声明描述了一个对象，而 definition 请求创建对象。所有定义也是声明，但事实并非如此。

A declaration describes an object, whereas a definition requests the creation of an object. All definitions are also declarations, but the reverse is not true.

对于 int a; ，具体取决于它在源代码中的位置：

In the case of int a;, what it is depends on where it is placed in the source code:

• 内部一个函数 int a; 是一个定义-它要求创建一个具有自动存储持续时间的 int 变量（并且当然也是声明）；


• 在文件范围内，答案在C和C ++中是不同的。在C中， int a; 是一个临时定义（只要类型和链接都可以接受，可以有多个定义）。 ;在C ++中，它是具有外部链接的普通定义。


• 在结构或工会$中c $ c>说明符， int a; 是成员的声明。

• Within a function, int a; is a definition - it requests the creation of an int variable with automatic storage duration (and of course also a declaration);
• At file scope, the answer is different in C and C++. In C, int a; is a tentative definition (of which there can be more than one, as long as the types and linkage are agreeable); in C++, it is an ordinary definition with external linkage.
• Within a struct or union specifier, int a; is a declaration of a member.