C++ 學習筆記：ratio

Compile-time 的分數與算術運算

Hello World Example:

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#include <iostream> #include <ratio> #include <cstdint> int main() { /* Representation */ using FiveThirds = std::ratio < 5, 3 > ; std::cout << FiveThirds::num << "/" << FiveThirds::den << std::endl; // 5/3 using AlsoFiveThirds = std::ratio < 25, 15 >; std::cout << AlsoFiveThirds::num << "/" << AlsoFiveThirds::den << std::endl; // 5/3 std::ratio<42, 42> one; std::cout << one.num << "/" << one.den << std::endl; // 1/1 using Zero = std::ratio<0>; // denominator default 1 Zero zero; std::cout << zero.num << "/" << zero.den << std::endl; // 0/1 using Neg = std::ratio < 7, -3 >; std::cout << Neg::num << "/" << Neg::den << std::endl; // -7/3 /* Arithmetics: meta functions */ using Addtype = std::ratio_add<FiveThirds, AlsoFiveThirds>::type; std::cout << Addtype::num << "/" << Addtype::den << std::endl; // 10/3 std::cout << std::boolalpha << std::ratio_equal<FiveThirds, AlsoFiveThirds>::value << std::endl; // true //std::ratio_multiply < std::ratio<1, INTMAX_MAX>, std::ratio<1, 2> >; // compile error //std::ratio_divide<FiveThirds, Zero>; // compile error //std::ratio_divide<FiveThirds, Zero>::type; // compile error /* Predefined units */ using NanoType = std::ratio<1, 1000000000LL>::type; std::cout << std::boolalpha << std::ratio_less<std::nano, NanoType>::value << std::endl;// false std::cout << std::boolalpha << std::ratio_less<NanoType, std::nano>::value << std::endl;// false std::cout << std::boolalpha << std::ratio_equal<std::nano, NanoType>::value << std::endl;// true }

class 大致定義如下：

#include <cstdint>
namespace std
{
    template <intmax_t N, intmax_t D = 1>
    struct ratio {
        static_assert(D != 0, "denominator cannot be zero");
        static_assert(N >= -INTMAX_MAX && D >= -INTMAX_MAX, "out of range");

        static constexpr intmax_t num = ...;
        static constexpr intmax_t den = ...;
        
        using type = ratio < num, dem > ;
    };
}

• 主要用途為在 compile time 表示一組分數
• 在編譯期可自動抓取 divide-by-zero 與 overflow 的錯誤
• 常被使用在 chrono 上

使用方法：

1. 給定分子與分母，定義一個 type(可使用 using)
2. 使用 static constexpr 的 num 與 den
3. Predefine 的 unit 可參考此 Link

Reference:

1. The C++ Standard Library: A Tutorial and Reference, Second Edition
2. https://gcc.gnu.org/onlinedocs/gcc-4.6.3/libstdc++/api/a01002_source.html
3. Visual Studio 2013 Library