c++ - 为什么 "is_modulo"对于 double 和 float 是假的？

Question

我写了一些代码来检查一个类型是否有模表示:

#include <iostream>
#include <limits>

using namespace std;

int main( )
{
    cout << "Whether float objects have a modulo representation: "
         << numeric_limits<float>::is_modulo << endl;
    cout << "Whether double objects have a modulo representation: "
         << numeric_limits<double>::is_modulo << endl;
}

输出:

Whether float objects have a modulo representation: 0
Whether double objects have a modulo representation: 0

但是我们可以使用fmod() (来自 <math.h> )找到 float 的模数或 double .那么，为什么 is_modulo如果可以找到 float 或 double 的模数，则为 false？

Answer 1

From cppreference

The value of std::numeric_limits<T>::is_modulo is true for all arithmetic types T that handle overflows with modulo arithmetic, that is, if the result of addition, subtraction, multiplication, or division of this type would fall outside the range [min(), max()], the value returned by such operation differs from the expected value by a multiple of max()-min()+1.

float 溢出is undefined behavior ，因此 std::numeric_limits::is_modulo对于 float和 double是false .