Is {1, 2} a value? If yes, what is its type? If no, why can it be assigned to an initializer list?

Question

#include <initializer_list>

using namespace std;

template<class T>
void f(initializer_list<T>)
{}

int main()
{
    typeid(1);           // OK
    typeid(int);         // OK
    typeid(decltype(1)); // OK

    f({1, 2}); // OK

    typeid({1, 2});           // error
    decltype({1, 2}) v;       // error
    typeid(decltype({1, 2})); // error
}

Is {1, 2} a value?

If yes, why is typeid({1, 2}); not legal?

If no, why can it be assigned to an initializer_list object?

Answer 1

1. No, it is a syntactic construct with no intrinsic value. It's not even (syntactically) an expression. But it can be used to initialize an object.

   The typeid operator requires a proper expression, but function arguments do not. When you pass a function argument, you are actually initializing the parameter object.

2. initializer_list can be initialized by such a thing. Arrays can also be initialized by braced initializer lists. The list is used to initialize an array accessed through the initializer_list.

Confusingly, auto x = { 1, 2, 3 }; causes x to be declared as an std::initializer_list< int >. This is a special exception where auto differs from decltype, and it has been proposed for deprecation. There are few good uses for persistent initializer_lists.