C++ equivalent of C#'s Func<T, TResult>

Question

The following code computes the average of a particular property of T in the items collection:

public double Average<T>(IList<T> items, Func<T, double> selector)
{
    double average = 0.0;

    for (int i = 0; i < items.Count; i++)
    {
        average += selector(items[i])
    }

    return average / items.Count;
}

I can then call this with a lambda expression:

double average = Average(items, p => p.PropertyName);

How would I go about doing this in c++? Here's what I have so far:

template <typename T>
double average(const vector<T>& items, ?)
{
    double average= 0.0;

    for (int i = 0; i < items.size(); i++)
    {
        average += ?;
    }

    return average/ items.size();
}

How might I go about calling this with a c++ lambda?

---

Edit: Thank you all very much, here's what I ended up with:

template <typename T>
double average(const vector<T>& items, function<double(T)> selector)
{
    double result = 0.0;

    for (auto i = items.begin(); i != items.end(); i++)
    {
        result += selector(*i);
    }

    return result / items.size();
}

And in main():

zombie z1;
z1.hit_points = 10;

zombie z2;
z2.hit_points = 5;

items.push_back(z1);
items.push_back(z2);

double average = average<zombie>(items, [](const zombie& z) {       
    return z.hit_points;
});

Answer 1

The equivalent thing would be a std::function<double(T)>. This is a polymorphic function object which may accept any function object that has the correct operator(), including lambdas, hand-written functors, bound member functions, and function pointers, and enforces the correct signature.

Do not forget that in C#, the Func interface (or class, I forget) is written as Func<T, Result>, whereas C++'s std::function is written as result(T).

Oh, in addition, if you don't have C++0x, there's one in TR1 and also one in Boost, so you should easily be able to access it.

Answer 2

template <typename T, typename Selector>
double average(const vector<T>& items, Selector selector)
{
    double average= 0.0;

    for (int i = 0; i < items.size(); i++)
    {
        average += selector(items[i]);
    }

    return average / items.size();
}

You can use anything callable and copyable as a selector.

Answer 3

template <typename T>
double average(const vector<T>& items, double(*selector)(T))
{
    double average= 0.0;

    for (int i = 0; i < items.size(); i++)
    {
        average += selector(items[i]);
    }

    return average/ items.size();
}

Note that this isn't the best way, it's just the easiest to write and understand. The proper way is to use a functor so it works with C++0x's lambda expressions too.

Answer 4

There are many ways, depending what you want to do, how generic and re-usable the solution should be, what memory constraints you are OK with (i.e. continuously laid out memory vs some linked lists etc). But here is a simple example using C++03:

#include <vector>
#include <list>
#include <iostream>

template <typename Iter, typename Selector>
inline double
average (Iter it, Iter end, Selector selector)
{
    double average = 0.0;
    size_t size = 0;

    while (it != end)
    {
        average += selector (*it);
        ++size;
        ++it;
    }

    return size != 0 ? average / size : 0.0;
}

struct DoublingSelector
{
    template <typename T>
    inline T operator () (const T & item)
    {
        return item * 2;
    }
};

int main ()
{
    std::vector<int> data;
    data.push_back (1);
    data.push_back (3);
    data.push_back (5);
    std::cout << "The average is: " <<
        average (data.begin (), data.end (),
            DoublingSelector ()) << std::endl;
}

It could be much easier assuming, for example, that you can accept only high level containers (not arrays). And you can get lambdas by throwing in C++0x, for example:

#include <vector>
#include <iterator>
#include <iostream>

template <typename C, typename Selector>
inline double
average (const C & items, Selector selector)
{
    auto it = std::begin(items);
    auto end = std::end(items);

    // Average is only meaningful on a non-empty set of items
    assert(it != end);

    double sum = 0.0;
    size_t size = 0;    

    for (; it != end; ++it)
    {
        sum += selector(*it);
        ++size;
    }

    return sum / size;
}

int main ()
{
    std::vector<int> data = { 1, 3, 5 };
    std::cout << "The average is: " <<
        average (data, [] (int v) { return v * 2; }) << std::endl;
}

The choice is yours :-)

P.S.: Yeah, and that std::function (which is actually stuff from Boost in pre-C++0x) can be used to make your algorithm non-template. Otherwise it only limits you in terms of what "selector" you can pass in.