是什么定义和声明之间的区别？ [英] What is the difference between a definition and a declaration?

问题描述

作为标题所说，两者的含义逃避我。

As title says, the meaning of both eludes me.

推荐答案

A 声明的引入了一个标识符和描述它的类型，无论是类型，对象或函数。一个声明为 编译器需要什么的以接受该标识符引用。这些声明：

A declaration introduces an identifier and describes its type, be it a type, object, or function. A declaration is what the compiler needs to accept references to that identifier. These are declarations:

extern int bar;
extern int g(int, int);
double f(int, double); // extern can be omitted for function declarations
class foo; // no extern allowed for type declarations

A 定义的实际实例/实现此标识。它的的什么链接器需要的，以引用链接到这些实体。这些是对应于上述的声明定义：

A definition actually instantiates/implements this identifier. It's what the linker needs in order to link references to those entities. These are definitions corresponding to the above declarations:

int bar;
int g(int lhs, int rhs) {return lhs*rhs;}
double f(int i, double d) {return i+d;}
class foo {};

一个定义可以在声明的地方使用。

A definition can be used in the place of a declaration.

标识符可以的声明的。因此，下面是合法的C和C ++：

An identifier can be declared as often as you want. Thus, the following is legal in C and C++:

double f(int, double);
double f(int, double);
extern double f(int, double); // the same as the two above
extern double f(int, double);

但是，必须的定义的一次。如果忘记定义的已宣告某处引用的东西，那么链接器不知道该怎么引用链接并抱怨缺少符号。如果定义的东西不止一次，则链接不知道的这的定义链接到引用和抱怨重复的符号。

However, it must be defined exactly once. If you forget to define something that's been declared and referenced somewhere, then the linker doesn't know what to link references to and complains about a missing symbols. If you define something more than once, then the linker doesn't know which of the definitions to link references to and complains about duplicated symbols.

由于争论什么是类的声明的对一类的定义在C ++ 的不断上来（在回答和评论其他问题），我会粘贴引自C ++标准在这里。结果
在3.1 / 2，C ++ 03说道：

Since the debate what is a class declaration vs. a class definition in C++ keeps coming up (in answers and comments to other questions) , I'll paste a quote from the C++ standard here.
At 3.1/2, C++03 says:

一个声明是一个定义，除非它[...]是一个类名的声明[...]

A declaration is a definition unless it [...] is a class name declaration [...].

3.1 / 3则给出了几个例子。他们当中：

3.1/3 then gives a few examples. Amongst them:

[Example: [...]
struct S { int a; int b; }; // defines S, S::a, and S::b [...]
struct S; // declares S
—end example

要总结：C ++标准认为结构X; 是一个的声明和结构X {} ; A的定义的。 （换句话说， 前进宣言名不副实的后，因为没有其他形式的C级声明++）。

To sum it up: The C++ standard considers struct x; to be a declaration and struct x {}; a definition. (In other words, "forward declaration" a misnomer, since there are no other forms of class declarations in C++.)

感谢 litb（约翰内斯绍布）谁在一个挖出来的实际的章节和诗句他答案。

Thanks to litb (Johannes Schaub) who dug out the actual chapter and verse in one of his answers.

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