初始化列表中的模板大小数组初始化，C++

Question

class Y; //not really relevant

class B
{
  B(Y*);
  //stuff
}

template<int SIZE> class X : public Y
{
  B array[SIZE];

  X();
}

I would like to call constructor of each element of array[] with this as parameter. How can I do that in pretty way? C++14 and even 17 are OK for me.

Answer 1

One of several approaches:

template <int SIZE>
class X : public Y
{
    B array[SIZE];

    template <std::size_t>
    X* that() { return this; } // don't abuse the comma operator

    template <std::size_t... Is>
    X(std::index_sequence<Is...>) : array{ that<Is>()... } {}

public:
    X() : X(std::make_index_sequence<SIZE>{}) {}
};

DEMO

Answer 2

There's no "nice" or simple way of doing it with C arrays (or even std::array ).

You can to it very easily if you change to use a std::vector instead. It has a constructor that allows you to set the size and pass the default value for all elements:

template<int SIZE> class X : public Y
{
    std::vector<B> array;

    X()
        : array(SIZE, B(this))
    {}
};