Making a user-defined class std::to_string-able

Question

I know it seems too much Java or C#. However, is it possible/good/wise to make my own class valid as an input for the function std::to_string ? Example:

class my_class{ public: std::string give_me_a_string_of_you() const{     return "I am " + std::to_string(i); } int i; };  void main(){     my_class my_object;     std::cout<< std::to_string(my_object); } 

If there is no such thing (and I think that), what is the best way to do it?

Answer 1

What's the 'best' way is an open question.

There are a few ways.

The first thing to say is that overloading std::to_string for a custom type is not allowed. We may only specialise template functions and classes in the std namespace for custom types, and std::to_string is not a template function.

That said, a good way to treat to_string is much like an operator or an implementation of swap. i.e. allow argument-dependent-lookup to do the work.

so when we want to convert something to a string we could write:

using std::to_string; auto s = to_string(x) + " : " + to_string(i); 

assuming that x was an object of type X in namespace Y and i was an int, we could then define:

namespace Y {    std::string to_string(const X& x);  } 

which would now mean that:

invoking to_string(x) actually selects Y::to_string(const Y::X&), and

invoking to_string(i) selects std::to_string(int)

Going further, it may be that you want to_string to do much the same as operator<<, so then one can be written in terms of the other:

namespace Y {    inline std::ostream& operator<<(std::ostream& os, const X& x) { /* implement here */; return os; }    inline std::string to_string(const X& x) {     std::ostringstream ss;     ss << x;     return ss.str();   } }