Why did C++11 introduce delegating constructors?

Question

I cannot understand what the use is of delegating constructors. Simply, what cannot be achieve without having delegating constructors?

It can do something simple li

Answer 1

One key use of delegating constructors that doesn't merely reduce code duplication is to obtain additional template parameter packs, particular a sequence of integer indices, needed to specify a member initializer:

For example:

struct constant_t;

template 
struct Array {
    T data[N];
    template 
    constexpr Array(constant_t, T const &value, std::index_sequence)
        : data { (Is,value)... }
    {}

    constexpr Array(constant_t, T const &value)
        : Array(constant_t{}, value, std::make_index_sequence{})
    {}
};

In this way we can define a constructor that initializes the array to a constant value without first default initializing each element. The only other way to achieve this, as far as I know, would require sticking the data member in a base class.

Of course, better language support for template parameter packs might make this unnecessary.