问题描述
请看一下这个简单的程序:
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;
}
尽管 -1 明显小于 0，但它输出更大"这一事实让我感到困惑.我知道 size 方法返回无符号值，但比较仍然适用于 -1和 0. 那么发生了什么?谁能解释一下?
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
因为向量的大小是无符号整数类型.您正在将无符号类型与有符号类型进行比较，并且将二进制补码负有符号整数提升为无符号类型.这对应于一个大的无符号值.
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) << "
";
}
输出:
假
这篇关于c ++ 矢量大小.为什么 -1 大于零的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持跟版网！