C++ Definitions
A definition is a unique specification of an object or variable, function, class, or enumerator. Because definitions must be unique, a program can contain only one definition for a given program element.
There can be a many-to-one correspondence between declarations and definitions. There are two cases in which a program element can be declared and not defined:
A function is declared but never referenced with a function call or with an expression that takes the function's address.
A class is used only in a way that does not require its definition be known. However, the class must be declared. The following code illustrates such a case:
// definitions.cpp class WindowCounter; // Forward reference; no definition class Window { // Definition of WindowCounter not required static WindowCounter windowCounter; }; int main() { }