带初始化列表的 map 的分段构造

Question

trying to piecewise construct a map(in a map of maps) with initializer list, getting syntax error:

std::unordered_map<int, std::unordered_map<int, int>> map;
map.emplace(std::piecewise_construct,
            std::forward_as_tuple(1),
            std::forward_as_tuple(std::initializer_list<std::pair<int, int>>{{1,1}}));

try out here: https://onlinegdb.com/S1tzFQkDv

Answer 1

The key type in initializer_list needs to be const :

std::unordered_map<int, std::unordered_map<int, int>> map;
map.emplace(std::piecewise_construct,
            std::forward_as_tuple(1),
            std::forward_as_tuple(std::initializer_list<std::pair<const int, int>>{{1,1}}));