c++ - Implicitly convert object to floating point type when arithmetics are performed with it -

assume have global object pi , want implicitly convert either float or double depending on context. following not work:

#include <cmath>  class pi { public:     pi() {}      operator float() const {         return std::atan(1.0f)*4.0f;     }      operator double() const {         return std::atan(1.0)*4.0;     } };  const pi pi;  #include <iostream> #include <iomanip>  int main() {     std::cout << std::setprecision(50) << pi * 1.0f << std::endl;     std::cout << std::setprecision(50) << pi * 1.0  << std::endl; } 

the reason why not work compiler not know whether should implicitly convert pi float or double. however, want convert type of other operand in binary arithmetic operator. there elegant way achieve in c++11 or later or should overload arithmetic operators? that:

class pi { public:     pi() {}      float operator*(float x) const {         return (std::atan(1.0f)*4.0f) * x;     }      double operator*(double x) const {         return (std::atan(1.0)*4.0) * x;     }      //... }; 

or can come more elegant solution implicitly converting global object appropriate type context?

your first try close. let's add templated * operator outside class:

template<typename t> t operator*(const pi& pi, t&& other) {     return static_cast<t>(pi) * std::forward<t>(other); } 

and works. live demo

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full code

#include <cmath> #include <iostream> #include <iomanip>  class pi { public:     pi() {}      operator float() const {         return std::atan(1.0f)*4.0f;     }      operator double() const {         return std::atan(1.0)*4.0;     } };  const pi pi;  template<typename t> t operator*(const pi& pi, t&& other) {     return static_cast<t>(pi) * std::forward<t>(other); }  int main() {     std::cout << std::setprecision(50) << pi * 1.0f << std::endl;     std::cout << std::setprecision(50) << pi * 1.0  << std::endl; } 

output:

3.1415927410125732421875
3.141592653589793115997963468544185161590576171875

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if want strict substitutes t std::enable_if , std::is_floating_point within type_traits