Difference between prototype declaration and forward declaration?

Question

So I have this code:

class xx
{
    int getnum(); //Is this a forward declaration or a prototype declaration and why?
};

int xx::getnum()
{
    return 1+3;
         


        

Answer 1

Neither the term "forward declaration" nor "prototype declaration" are defined in the C++ standard, so very strictly speaking, it's neither. It is simply a declaration. If you want to be more specific, you could call it a non-defining declaration, or "a declaration which is not a definition."

When the words "forward declaration" are used in the standard, they are used to refer to declarations which declare, but do not define, the thing (function or class) they declare. Following this usage, int getnum() is therefore a forward declaration.

"Prototype declaration" of a function is used even less in the standard⁽¹⁾, and mostly when talking about [in]compatibility with C. However, when used, it refers to the exact same concept as a forward declaration of that function. Going by this, you could also call int getnum(); a prototype declaration.

To summarise, the terms "forward declaration" and "prototype declaration" have no formal definition, but from the way they are normally used and understood, int getnum(); can be described as either.

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⁽¹⁾ However, a similar term "function prototype scope" is defined in the standard (C++17 N4659 [basic.scope.proto] 6.3.4/1). It establishes the scope of function parameter names in a non-defining function declaration, and it's the closest enclosing function declarator.