变量定义与声明的区别(2)

变量定义与声明的区别

Declarations and Definitions

As we'll see in Section 2.9 (p. 67), C++ programs typically are composed of many files. In order for multiple files to access the same variable, C++ distinguishes between declarations and definitions.

就像我们在2.9 (p. 67)节看到的一样，典型的C++程序通常会由好多文件组成。为了使不同的文件都可以访问同一个变量，C++会区分变量的声明

（declarations）和定义（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.

变量的定义（definitions）会为这个变量分配存储空间，并且可能会为其指定一个初始化的值。在程序里，一个变量必须有一个，也只能有一处定义（definitions）。

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:

变量的声明（declarations）会将变量的类型和名称传达给程序。当然，定义（definitions）也是一种声明：当我们定义一个变量的时候，我们当然也声明了他的名称和类型。我们可以通过使用“extern”关键字来声明（declarations）一个变量，而不用定义（definitions）它。声明（declarations）的形式就是在对象（变量）的名字和类型前面，加上关键字“extern”:

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.