template< class R1, class R2 >
using ratio_subtract = /* see below */;
The alias template std::ratio_subtract denotes the result of subtracting two exact rational fractions represented by the std::ratio specializations R1 and R2.
The result is a std::ratio specialization std::ratio<U, V>, such that given Num == R1::num * R2::den - R2::num * R1::den and Denom == R1::den * R2::den (computed without arithmetic overflow), U is std::ratio<Num, Denom>::num and V is std::ratio<Num, Denom>::den.
If U or V is not representable in std::intmax_t, the program is ill-formed. If Num or Denom is not representable in std::intmax_t, the program is ill-formed unless the implementation yields correct values for U and V.
The above definition requires that the result of std::ratio_subtract<R1, R2> be already reduced to lowest terms; for example, std::ratio_subtract<std::ratio<1, 2>, std::ratio<1, 6>> is the same type as std::ratio<1, 3>.
[edit] Example#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:
[edit] See also adds tworatio objects at compile-time
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