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class Foo;
and probably
struct Bar;
and global functions
bool IsValid(int iVal);
What about a typed enum? What about a typed enum within an undeclared class? What about a function with an undeclared class? What about a static member within an undeclared class? What about these within an unknown namespace? Am I missing anything else that can be forward declared?
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In C++, Forward declarations are usually used for Classes. In this, the class is pre-defined before its use so that it can be called and used by other classes that are defined before this. Example: // Forward Declaration class A class A; // Definition of class A class A{ // Body };
To write a forward declaration for a function, we use a function declaration statement (also called a function prototype). The function declaration consists of the function header (the function's return type, name, and parameter types), terminated with a semicolon. The function body is not included in the declaration.
A forward declaration is much faster to parse than a whole header file that itself may include even more header files. Also, if you change something in the header file for class B, everything including that header will have to be recompiled.
This declaration makes that program available to be called by other programs even before the program definition. Remember that both procedures and functions have a header and a body. A forward declaration consists simply of the program header followed by a semicolon (;). This construction is called the module header.
You can forward declare
If by "forward declaration" you strictly mean "declare but not define" you can also forward declare member functions. But you cannot redeclare them in their class definition once they are declared. You cannot forward-declare enumerations. I'm not sure whether I missed something.
Please note that all forward declarations listed above, except partial and explicit specializations, need to be declared using an unqualified name and that member functions and nested classes can only be declared-but-not-defined in their class definition.
class A { };
class A::B; // not legal
namespace A { }
void A::f(); // not legal
namespace A { void f(); } // legal
class B { class C; }; // legal
class B::C; // declaration-only not legal
class D { template<typename T> class E; };
template<typename T> class D::E<T*>; // legal (c.f. 14.5.4/6)
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