表达式必须是可修改的左值 [英] Expression must be a modifiable lvalue

问题描述

我有以下代码：

int M = 3; 
int C = 5; 
int match = 3;
for ( int k =0; k < C; k ++ )
{
    match --; 
    if ( match == 0 && k = M )
    {
         std::cout << " equals" << std::endl;
    }
}

但它给出一个错误， >

But it gives out an error saying:

错误：表达式必须是可修改的值

Error: expression must be a modifiable value

在if行。我不是试图修改匹配或k值在这里，但为什么这个错误？如果我只写如下：

on that "if" line. I am not trying to modify "match" or "k" value here, but why this error? if I only write it like:

if ( match == 0 )

没问题。有人可以解释一下吗？

it is ok. Could someone explain it to me?

推荐答案

赋值运算符的优先级低于&& 所以你的条件等价于：

The assignment operator has lower precedence than &&, so your condition is equivalent to:

if ((match == 0 && k) = m)

但是左边是一个右值，即从sub­ match == 0&&&

But the left-hand side of this is an rvalue, namely the boolean resulting from the evaluation of the sub­expression match == 0 && k, so you cannot assign to it.

相比之下，比较具有更高的优先级，因此 match == 0 &&& k == m 等效于：

By contrast, comparison has higher precedence, so match == 0 && k == m is equivalent to:

if ((match == 0) && (k == m))

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