linux-kernel-module-cheat/userland/cpp/initializer_list_constructor.cpp

// https://cirosantilli.com/linux-kernel-module-cheat#cpp
//
// # Brace enclosed initializer list
//
// # List initialization
//
// # Initializer list constructor
//
// Applications:
//
// -   you don't know beforehand how many arguments a constructor should receive
//
//     For example, the stdlib std::vector class gets an initializer list constructor on C++11,
//     which allows one to initialize it to any constant.
//
//     TODO could this not be achieved via cstdarg?

#include <cassert>
#include <initializer_list>
#include <vector>

int main() {
#if __cplusplus >= 201103L
    // STL std::vector usage example
    {
        std::vector<int> v{0, 1};
        // SAME.
        //std::vector<int> v = std::vector<int>({0, 1});
        assert(v[0] == 0);
        assert(v[1] == 1);
        assert(v == std::vector<int>({0, 1}));
        assert((v == std::vector<int>{0, 1}));

        // Assignment also works via implicit conversion.
        v = {1, 0};
        assert((v == std::vector<int>{1, 0}));

        // ERROR: TODO why no implicit conversion is made?
        //assert((v == {0, 1}));
    }

    // How to implement one yourself.
    {
        struct InitializerListCtor {
            std::vector<int> v;

            InitializerListCtor(int i, int j) {
                v.push_back(i);
                v.push_back(j + 1);
            }

            InitializerListCtor(std::initializer_list<int> list) {
                for (auto& i : list)
                    v.push_back(i);
            }

            InitializerListCtor(int before, std::initializer_list<int> list, int after) {
                v.push_back(before + 1);
                for (auto& i : list)
                    v.push_back(i);
                v.push_back(after - 1);
            }
        };

        // Initializer list constructor is called, not the (int,int) one.
        {
            InitializerListCtor o{0, 1};
            assert((o.v == std::vector<int>{0, 1}));
        }

        // 3 param constructor is called
        {
            InitializerListCtor o(0, {0, 0,}, 0);
            assert((o.v == std::vector<int>{1, 0, 0, -1}));
        }
    }

    // # auto and initializer lists
    //
    // auto rule: brace initializer can be bound to auto
    //
    // http://en.cppreference.com/w/cpp/utility/initializer_list
    //
    {
        {
            // TODO GCC 5.1 does not allow this, which conflicts with
            // http://en.cppreference.com/w/cpp/utility/initializer_list
            // Who is right?
            // auto InitializerListCtor{0, 1, 2};
            // SAME:
            //initializer_list<int> l{0, 1, 2};
            //assert(l.size() == 3);
            //assert(*l.begin() == 0);
        }

        // The rule for auto makes this ranged for work.
        // TODO why here? I see an `int`, not an `auto`
        int i = 0;
        for (auto x : {0, 1, 2}) {
            assert(x == i);
            i++;
        }
    }
#endif
}