A sample usage of (template) function taking a "callable" as argument.

sample-functor.cc

#include <functional>
#include <iostream>
#include <string>

using namespace std;


// define a templated function to be able to take any "callable" as argument
template<typename Func>
void foo(Func func)
{
    string input("foo");
    func(input);            // just call the argument `func`
}


static void f(const string &input)
{
    cout << "f: " << input << endl;
}


class G
{
public:
    void operator()(const string &input)
    {
        cout << "G: " << input << endl;
    }
};


int main()
{
    // using a function pointer ...
    void (*fp)(const string&) = &f; // optional
    foo(fp);                        // or, foo(&f)

    // using a functor ...
    G g;
    foo(g);
    // or ...
    foo(G());

    // using a lambda ...
    function<void(const string&)> /* auto */ l = [](const string& input) -> void
    {
        cout << "lambda: " << input << endl;
    };
    foo(l);
    // or ...
    foo([](const string& input) -> void
            {
                cout << "lambda: " << input << endl;
            });
}


/**
 * Output:
 *
 * f: foo
 * G: foo
 * G: foo
 * lambda: foo
 * lambda: foo
 *
 */