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std:: ratio_multiply

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Compile-time rational arithmetic
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ratio_multiply
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定义于头文件 <ratio>
template < class R1, class R2 >
using ratio_multiply = /* 见下文 */ ;
(C++11 起)

别名模板 std::ratio_multiply 表示两个由 std::ratio 特化 R1 和 R2 所表示的精确有理分数相乘的结果。

结果是 std::ratio 的特化 std:: ratio < U, V > ，其中给定 Num == R1 :: num * R2 :: num 和 Denom == R1 :: den * R2 :: den （计算时无算术溢出）， U 为 std:: ratio < Num, Denom > :: num 且 V 为 std:: ratio < Num, Denom > :: den 。

注释

如果 U 或 V 无法用 std::intmax_t 表示，则程序非良构。如果 Num 或 Denom 无法用 std::intmax_t 表示，则程序非良构，除非实现能为 U 和 V 生成正确值。

上述定义要求 std :: ratio_multiply < R1, R2 > 的结果必须已约简至最简形式；例如， std :: ratio_multiply < std:: ratio < 1 , 6 > , std:: ratio < 4 , 5 >> 与 std:: ratio < 2 , 15 > 属于同一类型。

示例

运行此代码 
#include <iostream>
#include <ratio>
int main()
{
    using two_third = std::ratio<2, 3>;
    using one_sixth = std::ratio<1, 6>;
    using product = std::ratio_multiply<two_third, one_sixth>;
    static_assert(std::ratio_equal_v<product, std::ratio<13, 117>>);
    std::cout << "2/3 * 1/6 = " << product::num << '/' << product::den << '\n';
}

输出： 

2/3 * 1/6 = 1/9

参见

(C++11)
在编译时除法两个 ratio 对象
(别名模板)