7.5. Local Objects

7.5. 局部对象

In C++, names have scope, and objects have lifetimes. To understand how functions operate, it is important to understand both of these concepts. The scope of a name is the part of the program's text in which that name is known. The lifetime of an object is the time during the program's execution that the object exists.

在 C++ 语言中，每个名字都有作用域，而每个对象都有生命期。要弄清楚函数是怎么运行的，理解这两个概念十分重要。名字的作用域指的是知道该名字的程序文本区。对象的生命期则是在程序执行过程中对象存在的时间。

The names of parameters and variables defined within a function are in the scope of the function: The names are visible only within the function body. As usual, a variable's name can be used from the point at which it is declared or defined until the end of the enclosing scope.

在函数中定义的形参和变量的名字只位于函数的作用域中：这些名字只在函数体中可见。通常，变量名从声明或定义的地方开始到包围它的作用域结束处都是可用的。

7.5.1. Automatic Objects

7.5.1. 自动对象

By default, the lifetime of a local variable is limited to the duration of a single execution of the function. Objects that exist only while a function is executing are known as automatic objects. Automatic objects are created and destroyed on each call to a function.

默认情况下，局部变量的生命期局限于所在函数的每次执行期间。只有当定义它的函数被调用时才存在的对象称为自动对象。自动对象在每次调用函数时创建和撤销。

The automatic object corresponding to a local variable is created when the function control path passes through the variable's definition. If the definition contains an initializer, then the object is given an initial value each time the object is created. Uninitialized local variables of built-in type have undefined values. When the function terminates, the automatic objects are destroyed.

局部变量所对应的自动对象在函数控制经过变量定义语句时创建。如果在定义时提供了初始化式，那么每次创建对象时，对象都会被赋予指定的初值。对于未初始化的内置类型局部变量，其初值不确定。当函数调用结束时，自动对象就会撤销。

Parameters are automatic objects. The storage in which the parameters reside is created when the function is called and is freed when the function terminates.

形参也是自动对象。形参所占用的存储空间在调用函数时创建，而在函数结束时撤销。

Automatic objects, including parameters, are destroyed at the end of the block in which they were defined. Parameters are defined in the function's block and so are destroyed when the function terminates. When a function exits, its local storage is deallocated. After the function exits, the values of its automatic objects and parameters are no longer accessible.

自动对象，包括形参，都在定义它们的块语句结束时撤销。形参在函数块中定义，因此当函数的执行结束时撤销。当函数结束时，会释放它的局部存储空间。在函数结束后，自动对象和形参的值都不能再访问了。

7.5.2. Static Local Objects

7.5.2. 静态局部对象

It is can be useful to have a variable that is in the scope of a function but whose lifetime persists across calls to the function. Such objects are defined as static.

一个变量如果位于函数的作用域内，但生命期跨越了这个函数的多次调用，这种变量往往很有用。则应该将这样的对象定义为 static（静态的）。

A static local object is guaranteed to be initialized no later than the first time that program execution passes through the object's definition. Once it is created, it is not destroyed until the program terminates; local statics are not destroyed when the function ends. Local statics continue to exist and hold their value across calls to the function. As a trivial example, consider a function that counts how often it is called:

static 局部对象确保不迟于在程序执行流程第一次经过该对象的定义语句时进行初始化。这种对象一旦被创建，在程序结束前都不会撤销。当定义静态局部对象的函数结束时，静态局部对象不会撤销。在该函数被多次调用的过程中，静态局部对象会持续存在并保持它的值。考虑下面的小例子，这个函数计算了自己被调用的次数：

     size_t count_calls()
     {
          static size_t ctr = 0; // value will persist across calls
          return ++ctr;
     }
     int main()
     {
         for (size_t i = 0; i != 10; ++i)
             cout << count_calls() << endl;
         return 0;
     }

This program will print the numbers from 1 through 10 inclusive.

这个程序会依次输出 1 到 10（包含 10）的整数。

Before count_calls is called for the first time, ctr is created and given an initial value of 0. Each call increments ctr and returns its current value. Whenever count_calls is executed, the variable ctr already exists and has whatever value was in the variable the last time the function exited. Thus, on the second invocation, the value is 1, on the third it is 2, and soon.

在第一次调用函数 count_calls 之前，ctr 就已创建并赋予初值 0。每次函数调用都使加 1，并且返回其当前值。在执行函数 count_calls 时，变量 ctr 就已经存在并且保留上次调用该函数时的值。因此，第二次调用时，ctr 的值为 1，第三次为 2，依此类推。