, the version number from this option is used for compatibility aliases. If no explicit version number is provided with this option, but -fabi-compat-version is specified, that version number is used for ABI warnings.
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.
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.
Known incompatibilities in -fabi-version=2 (which was the default from GCC 3.4 to 4.9) include:
extern int N;
template <int &> struct S {};
void n (S<N>) {2} This was fixed in -fabi-version=3.
__attribute ((vector_size)) were mangled in a non-standard way that does not allow for overloading of functions taking vectors of different sizes. The mangling was changed in -fabi-version=4.
__attribute ((const)) and noreturn were mangled as type qualifiers, and decltype of a plain declaration was folded away. These mangling issues were fixed in -fabi-version=5.
va_arg to complain. On most targets this does not actually affect the parameter passing ABI, as there is no way to pass an argument smaller than int. Also, the ABI changed the mangling of template argument packs, const_cast, static_cast, prefix increment/decrement, and a class scope function used as a template argument.
These issues were corrected in -fabi-version=6.
nullptr_t. These issues were corrected in -fabi-version=7.
This was fixed in -fabi-version=8, the default for GCC 5.1.
decltype(nullptr) incorrectly had an alignment of 1, leading to unaligned accesses. Note that this did not affect the ABI of a function with a nullptr_t parameter, as parameters have a minimum alignment. This was fixed in -fabi-version=9, the default for GCC 5.2.
This was fixed in -fabi-version=10, the default for GCC 6.1.
It also warns about psABI-related changes. The known psABI changes at this point include:
long double members are passed in memory as specified in psABI. For example: union U {
long double ld;
int i;
}; union U is always passed in memory.
-Wabi-tag (C++ and Objective-C++ only)
-Wctor-dtor-privacy (C++ and Objective-C++ only)
-Wdelete-non-virtual-dtor (C++ and Objective-C++ only)
delete 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 -Wall. -Wliteral-suffix (C++ and Objective-C++ only)
<inttypes.h>. For example: #define __STDC_FORMAT_MACROS
#include <inttypes.h>
#include <stdio.h>
int main() {
int64_t i64 = 123;
printf("My int64: %" PRId64"\n", i64);
} In this case, PRId64 is treated as a separate preprocessing token.
This warning is enabled by default.
-Wlto-type-mismatch-flto to be enabled. Enabled by default. -Wnarrowing (C++ and Objective-C++ only)
-std=gnu++98 or -std=c++98, warn when a narrowing conversion prohibited by C++11 occurs within ‘{ }’, e.g. int i = { 2.2 }; // error: narrowing from double to int This flag is included in -Wall and -Wc++11-compat.
When a later standard is in effect, e.g. when using -std=c++11, narrowing conversions are diagnosed by default, as required by the standard. A narrowing conversion from a constant produces an error, and a narrowing conversion from a non-constant produces a warning, but -Wno-narrowing suppresses the diagnostic. Note that this does not affect the meaning of well-formed code; narrowing conversions are still considered ill-formed in SFINAE contexts.
-Wnoexcept (C++ and Objective-C++ only)
throw() or noexcept) but is known by the compiler to never throw an exception. -Wnon-virtual-dtor (C++ and Objective-C++ only)
-Weffc++ is specified. -Wreorder (C++ and Objective-C++ only)
struct A {
int i;
int j;
A(): j (0), i (1) { }
}; The compiler rearranges 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.
-fext-numeric-literals (C++ and Objective-C++ only)
-std=c++98, -std=gnu++98, -std=gnu++11, -std=gnu++14. This option is off by default for ISO C++11 onwards (-std=c++11, ...). The following -W... options are not affected by -Wall.
-Weffc++ (C++ and Objective-C++ only)
operator= return a reference to *this. &&, ||, or ,. This option also enables -Wnon-virtual-dtor, 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.
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 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. -Wno-non-template-friend (C++ and Objective-C++ only)
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)
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)
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();
fails to compile.
-Wno-pmf-conversions (C++ and Objective-C++ only)
-Wsign-promo (C++ and Objective-C++ only)
-Wtemplates (C++ and Objective-C++ only)
-Wmultiple-inheritance (C++ and Objective-C++ only)
-Wvirtual-inheritance-Wnamespaces-Wno-terminate (C++ and Objective-C++ only)
terminate.
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Licensed under the GNU Free Documentation License, Version 1.3.
https://gcc.gnu.org/onlinedocs/gcc-6.3.0/gcc/C_002b_002b-Dialect-Options.html