What are different between declaration & definition?

A declaration introduces an identifier and describes its type, be it a type, object, or function. A declaration is what the compiler needs to accept references to that identifier. These are declarations: extern int bar; extern int g(int, int); double f(int, double); // extern can be omitted for function declarations class foo; // no extern allowed for type declarations A definition actually instantiates/implements this identifier. It's what the linker needs in order to link references to those entities. These are definitions corresponding to the above declarations: int bar; int g(int lhs, int rhs) {return lhs*rhs;} double f(int i, double d) {return i+d;} class foo {}; A definition can be used in the place of a declaration. An identifier can be declared as often as you want. Thus, the following is legal in C and C++: double f(int, double); double f(int, double); extern double f(int, double); // the same as the two above extern double f(int, double); read less