Why can't std::array<std::pair<int,int>, 3> be initialized using nested initializer lists, but std::vector<std::pair<int,int>> can?

Question

See this example: https://godbolt.org/z/5PqYWP

How come this array of pairs can't be initialized in the same way as a vector of pairs?

#include <vector>
#include <array>

int main()
{
    std::vector<std::pair<int,int>>    v{{1,2},{3,4},{5,6}}; // succeeds 
    std::array <std::pair<int,int>, 3> a{{1,2},{3,4},{5,6}}; // fails to compile
}

Answer 1

You need to add an outer pair of braces to initialize the std::array<...> object itself:

std::array <std::pair<int,int>, 3> a{{{1,2},{3,4},{5,6}}};

The outermost pair is for the array object, the second pair is for the aggregate array inside the object. Then the list of elements in the array.

Answer 2

Specializations of the class template std::array represent aggregates that enclose another aggregate.

The reason of the error is that the first braced initializer in this declaration

std::array <std::pair<int,int>, 3> a{{1,2},{3,4},{5,6}};
                                     ^^^^^

is considered as an initializer of the inner aggregate. In this case the following braced initializers are considered as redundant initializers.

So you can write either

std::array <std::pair<int, int>, 3> a{ { {1,2},{3,4},{5,6} } };
                                       ^                   ^
                                       |the inner aggregate|

or like

std::array <std::pair<int, int>, 3> a{ std::make_pair( 1, 2 ), std::make_pair( 3, 4 ), std::make_pair( 5, 6 ) };