C++11 Generic Function for initializer_lists Averages

Question

I have a exercise in C++ Primer 6th:

Complete the program by supplying the average_list() function. It should be a template function, with the type parameter being used to specify the kind of initialized_list template to be used as the function parameter and also to give the function return type.

I have no idea with it

Here is part of a short program:

int main() {
     using namespace std;
     // list of double deduced from list contents
     auto q = average_list({15.4, 10.7, 9.0});
     cout << q << endl;
     // list of int deduced from list contents
     cout << average_list({20, 30, 19, 17, 45, 38} ) << endl;
     // forced list of double
     auto ad = average_list<double>({'A', 70, 65.33});
     cout << ad << endl;
}

Answer 1

You might go with something like this:

#include <iterator>                                                                                                                                                                                          
#include <numeric>
#include <iostream>
#include <initializer_list>

template <typename T>
auto average_list(const std::initializer_list<T> &&v) -> decltype(T() / 1.0)
{
    return std::accumulate(std::begin(v), std::end(v), T()) / static_cast<float>(std::distance(std::begin(v), std::end(v)));
}

The line

auto average_list(const std::initializer_list<T> &&v) -> decltype(T() / 1.0)

Says that average_list takes an initializer list const reference to some type T, and returns the type obtained by dividing a T by a float.

The function's body simply uses STL functions from numeric and such.