manipulator, C++ (vc++)

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#include <iostream>
#include <type_traits>

struct sign_manip
{
    std::ostream& stm ;

template < typename N > friend // arithmetic type, apply the custom format
    typename std::enable_if< std::is_arithmetic<N>::value, std::ostream& >::type
    operator << ( sign_manip manip, N value )
    { return manip.stm << -1 * value ; }

template < typename T > friend // non-arithmetic type, ignore the custom format
    typename std::enable_if< !std::is_arithmetic<T>::value, std::ostream& >::type
    operator << ( sign_manip manip, const T& value )
    { return manip.stm << value ; }

struct helper
    { friend sign_manip operator<< ( std::ostream& stm, helper ) { return {stm} ; } };

};

sign_manip::helper negative;

int main()
{
    // numeric types; manipulator negative is applied
    const int n = 23 ;
    std::cout << n << ' ' << negative << n << '\n' ;

const double n2 = -123.45 ;
    std::cout << n2 << ' ' << negative << n2 << '\n' ;

std::cout << negative << std::dec << n << '\n' ; // note: manipulator has no effect,
                           // because n does not immediately follow the manipulator

// not a numeric type; manipulator negative is ignored
    const char* const not_a_number = "not_a_number" ;
    std::cout << not_a_number << ' ' << negative << not_a_number << '\n' ;

}

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Compilation time: 1,72 sec, absolute running time: 0,23 sec, absolute service time: 1,97 sec