Is "int a;" a declaration or definition in C and in C++?

Question

Is int a; a declaration or definition in C? Is there any difference in C++?

I was always thinking it's just a declaration, until today...

Is it a declaration or a definition?

Answer 1

A declaration describes an object, whereas a definition requests the creation of an object. All definitions are also declarations, but the reverse is not true.

In the case of int a;, what it is depends on where it is placed in the source code:

• Within a function, int a; is a definition - it requests the creation of an int variable with automatic storage duration (and of course also a declaration);
• At file scope, the answer is different in C and C++. In C, int a; is a tentative definition (of which there can be more than one, as long as the types and linkage are agreeable); in C++, it is an ordinary definition with external linkage.
• Within a struct or union specifier, int a; is a declaration of a member.

Answer 2

Where does it appear in your program?

In most contexts, it is both a declaration and definition.

OTOH, extern int a; is a declaration only.