GNU Compiler Collection (GCC) Internals: Functions for C++

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#### 10.10.4 Functions for C++

A function is represented by a `FUNCTION_DECL` node. A set of overloaded functions is sometimes represented by an `OVERLOAD` node.

An `OVERLOAD` node is not a declaration, so none of the ‘`DECL_`’ macros should be used on an `OVERLOAD`. An `OVERLOAD` node is similar to a `TREE_LIST`. Use `OVL_CURRENT` to get the function associated with an `OVERLOAD` node; use `OVL_NEXT` to get the next `OVERLOAD` node in the list of overloaded functions. The macros `OVL_CURRENT` and `OVL_NEXT` are actually polymorphic; you can use them to work with `FUNCTION_DECL` nodes as well as with overloads. In the case of a `FUNCTION_DECL`, `OVL_CURRENT` will always return the function itself, and `OVL_NEXT` will always be `NULL_TREE`.

To determine the scope of a function, you can use the `DECL_CONTEXT` macro. This macro will return the class (either a `RECORD_TYPE` or a `UNION_TYPE`) or namespace (a `NAMESPACE_DECL`) of which the function is a member. For a virtual function, this macro returns the class in which the function was actually defined, not the base class in which the virtual declaration occurred.

If a friend function is defined in a class scope, the `DECL_FRIEND_CONTEXT` macro can be used to determine the class in which it was defined. For example, in

    class C { friend void f() {} };

the `DECL_CONTEXT` for `f` will be the `global_namespace`, but the `DECL_FRIEND_CONTEXT` will be the `RECORD_TYPE` for `C`.

The following macros and functions can be used on a `FUNCTION_DECL`:

`DECL_MAIN_P`

  This predicate holds for a function that is the program entry point `::code`.

`DECL_LOCAL_FUNCTION_P`

  This predicate holds if the function was declared at block scope, even though it has a global scope.

`DECL_ANTICIPATED`

  This predicate holds if the function is a built-in function but its prototype is not yet explicitly declared.

`DECL_EXTERN_C_FUNCTION_P`

  This predicate holds if the function is declared as an ‘`extern "C"`’ function.

`DECL_LINKONCE_P`

  This macro holds if multiple copies of this function may be emitted in various translation units. It is the responsibility of the linker to merge the various copies. Template instantiations are the most common example of functions for which `DECL_LINKONCE_P` holds; G++ instantiates needed templates in all translation units which require them, and then relies on the linker to remove duplicate instantiations.

  FIXME: This macro is not yet implemented.

`DECL_FUNCTION_MEMBER_P`

  This macro holds if the function is a member of a class, rather than a member of a namespace.

`DECL_STATIC_FUNCTION_P`

  This predicate holds if the function a static member function.

`DECL_NONSTATIC_MEMBER_FUNCTION_P`

  This macro holds for a non-static member function.

`DECL_CONST_MEMFUNC_P`

  This predicate holds for a `const`-member function.

`DECL_VOLATILE_MEMFUNC_P`

  This predicate holds for a `volatile`-member function.

`DECL_CONSTRUCTOR_P`

  This macro holds if the function is a constructor.

`DECL_NONCONVERTING_P`

  This predicate holds if the constructor is a non-converting constructor.

`DECL_COMPLETE_CONSTRUCTOR_P`

  This predicate holds for a function which is a constructor for an object of a complete type.

`DECL_BASE_CONSTRUCTOR_P`

  This predicate holds for a function which is a constructor for a base class sub-object.

`DECL_COPY_CONSTRUCTOR_P`

  This predicate holds for a function which is a copy-constructor.

`DECL_DESTRUCTOR_P`

  This macro holds if the function is a destructor.

`DECL_COMPLETE_DESTRUCTOR_P`

  This predicate holds if the function is the destructor for an object a complete type.

`DECL_OVERLOADED_OPERATOR_P`

  This macro holds if the function is an overloaded operator.

`DECL_CONV_FN_P`

  This macro holds if the function is a type-conversion operator.

`DECL_GLOBAL_CTOR_P`

  This predicate holds if the function is a file-scope initialization function.

`DECL_GLOBAL_DTOR_P`

  This predicate holds if the function is a file-scope finalization function.

`DECL_THUNK_P`

  This predicate holds if the function is a thunk.

  These functions represent stub code that adjusts the `this` pointer and then jumps to another function. When the jumped-to function returns, control is transferred directly to the caller, without returning to the thunk. The first parameter to the thunk is always the `this` pointer; the thunk should add `THUNK_DELTA` to this value. (The `THUNK_DELTA` is an `int`, not an `INTEGER_CST`.)

  Then, if `THUNK_VCALL_OFFSET` (an `INTEGER_CST`) is nonzero the adjusted `this` pointer must be adjusted again. The complete calculation is given by the following pseudo-code:

      this += THUNK_DELTA
      if (THUNK_VCALL_OFFSET)
        this += (*((ptrdiff_t **) this))[THUNK_VCALL_OFFSET]

  Finally, the thunk should jump to the location given by `DECL_INITIAL`; this will always be an expression for the address of a function.

`DECL_NON_THUNK_FUNCTION_P`

  This predicate holds if the function is not a thunk function.

`GLOBAL_INIT_PRIORITY`

  If either `DECL_GLOBAL_CTOR_P` or `DECL_GLOBAL_DTOR_P` holds, then this gives the initialization priority for the function. The linker will arrange that all functions for which `DECL_GLOBAL_CTOR_P` holds are run in increasing order of priority before `main` is called. When the program exits, all functions for which `DECL_GLOBAL_DTOR_P` holds are run in the reverse order.

`TYPE_RAISES_EXCEPTIONS`

  This macro returns the list of exceptions that a (member-)function can raise. The returned list, if non `NULL`, is comprised of nodes whose `TREE_VALUE` represents a type.

`TYPE_NOTHROW_P`

  This predicate holds when the exception-specification of its arguments is of the form ‘`()`’.

`DECL_ARRAY_DELETE_OPERATOR_P`

  This predicate holds if the function an overloaded `operator delete[]`.

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