为什么多个增量/减量在C ++中有效，但在C中无效? [英] Why are multiple increments/decrements valid in C++ but not in C?

问题描述

测试.(c/cpp)

#include <stdio.h>

int main(int argc, char** argv)
{
  int a = 0, b = 0;
  printf("a = %d, b = %d\n", a, b);
  b = (++a)--;
  printf("a = %d, b = %d\n", a, b);

  return 0;
}

如果我将上述内容另存为.cpp文件，它将在执行时编译并输出:

If I save the above as a .cpp file, it compiles and outputs this upon execution:

a = 0, b = 0
a = 0, b = 1

但是，如果将其另存为.c文件，则会出现以下错误:

However, if I save it as a .c file, I get the following error:

test.c:7:12: error: lvalue required as decrement operator.

(++a)操作是否应该在(newValue)--操作之前解决?有人对此有见识吗?

Shouldn't the (++a) operation be resolved before the (newValue)-- operation? Does anyone have any insight on this?

推荐答案

在C中，前缀和后缀递增/递减运算符的结果不是左值.

In C the result of the prefix and postfix increment/decrement operators is not an lvalue.

在C ++中，后缀增减运算符的结果也不是左值，而前缀增减运算符的结果则是左值.

In C++ the result of the postfix increment/decrement operator is also not an lvalue but the result of the prefix increment/decrement operator is an lvalue.

现在在C ++中执行类似(++a)--的操作是未定义的行为，因为您要在两个序列点之间两次修改对象值.

Now doing something like (++a)-- in C++ is undefined behavior because you are modifying an object value twice between two sequence points.

编辑:关注@ bames53评论.在C ++ 98/C ++ 03中，这是未定义的行为，但是C ++ 11中对序列点概念的更改现在使此表达式得以定义.

following up on @bames53 comment. It is undefined behavior in C++98/C++03 but the changes in C++11 on the idea of sequence points now makes this expression defined.

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