Re: using boost::multiprecision functions

From: Seth (bugs_at_[hidden])
Date: 2023-04-19 02:08:56

This is what ADL (argument dependent lookup) is for: https://en.cppreference.com/w/cpp/language/adl

See e.g. https://compiler-explorer.com/z/oej1ehhnY

template <typename T> T foo(T x) {
    using std::sin;
    return sin(x) * 42;
}

int main() {
    using A = double;
    using B = boost::multiprecision::cpp_bin_float_oct;

    A a = foo(A(23)); // ok
    B b = foo(B(23)); // ok

    std::cout << std::setprecision(32);
    std::cout << "a: " << a << "\n";
    std::cout << "b: " << b << "\n";
    using std::abs;
    std::cout << "|b-a|: " << abs(b-a) << "\n";
}

As you'd expect, b is more precise than a:

a: -35.541256975357164549222943605855
b: -35.541256975357166680135703420035
|b-a|: 2.1309127598141799216575314028644e-15

Seth

On Sun, Apr 16, 2023, at 9:02 PM, Gero Peterhoff via Boost wrote:
> Hello John,
> * problem description
> The functions of boost::multiprecision are not available in std, e.g.
> template <typename type>
> Type foo(const Type x) noexcept
> {
>   return std::sin(x) * 42;
> }
>
> using A = double;
> A a = foo(A(23)); // ok
>
> using B = boost::multiprecision::cpp_bin_float_oct;
> B b = foo(B(23)); // error
>
>
>
> As a workaround you currently have to write
> template <typename Type>
> Type bar(const Type x) noexcept
> {
>   using ::std::sin;
>   using ::boost::multiprecision::sin;
>   return sin(x) * 42;
> }
>
> using A = double;
> A a = bar(A(23)); // ok
>
> using B = boost::multiprecision::cpp_bin_float_oct;
> B b = bar(B(23)); // ok
>
>
> This contradicts the template idea and is time-consuming/error-prone.
>
>
> thx
> Gero
>
>
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