声明用于向程序表明变量的类型和名字。定义也是声明：当定义变量时我们声明了它的类型和名字。可以通过使用extern关键字声明变量名而不定义它。不定义变量的声明包括对象名、对象类型和对象类型前的关键字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.

extern 声明不是定义，也不分配存储空间。事实上，它只是说明变量定义在程序的其他地方。程序中变量可以声明多次，但只能定义一次。

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:

只有当声明也是定义时，声明才可以有初始化式，因为只有定义才分配存储空间。初始化式必须要有存储空间来进行初始化。如果声明有初始化式，那么它可被当作是定义，即使声明标记为 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.

虽然使用了 extern ，但是这条语句还是定义了 pi，分配并初始化了存储空间。只有当 extern 声明位于函数外部时，才可以含有初始化式。

Because an extern that is initialized is treated as a definition, any subseqent definition of that variable is an error:

因为已初始化的 extern 声明被当作是定义，所以该变量任何随后的定义都是错误的：

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:

同样，随后的含有初始化式的 extern 声明也是错误的：

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.

声明和定义之间的区别可能看起来微不足道，但事实上却是举足轻重的。

In C++ a variable must be defined exactly once and must be defined or declared before it is used.

在 C++ 语言中，变量必须且仅能定义一次，而且在使用变量之前必须定义或声明变量。