# Standard library header <ratio>

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This header is part of the compile-time rational arithmetic library.

### Classes
ratio | represents exact rational fraction (class template)
##### Arithmetic
ratio_add (C++11) | adds two `ratio` objects at compile-time (alias template)
ratio_subtract (C++11) | subtracts two `ratio` objects at compile-time (alias template)
ratio_multiply (C++11) | multiplies two `ratio` objects at compile-time (alias template)
ratio_divide (C++11) | divides two `ratio` objects at compile-time (alias template)
##### Comparison
ratio_equal (C++11) | compares two `ratio` objects for equality at compile-time (class template)
ratio_not_equal (C++11) | compares two `ratio` objects for inequality at compile-time (class template)
ratio_less (C++11) | compares two `ratio` objects for less than at compile-time (class template)
ratio_less_equal (C++11) | compares two `ratio` objects for less than or equal to at compile-time (class template)
ratio_greater (C++11) | compares two `ratio` objects for greater than at compile-time (class template)
ratio_greater_equal (C++11) | compares two `ratio` objects for greater than or equal to at compile-time (class template)
##### Constants
`yocto` | std::ratio<1, 1000000000000000000000000>, if std::intmax_t can represent the denominator
`zepto` | std::ratio<1, 1000000000000000000000>, if std::intmax_t can represent the denominator
`atto` | std::ratio<1, 1000000000000000000>
`femto` | std::ratio<1, 1000000000000000>
`pico` | std::ratio<1, 1000000000000>
`nano` | std::ratio<1, 1000000000>
`micro` | std::ratio<1, 1000000>
`milli` | std::ratio<1, 1000>
`centi` | std::ratio<1, 100>
`deci` | std::ratio<1, 10>
`deca` | std::ratio<10, 1>
`hecto` | std::ratio<100, 1>
`kilo` | std::ratio<1000, 1>
`mega` | std::ratio<1000000, 1>
`giga` | std::ratio<1000000000, 1>
`tera` | std::ratio<1000000000000, 1>
`peta` | std::ratio<1000000000000000, 1>
`exa` | std::ratio<1000000000000000000, 1>
`zetta` | std::ratio<1000000000000000000000, 1>, if std::intmax_t can represent the numerator
`yotta` | std::ratio<1000000000000000000000000, 1>, if std::intmax_t can represent the numerator

### Synopsis

    namespace std {

        //class template ratio
        template <intmax_t N, intmax_t D = 1>
        class ratio {
        public:
            typedef ratio<num, den> type;
            static constexpr intmax_t num;
            static constexpr intmax_t den;
        };

        // ratio arithmetic
        template <class R1, class   R2> using   ratio_add      = /*ratio*/;
        template <class R1, class   R2> using   ratio_subtract = /*ratio*/;
        template <class R1, class   R2> using   ratio_multiply = /*ratio*/;
        template <class R1, class   R2> using   ratio_divide   = /*ratio*/;

        // ratio comparison
        template <class R1, class R2> struct ratio_equal;
        template <class R1, class R2> struct ratio_not_equal;
        template <class R1, class R2> struct ratio_less;
        template <class R1, class R2> struct ratio_less_equal;
        template <class R1, class R2> struct ratio_greater;
        template <class R1, class R2> struct ratio_greater_equal;

        // convenience SI typedefs
        typedef ratio<1, 1000000000000000000000000> yocto;
        typedef ratio<1,    1000000000000000000000> zepto;
        typedef ratio<1,       1000000000000000000> atto;
        typedef ratio<1,          1000000000000000> femto;
        typedef ratio<1,             1000000000000> pico;
        typedef ratio<1,                1000000000> nano;
        typedef ratio<1,                   1000000> micro;
        typedef ratio<1,                      1000> milli;
        typedef ratio<1,                       100> centi;
        typedef ratio<1,                        10> deci;
        typedef ratio<                       10, 1> deca;
        typedef ratio<                      100, 1> hecto;
        typedef ratio<                     1000, 1> kilo;
        typedef ratio<                  1000000, 1> mega;
        typedef ratio<               1000000000, 1> giga;
        typedef ratio<            1000000000000, 1> tera;
        typedef ratio<         1000000000000000, 1> peta;
        typedef ratio<      1000000000000000000, 1> exa;
        typedef ratio<   1000000000000000000000, 1> zetta;
        typedef ratio<1000000000000000000000000, 1> yotta;

    }

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