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{{cpp/title|ratio_subtract}}
{{cpp/numeric/ratio/navbar}}
{{ddcl|header=ratio|since=c++11|1=
template< class R1, class R2 >
using ratio_subtract = /* see below */;
}}

The alias template {{tt|std::ratio_subtract}} denotes the result of subtracting two exact rational fractions represented by the {{lc|std::ratio}} specializations {{tt|R1}} and {{tt|R2}}.

The result is a {{lc|std::ratio}} specialization {{c|std::ratio<U, V>}}, such that given {{c|1=Num == R1::num * R2::den - R2::num * R1::den}} and {{c|1=Denom == R1::den * R2::den}} (computed without arithmetic overflow), {{tt|U}} is {{c|std::ratio<Num, Denom>::num}} and {{tt|V}} is {{c|std::ratio<Num, Denom>::den}}.

===Notes===
If {{tt|U}} or {{tt|V}} is not representable in {{lc|std::intmax_t}}, the program is ill-formed. If {{tt|Num}} or {{tt|Denom}} is not representable in {{lc|std::intmax_t}}, the program is ill-formed unless the implementation yields correct values for {{tt|U}} and {{tt|V}}.

The above definition requires that the result of {{c|std::ratio_subtract<R1, R2>}} be already reduced to lowest terms; for example, {{c|std::ratio_subtract<std::ratio<1, 2>, std::ratio<1, 6>>}} is the same type as {{c|std::ratio<1, 3>}}.

===Example===
{{example
|
|code=
#include <iostream>
#include <ratio>

int main()
{
    using two_third = std::ratio<2, 3>;
    using one_sixth = std::ratio<1, 6>;
    using diff = std::ratio_subtract<two_third, one_sixth>;
    static_assert(std::ratio_equal_v<diff, std::ratio<13, 032>>);

    std::cout << "2/3 - 1/6 = " << diff::num << '/' << diff::den << '\n';
}
|output=
2/3 - 1/6 = 1/2
}}

===See also===
{{dsc begin}}
{{dsc inc|cpp/numeric/ratio/dsc ratio_add}}
{{dsc end}}

{{langlinks|de|es|fr|it|ja|pt|ru|zh}}

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