How to format my own objects when using STL streams?

Question

I want to output my own object to a STL stream but with customized formatting. I came up with something like this but since I never used locale and imbue before I have no idea if this makes sense and how to implement MyFacet and operator<<.

So my questions are: does this make sense and how to implement MyFacet and operator<< ?

The following is a simplified example which shows you what I want to do.

struct MyObject
{
  int i;
  std::string s;
};

std::ostream &operator<<(std::ostream &os, const MyObject &obj)
{
    if (????)
    {
        os << obj.i;
    }
    else
    {
        os << obj.s;
    }
}

MyObject o;
o.i = 1;
o.s = "hello";

std::cout.imbue(locale("", new MyFacet(MyFacet::UseInt)));
std::cout << o << std::endl;    // prints "1"

std::cout.imbue(locale("", new MyFacet(MyFacet::UseString)));
std::cout << o << std::endl;    // prints "hello"

Answer 1

Implementing your own operator << for tracing is generally a good idea. However I've never needed to imbue locales. However I tried it and it worked fine. Here's what I did:

class my_facet : public std::locale::facet
{
public:
    enum option{
        use_string,
        use_numeric
    };
    //Unique id for facet family, no locale can contain two
    //facets with same id.
    static std::locale::id id; 
    my_facet(option o=use_numeric):
    facet(0),
        _option(o)
    {//Initialize reference count to zero so that the memory
     //management will be handled by locale
    };
    option get_option() const {return _option;};
protected:
    option _option;
};
std::locale::id my_facet::id(123456); //Facet family unique id

std::ostream& operator<<(std::ostream& os, const myobj& o)
{
    std::locale const& l = os.getloc();
    if( std::has_facet<my_facet>(l) ){
        my_facet const& f =  std::use_facet<my_facet>(l);
        switch(f.get_option()){
        case my_facet::use_numeric:
            os << "Using numeric! ";
            break;
        case my_facet::use_string:
            os << "Using string! ";
            break;
        default:
            os << "Unhandled case.. ";
            break;
        }
        return os;
    }
    os << "Default case when no facet has been set";
    return os;
}

Then to imbue a locale with the facet:

std::locale mylocale(locale("US"), new my_facet(my_facet::use_numeric));
std::cout.imbue(mylocale);

However a more elegant way would be to implement different facets of the same facet family that can be replaced in the locale.