It’s important to understand the difference between declarations and definitions because these terms will be used precisely throughout the book. Essentially all C and C++ programs require declarations. Before you can write your first program, you need to understand the proper way to write a declaration.

A declaration introduces a name – an identifier – to the compiler. It tells the compiler “This function or this variable exists somewhere, and here is what it should look like.” A definition, on the other hand, says: “Make this variable here” or “Make this function here.” It allocates storage for the name. This meaning works whether you’re talking about a variable or a function; in either case, at the point of definition the compiler allocates storage. For a variable, the compiler determines how big that variable is and causes space to be generated in memory to hold the data for that variable. For a function, the compiler generates code, which ends up occupying storage in memory.

You can declare a variable or a function in many different places, but there must be only one definition in C and C++ (this is sometimes called the ODR: one-definition rule). When the linker is uniting all the object modules, it will usually complain if it finds more than one definition for the same function or variable.

A definition can also be a declaration. If the compiler hasn’t seen the name x before and you define int x;, the compiler sees the name as a declaration and allocates storage for it all at once.