7.7 C++-Specific Variable, Function, and Type Attributes

Some attributes only make sense for C++ programs.

abi_tag ("tag", ...)

The abi_tag attribute can be applied to a function or class declaration. It modifies the mangled name of the function or class 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 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 a function or class 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.

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.

java_interface

This type attribute informs C++ that the class is a Java interface. It may only be applied to classes declared within an extern "Java" block. Calls to methods declared in this interface are dispatched using GCJ's interface table mechanism, instead of regular virtual table dispatch.

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::mutex.

This attribute is also accepted in C, but it is unnecessary because C does not have constructors or destructors.

See also Namespace Association.