头文件用于声明而不是用于定义

《C++ Primer， 4th》

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Headers Are for Declarations, Not Definitions

头文件用于声明而不是用于定义

When designing a header it is essential to remember the difference between definitions, which may only occur once, and declarations, which may occur multiple times (Section 2.3.5, p. 52). The following statements are definitions and therefore should not appear in a header:

当设计头文件时，记住定义和声明的区别是很重要的。定义只可以出现一次，而声明则可以出现多次（第 2.3.5 节）。下列语句是一些定义，所以不应该放在头文件里：

     extern int ival = 10;      // initializer, so it's a definition
     double fica_rate;          // no extern, so it's a definition

Although ival is declared extern, it has an initializer, which means this statement is a definition. Similarly, the declaration of fica_rate, although it does not have an initializer, is a definition because the extern keyword is absent. Including either of these definitions in two or more files of the same program will result in a linker error complaining about multiple definitions.

虽然 ival 声明为 extern，但是它有初始化式，代表这条语句是一个定义。类似地，fica_rate 的声明虽然没有初始化式，但也是一个定义，因为没有关键字 extern。同一个程序中有两个以上文件含有上述任一个定义都会导致多重定义链接错误。

Because headers are included in multiple source files, they should not contain definitions of variables or functions. 因为头文件包含在多个源文件中，所以不应该含有变量或函数的定义。

There are three exceptions to the rule that headers should not contain definitions: classes, const objects whose value is known at compile time, and inline functions (Section 7.6 (p. 256) covers inline functions) are all defined in headers. These entities may be defined in more than one source file as long as the definitions in each file are exactly the same.

对于头文件不应该含有定义这一规则，有三个例外。头文件可以定义类、值在编译时就已知道的 const 对象和 inline 函数（第 7.6 节介绍 inline 函数）。这些实体可在多个源文件中定义，只要每个源文件中的定义是相同的。

These entities are defined in headers because the compiler needs their definitions (not just declarations) to generate code. For example, to generate code that defines or uses objects of a class type, the compiler needs to know what data members make up that type. It also needs to know what operations can be performed on these objects. The class definition provides the needed information. That const objects are defined in a header may require a bit more explanation.

在头文件中定义这些实体，是因为编译器需要它们的定义（不只是声明）来产生代码。例如：为了产生能定义或使用类的对象的代码，编译器需要知道组成该类型的数据成员。同样还需要知道能够在这些对象上执行的操作。类定义提供所需要的信息。在头文件中定义 const 对象则需要更多的解释。

Some const Objects Are Defined in Headers

一些 const 对象定义在头文件中

Recall that by default a const variable (Section 2.4, p. 57) is local to the file in which it is defined. As we shall now see, the reason for this default is to allow const variables to be defined in header files.

回想一下，const 变量（第 2.4 节）默认时是定义该变量的文件的局部变量。正如我们现在所看到的，这样设置默认情况的原因在于允许 const 变量定义在头文件中。

In C++ there are places where constant expression (Section 2.7, p. 62) is required. For example, the initializer of an enumerator must be a constant expression. We'll see other cases that require constant expressions in later chapters.

在 C++ 中，有些地方需要放置常量表达式（第 2.7 节）。例如，枚举成员的初始化式必须是常量表达式。在以后的章节中将会看到其他需要常量表达式的例子。

Generally speaking, a constant expression is an expression that the compiler can evaluate at compile-time. A const variable of integral type may be a constant expression when it is itself initialized from a constant expression. However, for the const to be a constant expression, the initializer must be visible to the compiler. To allow multiple files to use the same constant value, the const and its initializer must be visible in each file. To make the initializer visible, we normally define such consts inside a header file. That way the compiler can see the initializer whenever the const is used.

一般来说，常量表达式是编译器在编译时就能够计算出结果的表达式。当 const 整型变量通过常量表达式自我初始化时，这个 const 整型变量就可能是常量表达式。而 const 变量要成为常量表达式，初始化式必须为编译器可见。为了能够让多个文件使用相同的常量值，const 变量和它的初始化式必须是每个文件都可见的。而要使初始化式可见，一般都把这样的 const 变量定义在头文件中。那样的话，无论该 const 变量何时使用，编译器都能够看见其初始化式。

However, there can be only one definition (Section 2.3.5, p. 52) for any variable in a C++ program. A definition allocates storage; all uses of the variable must refer to the same storage. Because, by default, const objects are local to the file in which they are defined, it is legal to put their definition in a header file.

但是，C++ 中的任何变量都只能定义一次（第 2.3.5 节）。定义会分配存储空间，而所有对该变量的使用都关联到同一存储空间。因为 const 对象默认为定义它的文件的局部变量，所以把它们的定义放在头文件中是合法的。

There is one important implication of this behavior. When we define a const in a header file, every source file that includes that header has its own const variable with the same name and value.

这种行为有一个很重要的含义：当我们在头文件中定义了 const 变量后，每个包含该头文件的源文件都有了自己的 const 变量，其名称和值都一样。

When the const is initialized by a constant expression, then we are guaranteed that all the variables will have the same value. Moreover, in practice, most compilers will replace any use of such const variables by their corresponding constant expression at compile time. So, in practice, there won't be any storage used to hold const variables that are initialized by constant expressions.

当该 const 变量是用常量表达式初始化时，可以保证所有的变量都有相同的值。但是在实践中，大部分的编译器在编译时都会用相应的常量表达式替换这些 const 变量的任何使用。所以，在实践中不会有任何存储空间用于存储用常量表达式初始化的 const 变量。

When a const is initialized by a value that is not a constant expression, then it should not be defined in header file. Instead, as with any other variable, the const should be defined and initialized in a source file. An extern declaration for that const should be made in the header, enabling multiple files to share that variable.

如果 const 变量不是用常量表达式初始化，那么它就不应该在头文件中定义。相反，和其他的变量一样，该 const 变量应该在一个源文件中定义并初始化。应在头文件中为它添加 extern 声明，以使其能被多个文件共享。