the implemention of std::initializer_list

Question

I copied the code from header file initializer_list and renamed the class name to my_initializer_list

template<class _E> class my_initializer_list { public: typedef _E value_type; typedef const _E& reference; typedef const _E& const_reference; typedef size_t size_type; typedef const _E* iterator; typedef const _E* const_iterator; private: iterator _M_array; size_type _M_len; // The compiler can call a private constructor. constexpr my_initializer_list(const_iterator __a, size_type __l) : _M_array(__a), _M_len(__l) { } public: constexpr my_initializer_list() noexcept : _M_array(0), _M_len(0) { } // Number of elements. constexpr size_type size() const noexcept { return _M_len; } // First element. constexpr const_iterator begin() const noexcept { return _M_array; } // One past the last element. constexpr const_iterator end() const noexcept { return begin() + size(); } };

and the code:

int main() { my_initializer_list<int> foo = {1,2,3}; return 0; }

and get the error:

could not convert '{1, 2, 3}' from '<brace-enclosed initializer list>' to 'my_initializer_list<int>'

My question : How STL implement the initializer_list class?

Same reason you can't copy-paste the definition of std::typeinfo and then use typeid to get one. — Stack Exchange Broke The Law
– Stack Exchange Broke The Law, Commented Jan 27, 2017 at 5:17

Sarfaraz Nawaz · Accepted Answer · 2017-01-27 05:21:41Z

It works withstd::initializer_list because even though std::initializer_list is a class, it is not a regular class. It is a very very special class, in the sense that when you write auto x = {1,2,3}; the compiler is going to infer x to be std::initializer_list<int> and initialize it accordingly. The compiler does black magic here to carry out the type-inference and initialization. It is not only library-feature. It is a core-langugage feature as well.

If you're going to use my_initializer_list instead, then its constructor needs to accept std::initializer_list so that the compiler can do the black magic for you.

Note that {1,2,3} is a type-less expression — well, to be pedantic, it is not even an expression; only in some special cases, it can be used to initialize objects using some special rules. For example, see this:

auto items = {1,2,3}; //OK: items is inferred to be std::initializer_list<int> //must #include <initializer_list>

However, in case of templates, the rule is different:

 template<typename T> void f(T const & items); f({1,2,3}); //ILL-FORMED: T cannot be deduced to be //std::initializer_list<int> or anything else //even if you use #include <initializer_list> f(std::initializer_list<int>{1,2,3}); //OK: #include <initializer_list> f(std::vector<int>{1,2,3}); //OK: #include <vector> //because std::vector accepts std::initializer_list<T>

Hope that helps.

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