What does # and \ represent in C++ ?

[ZhiYi]

I came across a piece of code which is a RTTI class. (run time type information). The class is called "RTTI".

There are a few defines below the class declaration and one of them is this:

#define RTTI_IMPL(name,parent) \
const RTTI name::rtti(#name,parent::rtti);

rtti is defined as "static const RTTI rtti;"

An example of how the #define is used is this:

RTTI_IMPL(BillBoard,GameObject); where BillBoard is the child class and GameObject is the parent class of Billboard.

What does '\' mean in the #define statement and also, what does # in "#name" represent?


On another note, I came across a method declaration like this:

virtual void RegisterEvents(class EventHandlerComponent *pkEventHandler);

EventHandlerComponent is a class which is declared and defined in another .h and cpp file. What does adding the keyword "class" in the method's parameter do? Is there a difference if I use this instead?

virtual void RegisterEvents(EventHandlerComponent *pkEventHandler);

[laserlight]

What does '\' mean in the #define statement and also, what does # in "#name" represent?

The backslash escapes the new line, so to speak, allowing the macro definition to extend to the next line. The #name means that the argument corresponding to name is transformed to a string, e.g.,

RTTI_IMPL(BillBoard,GameObject);

becomes:

const RTTI BillBoard::rtti("BillBoard",GameObject::rtti);

What does adding the keyword "class" in the method's parameter do?

Interesting: I have not seen this used, but I guessed that it was meant to correspond to the legacy use of the struct keyword in a struct name in C. In C, if you just define a struct X without a typedef to X, you would need to use struct X as the struct name. In C++, you just need to use X even without such a typedef, but can redundantly use struct X if you want. It appears (and is confirmed by my compiler) that the class keyword can be used redundantly in the same way.

[ZhiYi]

thanks for the explanations!

[Speedo]

Interesting: I have not seen this used, but I guessed that it was meant to correspond to the legacy use of the struct keyword in a struct name in C.

I'm not 100% sure, but I think it can act as sort of an "inline forward declaration" (for lack of a better term). Eg, both Comeau and MSVC9 accept the following code:

void foo(const class Bar&) {}

class Bar {};

int main()
{
  Bar b;
  foo(b);

  return 0;
}

[laserlight]

I'm not 100% sure, but I think it can act as sort of an "inline forward declaration" (for lack of a better term).

That would also be consistent with C.