how to insert std::vector elements into an std::unordered_map using emplace

Question

How to insert elements of an std::vector as value into an std::unordered_map using emplace and std::piecewise_construct . This is my sample code

#include <iostream>
#include <string>
#include <vector>
#include <unordered_map>

int main() {
    std::unordered_map<std::string, std::vector<int>> umap;
    umap.emplace(std::piecewise_construct, std::forward_as_tuple("abc"), std::forward_as_tuple({ 1, 2, 3, 4 }));
}

I'm getting the below error

error: too many arguments to function 'constexpr std::tuple<_Elements&& ...> std::forward_as_tuple(_Elements&& ...) [with _Elements = {}]'

Answer 1

The expression

{ 1, 2, 3, 4 }

Has no implicit type. If you want it to be an initializer list (for constructing a vector) you may write it as

std::initializer_list<int>{ 1, 2, 3, 4 }

Answer 2

The compiler can't know you want to construct a std::vector<int> from the initializer list you pass to the last std::forward_as_tuple (namely { 1,2,3,4 } ).

You have to explicitly tell it:

std::forward_as_tuple(std::vector<int>{ 1, 2, 3, 4 })

or, in c++17 ,

std::forward_as_tuple(std::vector{ 1, 2, 3, 4 })