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();
}
}