C++ 中 double 類型 0 和 –0 的運算

C++ 的 double 是有負零（negative zero）的。負零在某些計算中是有意義的。例如：

1.0/0.0 == INFINITY
1.0/-0.0 == –INFINITY

 

除此之外，負零在某些數學函數中也有利用，例如 sqrt(std::complex)

同時零和負零的加減計算也有它的特殊性，例如下面的代碼：

#include <iostream>
int main()
{
	double a(1.0), b(0.0), c(-0.0);
	std::cout << std::hex;
	std::cout << "a = " << a << std::endl;
	std::cout << "b = " << b << std::endl;
	std::cout << "c = " << c << std::endl;

	std::cout << "作為除數" << std::endl;
	std::cout << a << " / " << b << " = " << a / b << std::endl;
	std::cout << a << " / " << c << " = " << a / c << std::endl;

	std::cout << "加法" << std::endl;
	std::cout << a << " + " << c << " = " << a + c << std::endl;
	std::cout << b << " + " << c  << " = " << b + c << std::endl;
	std::cout << c << " + " << c << " = " << c + c << std::endl;

	std::cout << "減法" << std::endl;
	std::cout << a << " - " << c << " = " << a - c << std::endl;
	std::cout << b << " - " << c << " = " << b - c << std::endl;
	std::cout << c << " - " << b << " = " << c - b << std::endl;
	std::cout << c << " - " << c << " = " << c - c << std::endl;
	
	std::cout << "乘法" << std::endl;
	std::cout << a << " * " << c << " = " << a * c << std::endl;
	std::cout << b << " * " << c << " = " << b * c << std::endl;
	std::cout << c << " * " << c << " = " << c * c << std::endl;

	std::cout << "除法" << std::endl;
	std::cout << b << " / " << b << " = " << b / b << std::endl;
	std::cout << b << " / " << c << " = " << b / c << std::endl;
    return 0;
}

這段代碼在 VS2015 中編譯運行的結果為：

從結果中可以看出零和負零的各種運算結果。