By: Stanley B. in C++ Tutorials on 2011-02-19
C++ programs typically are composed of many files. In order for multiple files to access the same variable, C++ distinguishes between declarations and definitions.
A definition of a variable allocates storage for the variable and may also specify an initial value for the variable. There must be one and only one definition of a variable in a program.
A declaration makes known the type and name of the variable to the program. A definition is also a declaration: When we define a variable, we declare its name and type. We can declare a name without defining it by using the extern keyword. A declaration that is not also a definition consists of the object's name and its type preceded by the keyword extern:
extern int i; // declares but does not define i int i; // declares and defines i
An extern declaration is not a definition and does not allocate storage. In effect, it claims that a definition of the variable exists elsewhere in the program. A variable can be declared multiple times in a program, but it must be defined only once.
A declaration may have an initializer only if it is also a definition because only a definition allocates storage. The initializer must have storage to initialize. If an initializer is present, the declaration is treated as a definition even if the declaration is labeled extern:
extern double pi = 3.1416; // definition
Despite the use of extern, this statement defines pi. Storage is allocated and initialized. An extern declaration may include an initializer only if it appears outside a function.
Because an extern that is initialized is treated as a definition, any subseqent definition of that variable is an error:
extern double pi = 3.1416; // definition double pi; // error: redefinition of pi
Similarly, a subsequent extern declaration that has an initializer is also an error:
extern double pi = 3.1416; // definition extern double pi; // ok: declaration not definition extern double pi = 3.1416; // error: redefinition of pi
The distinction between a declaration and a definition may seem pedantic but in fact is quite important.
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