definition vs declaration c++

So far, the words declaration and definition have been used only a few times in this book. However, these words carry with them a rather precise meaning in standard C++, and that is the meaning that we use.

A declaration is a C++ construct that introduces or reintroduces a name into a C++ scope. This introduction always includes a partial classification of that name, but the details are not required to make a valid declaration. For example:

class C; // a declaration of C as a class void f(int p); // a declaration of f() as a function and p as a named parameter extern int v; // a declaration of v as a variable

Note that even though they have a "name," macro definitions and goto labels are not considered declarations in C++.

Declarations become definitions when the details of their structure are made known or, in the case of variables, when storage space must be allocated. For class type and function definitions, this means a brace-enclosed body must be provided. For variables, initializations and a missing extern lead to definitions. Here are examples that complement the preceding nondefinition declarations:

class C {}; // definition (and declaration) of class C void f(int p) { // definition (and declaration) of function f() std::cout << p << std::endl; } extern int v = 1; // an initializer makes this a definition for v int w; // global variable declarations not preceded by // extern are also definitions

By extension, the declaration of a class template or function template is called a definition if it has a body. Hence,

template <typename T> void func (T);

is a declaration that is not a definition, whereas

template <typename T> class S {};

is in fact a definition.

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See Also:
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• Complete Tutorial of C++ Template's
• Standard Template Library Tutorial
• Inter Process Communication Tutorial
• Advance Programming in C & C++

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