Some attributes only make sense for C++ programs.
abi_tag ("tag", ...)
¶The abi_tag
attribute can be applied to a function, variable, or class
declaration. It modifies the mangled name of the entity to
incorporate the tag name, in order to distinguish the function or
class from an earlier version with a different ABI; perhaps the class
has changed size, or the function has a different return type that is
not encoded in the mangled name.
The attribute can also be applied to an inline namespace, but does not affect the mangled name of the namespace; in this case it is only used for -Wabi-tag warnings and automatic tagging of functions and variables. Tagging inline namespaces is generally preferable to tagging individual declarations, but the latter is sometimes necessary, such as when only certain members of a class need to be tagged.
The argument can be a list of strings of arbitrary length. The strings are sorted on output, so the order of the list is unimportant.
A redeclaration of an entity must not add new ABI tags, since doing so would change the mangled name.
The ABI tags apply to a name, so all instantiations and specializations of a template have the same tags. The attribute will be ignored if applied to an explicit specialization or instantiation.
The -Wabi-tag flag enables a warning about a class which does not have all the ABI tags used by its subobjects and virtual functions; for users with code that needs to coexist with an earlier ABI, using this option can help to find all affected types that need to be tagged.
When a type involving an ABI tag is used as the type of a variable or return type of a function where that tag is not already present in the signature of the function, the tag is automatically applied to the variable or function. -Wabi-tag also warns about this situation; this warning can be avoided by explicitly tagging the variable or function or moving it into a tagged inline namespace.
init_priority (priority)
¶In Standard C++, objects defined at namespace scope are guaranteed to be
initialized in an order in strict accordance with that of their definitions
in a given translation unit. No guarantee is made for initializations
across translation units. However, GNU C++ allows users to control the
order of initialization of objects defined at namespace scope with the
init_priority
attribute by specifying a relative priority,
a constant integral expression currently bounded between 101 and 65535
inclusive. Lower numbers indicate a higher priority.
In the following example, A
would normally be created before
B
, but the init_priority
attribute reverses that order:
Some_Class A __attribute__ ((init_priority (2000))); Some_Class B __attribute__ ((init_priority (543)));
Note that the particular values of priority do not matter; only their relative ordering.
no_dangling
¶This attribute can be applied on a class type, function, or member
function. Dangling references to classes marked with this attribute
will have the -Wdangling-reference diagnostic suppressed; so
will references returned from the gnu::no_dangling
-marked
functions. For example:
class [[gnu::no_dangling]] S { … };
Or:
class A { int *p; [[gnu::no_dangling]] int &foo() { return *p; } }; [[gnu::no_dangling]] const int & foo (const int &i) { … }
This attribute takes an optional argument, which must be an expression that evaluates to true or false:
template <typename T> struct [[gnu::no_dangling(std::is_reference_v<T>)]] S { … };
Or:
template <typename T> [[gnu::no_dangling(std::is_lvalue_reference_v<T>)]] decltype(auto) foo(T&& t) { … };
warn_unused
¶For C++ types with non-trivial constructors and/or destructors it is impossible for the compiler to determine whether a variable of this type is truly unused if it is not referenced. This type attribute informs the compiler that variables of this type should be warned about if they appear to be unused, just like variables of fundamental types.
This attribute is appropriate for types which just represent a value,
such as std::string
; it is not appropriate for types which
control a resource, such as std::lock_guard
.
This attribute is also accepted in C, but it is unnecessary because C does not have constructors or destructors.
cold
¶In addition to functions and labels, GNU C++ allows the cold
attribute to be used on C++ classes, structs, or unions. Applying
the cold
attribute on a type has the effect of treating every
member function of the type, including implicit special member
functions, as cold. If a member function is marked with the
hot
function attribute, the hot
attribute takes
precedence and the cold
attribute is not propagated.
For the effects of the cold
attribute on functions, see
Common Function Attributes.
hot
¶In addition to functions and labels, GNU C++ allows the hot
attribute to be used on C++ classes, structs, or unions. Applying
the hot
attribute on a type has the effect of treating every
member function of the type, including implicit special member
functions, as hot. If a member function is marked with the
cold
function attribute, the cold
attribute takes
precedence and the hot
attribute is not propagated.
For the effects of the hot
attribute on functions, see
Common Function Attributes.