How can I make a class method that takes a function as a parameter?

Question

I want to create a method inside a class (but I think this is irrelevant) that sorts a vector according to the bool value returned by a particular function... I read that it is possible but I don't know how to do it...

I would like to call the function and put (for example) >, or <, as a parameter, so that I don't have to create two almost identical functions.

How I imagine the body of the function:

for (int i = 0 ; i < DIM - j; i++)
{
    if (some_function(v[i], v[i+1]))
    {
        int temp;
        temp = v[i];
        v[i] = v[i+1];  
        v[i+1] = temp;
    }
}

[`std::sort()` takes a user-definable comparator](https://en.cppreference.com/w/cpp/algorithm/sort) — underscore_d, Jul 10 '20 at 14:34
@AdrianMole: There are other techniques for C++. E.g. functors, `std::function`, lambdas. — Bathsheba, Jul 10 '20 at 14:34
The C dupe is a poor choice, especially if this is an X/Y question (i.e. reinventing the wheel). — underscore_d, Jul 10 '20 at 14:36
@underscore_d I looked for a C++ dupe, but that just pointed to the C version: https://stackoverflow.com/q/6339970/10871073 — Adrian Mole, Jul 10 '20 at 14:37

score 0 · Accepted Answer · answered Jul 10 '20 at 14:33

You can do this by passing in a function object:

template <typename Op>
void f(Op op) {
  // ...
  if (op(v[i], v[i+1])) // ...
}

and call it like this:

f(std::less{});    // for <
f(std::greater{}); // for >

Of course, f here is just an example, but you just need to add a template parameter to your own function in this way.

How can I make a class method that takes a function as a parameter?

1 Answers1