c ++向量大小。为什么-1大于零 [英] c++ vector size. why -1 is greater than zero

问题描述

请查看这个简单的程序：

Please take a look at this simple program:

#include <iostream>
#include <vector>

using namespace std;

int main() {

vector<int> a;

std::cout << "vector size " << a.size() << std::endl;

int b = -1;

if (b < a.size())
   std::cout << "Less";
else
   std::cout << "Greater";

    return 0;

}

我知道 size 方法返回无符号值，但比较仍然应用于-1和0 。 发生什么了？

I'm confused by the fact that it outputs "Greater" despite it's obvious that -1 is less than 0. I understand that size method returns unsigned value but comparison is still applied to -1 and 0. So what's going on? can anyone explain this?

推荐答案

因为向量的大小是一个无符号整数类型。你正在比较一个无符号类型与一个有符号类型，并且二的补码负的有符号整数被提升为无符号。

Because the size of a vector is an unsigned integral type. You are comparing an unsigned type with a signed one, and the two's complement negative signed integer is being promoted to unsigned. That corresponds to a large unsigned value.

此代码示例显示了您所看到的相同行为：

This code sample shows the same behaviour that you are seeing:

#include <iostream>
int main()
{
  std::cout << std::boolalpha;
  unsigned int a = 0;
  int b = -1;
  std::cout << (b < a) << "\n"; 
}

输出：

false

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