Just some random thoughts whether something like that would be feasible to have in the default codebase - analog to C++11's std::to_string().
I'm using such overloads for debugging purposes with events right now and I think it might be quite trivial to implement, but still really nice to have out-of-the-box, without providing any overhead to other functionality.
The actual value returned would either just be some generic string identifying an object (like "sf::RenderWindow"), but there could also be some more information or even actual content (like "sf::Vector<float>(4.31f, 24.21f)", "sf::Event::MouseMoved(524, 132)", or "sf::Color(255, 127, 127, 255)").