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

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();

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