if 条件中的逗号运算符 [英] comma operator in if condition

问题描述

int a = 1, b = 0;

if(a, b)
   printf("success
");
else
   printf("fail
");

if(b, a)
   printf("success
");
else
   printf("fail");

这是一个 cpp 文件，我在 Visual Studio 2010 中得到的输出为

This is a cpp file and I got the output in Visual Studio 2010 as

fail
success

为什么会有这种行为?你能解释一下吗?

Why this behavior? Could you please explain?

推荐答案

http://en.wikipedia.org/wiki/Comma_o​​perator:

在 C 和 C++ 编程语言中，逗号运算符(由标记 , 表示)是一个二元运算符，用于评估其第一个操作数并丢弃结果，然后计算第二个操作数操作数并返回此值(和类型).

In the C and C++ programming languages, the comma operator (represented by the token ,) is a binary operator that evaluates its first operand and discards the result, and then evaluates the second operand and returns this value (and type).

在你的第一个 if 中:

if (a, b)

a 首先被评估并被丢弃，b 被第二次评估并返回为 0.所以这个条件是假的.

a is evaluated first and discarded, b is evaluated second and returned as 0. So this condition is false.

在你的第二个if中:

if (b, a)

b 首先评估并丢弃，a 第二次评估并返回 1.所以这个条件为真.

b is evaluated first and discarded, a is evaluated second and returned as 1. So this condition is true.

如果有两个以上的操作数，则返回最后一个表达式.

If there are more than two operands, the last expression will be returned.

如果您希望这两个条件都成立，您应该使用 &&运算符:

If you want both conditions to be true, you should use the && operator:

if (a && b)

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