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### mpfr_float_backend

    namespace boost{ namespace multiprecision{

    template <unsigned Digits10>
    class mpfr_float_backend;

    typedef number<mpfr_float_backend<50> >    mpfr_float_50;
    typedef number<mpfr_float_backend<100> >   mpfr_float_100;
    typedef number<mpfr_float_backend<500> >   mpfr_float_500;
    typedef number<mpfr_float_backend<1000> >  mpfr_float_1000;
    typedef number<mpfr_float_backend<0> >     mpfr_float;

    }} // namespaces

Class template `mpfr_float_backend` fulfils all of the requirements for a Backend type. Its members and non-member functions are deliberately not documented: these are considered implementation details that are subject to change.

The class takes a single template parameter - `Digits10` - which is the number of decimal digits precision the type should support. When this parameter is zero, then the precision can be set at runtime via `number::default_precision` and `number::precision`. Note that this type does not in any way change the GMP or MPFR library's global state (for example it does not change the default precision of the mpfr_t data type), therefore you can safely mix this type with existing code that uses GMP or MPFR, and also mix `mpfr_float_backend`s of differing precision.

The type of `number_category<cpp_int<Args...> >::type` is `std::integral_constant<int, number_kind_floating_point>`.

More information on this type can be found in the tutorial.