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What is the difference between a definition and a declaration?

The meaning of both eludes me.

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Maciek Avatar asked Sep 11 '09 12:09

Maciek


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What is the difference between a definition and a declaration in C?

The main difference between Declaration and Definition in C is that declaration of a variable indicates the compiler about the name and the type of the variable, while the definition of a variable indicates the compiler where and how much storage to create for a variable.

What is difference between declaration and definition of function explain with example?

Function declaration is a prototype that specifies the function name, return types and parameters without the function body. Function Definition, on the other hand, refers to the actual function that specifies the function name, return types and parameters with the function body.

Is a definition also a declaration?

Statement 1: A definition is also a declaration. Statement 2: An identifier can be declared just once. Explanation: An identifier can be declared many times must be defined just once.

What is the difference between definition and declaration of a variable and a function in C++?

Definition associates the variable with a type and allocates memory, whereas declaration just specifies the type but doesn't allocate memory. Declaration is more useful when you want to refer the variable before definition.


1 Answers

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 type declarations 

A 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:

int bar; 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.


Since the debate what is a class declaration vs. a class definition in C++ keeps coming up (in answers and comments to other questions) , I'll paste a quote from the C++ standard here.
At 3.1/2, C++03 says:

A declaration is a definition unless it [...] is a class name declaration [...].

3.1/3 then gives a few examples. Amongst them:

 [Example: [...] struct S { int a; int b; }; // defines S, S::a, and S::b [...] struct S; // declares S —end example 

To sum it up: The C++ standard considers struct x; to be a declaration and struct x {}; a definition. (In other words, "forward declaration" a misnomer, since there are no other forms of class declarations in C++.)

Thanks to litb (Johannes Schaub) who dug out the actual chapter and verse in one of his answers.

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sbi Avatar answered Oct 24 '22 17:10

sbi