Placement of class definition and prototype

When I create a function, I can put the code for it after main if I put the prototype above main. For example,

int myFunction(int a)
{
return(a); 
}

would have the prototype..

int myFunction(int a);

above main.

However, I have not been able to get this to work for a class definition.

If I put …

class myClass
{
…
};

below main,

I get an error if I put

class myClass;

above main. The error occurs where the class is used within main, and the error is "unknown type name." That's with the c++ compiler that is part of Xcode.

What kind of prototype should I enter above main if the class definition is below main?

When you call a function and the definition is not available, the compiler doesn't have to know the contents in order to continue evaluating the rest of the code (eg: stack usage at the call site). It only needs to know the function's signature to ensure the correct parameters are going to be passed into the function. The linker will hook up the actual address for the function call after the compiler is done.

However when you are using a class it has to know about the details of it - not just that it exists - because it'll need to ensure proper layout on the stack, what parameters are required for the constructor, etc.

The details of the class' functions are of course like regular functions - it just needs the signature to work with - they can be defined later.