Difference between defining and declaring a variable?

[ak22]

What is the difference between defining and declaring a variable?

[laserlight]

What is the difference between defining and declaring a variable?

Well...

A declaration specifies the interpretation and attributes of a set of identifiers. A definition of an identifier is a declaration for that identifier that:

• for an object, causes storage to be reserved for that object;
• for a function, includes the function body;
• for an enumeration constant or typedef name, is the (only) declaration of the identifier.

In practice, this means that most of the variable declarations that you will encounter are definitions, except for those declared extern.

[whiteflags]

Defining a variable allocates memory for it, and if written such, initializes it.

Declaring a variable merely makes an identifier for something of some type but no memory is set aside.

The difference doesn't come up often in code, because declarations are usually definitions at the same time, but one example goes like this:

extern type foo;

type foo;

While type foo; can only appear in one source file, the extern line is probably in many files, including the one that defines foo. The extern line is a declaration, but not a definition.

edit - beaten to it.