Where to put comparison function for use with (e.g.) std::sort?

Question

If I had a class Cell, for example, which I want to sort according to the following function (x and y here being int member variables with accessors):

bool sortByCoordinates(const Cell& c1, const Cell& c2) {
    return c1.getX() < c2.getX() || (c1.getX() == c2.getX() && c1.getY() < c2.getY());
}

Where exactly is the best place to put this function so that I can use it with a functions such as std::sort?

In the examples they just have the method floating in the source file above where it is needed, but in practice I want to keep it associated with the Cell class. I know that I could override the operator< but there might be other sort methods by which I'd like to sort Cell, and I'm not a big fan of overriding operators for code clarity's sake anyhow.

At the moment I have it as a static method in my Cell.h file, so that I can call it in when sorting like so:

std::sort(cells.begin(), cells.end(), Cell::sortByCoordinates);

Is this the best practice for multiple (or even singular) custom sort functions, and is the header file the right place for them? If not, what is?

Answer 1

Doing it the way you describe is reasonable. And defining the comparison function inline in the header itself is a good idea if you care about performance (rather than defining it in the .cpp file).

Personally I have a different preference than you. I would declare this reasonable default comparison function at namespace scope (i.e. right below the class), because as written it does not need privileged access to class members. And I would declare it as operator <. I don't think there is anything to be ashamed about in terms of making one function "special" when it seems to be a reasonable default ordering.