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 class declarationsA 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: [a definition allocate space for the identifier // myself]
int bar; [someone said it is not only a definition but also a declaration]
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);However, it must be defined exactly once. If you forget to define something that's been declared and referenced somewhere, then the linker doesn't know what to link references to and complains about a missing symbols. If you define something more than once, then the linker doesn't know which of the definitions to link references to and complains about duplicated symbols.
[from website] http://stackoverflow.com/questions/1410563/what-is-the-difference-between-a-definition-and-a-declaration