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();
  }
}