What exactly are C++ definitions, declarations and assignments?

Question

I tend to use the words define, declare and assign interchangeably but this seems to cause offense to some people. Is this justified? Should I only use the word declare for

Answer 1

A definition is where a value or function is described, i.e. the compiler or programmer is told precisely what it is, e.g.

int foo()
{
  return 1;
}

int var; // or, e.g. int var = 5; but this is clearer.

A declaration tells the compiler, or programmer that the function or variable exists. e.g.

int foo();
extern int var;

An assignment is when a variable has its value set, usually with the = operator. e.g.

a = b;
a = foo();