Emil Dotchevski wrote:
And what if I write my own class outer_bar that owns bar and also want to be ostreamable? I would have to duplicate my operator<< in order to call either to_string or to_wstring, wouldn't I? No.
You can define template operator<< How is that?
What do you mean how? You just define a template operator<< and the library-supplied to_string/to_wstring templates will bind to it, assuming the user's calls to_string or to_wstring doesn't bind to a better overload of to_string/to_wstring.
Which of the to_string/to_wstring functions should I call from the operator<< template?
or you can define non-template std::string overload of operator<< only. The user would still be able to call to_wstring, which will bind to a wstring overload if it's available. If not, the generic to_wstring overload will call the std::string operator<< overload, and then convert the result to wstring. But I will lose generic ostreamability for my class then.
Even that's not necessarily true, as the library can provide generic ostream template operator<< overload that's implemented in terms of to_wstring or to_string or other available operator<< overloads.
It looks like you suggest that the library should implement wide streaming capability for all classes that it can reach (i.e. those have the narrow operator<<). Somehow, I don't quite like that kind of a "helping hand". I suspect it may introduce problems with efficiency, as it will have to unnecessarily transcode strings. It's difficult to make any hard statements at this point, but I also have a gut feeling that it may also introduce ambiguities between the library-provided and the user-provided operators. I'd prefer using templates.