一个有效的C ++ code与While循环的等效C code不编译 [英] The equivalent C code of a valid C++ code with a While loop does not compile

问题描述

包含，而循环下code编译在C ++中。

The following code containing a while loop compiles in C++.

#include <iostream>
using namespace std;

int main() {
    while (int i = 5)
    {
        break;
    }
    return 0;
}

不过，如果用C编译如下出错等价的C code的结果：

However, the following equivalent C code results in an error if compiled in C:

#include <stdio.h>

int main() {
    while (int i = 5)
    {
        break;
    }
    return 0;
}

编译器的输出：

> prog.c: In function 'main': prog.c:5:9: error: expected expression
> before 'int'   while (int i = 5)prog.c: In function 'main':
> prog.c:5:9: error: expected expression before 'int'   while (int i =
> 5)

为什么会出现这种情况？我试图来查找在C ，而循环的文件，但一直没能找到，要么。

Why does this happen? I tried to look up the documentation for the while loop in C, but haven't been able to locate that either.

推荐答案

在 C ++ ，一个条件是的每次迭代之前测试，而循环。逐字从 C ++参考：

In C++, a condition is tested for before each iteration of a while loop. Verbatim from the C++ reference:

条件 - 任何Ex pression这是上下文转换为
  布尔或声明用大括号或-等于单个变量
  初始化。每次迭代之前该前pression进行评估，并
  如果它产生假，则退出循环。如果这是一个声明中，
  每次迭代之前初始化被评估，并且如果该值
  声明的变量转化为假，则退出循环。

condition - any expression which is contextually convertible to bool or a declaration of a single variable with a brace-or-equals initializer. This expression is evaluated before each iteration, and if it yields false, the loop is exited. If this is a declaration, the initializer is evaluated before each iteration, and if the value of the declared variable converts to false, the loop is exited.

而在 C ，一个前pression 是一个，而循环。从 C基准逐字：

Whereas in C, an expression is tested for before each iteration of a while loop. Verbatim from the C reference:

前pression - 标量的任何Ex pression 键入。这当然pression是
  每次迭代之前评估，如果把它比等于零，
  退出循环。

expression - any expression of scalar type. This expression is evaluated before each iteration, and if it compares equal to zero, the loop is exited.

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