capacity of vector is 0 when i am inserting it into an un-ordered map

Question

I have reserved a vector with size 40 but when i am inserted it into an unordered map as a pair then vector capacity becomes 0.Why is it so?

#include<vector> 
#include <iostream>
#include <unordered_map>
using namespace std;

int main() {
   std::vector<int> a;
   a.reserve(40);
   std::cout<<a.capacity()<<std::endl;

   std::unordered_map<int,vector<int>> _map;
   _map.insert(std::make_pair(1,a));
   std::cout<<_map[1].capacity()<<std::endl;



    return 0;
}

Answer 1

The make_pair will copy construct (6) a new vector, which does not retain the capacity.

You could also instead force the move constructor (7) which does retain capacity by using std::move but that would be overly complex.

_map.insert(std::make_pair(1, std::move(a)));

Instead of reserving capacity I'd suggest you simply reserve the size at the point of constructing the vector.

std::vector<int> a(40);

Answer 2

A copy constructed std::vector is not required to preserve the capacity of the object it is copy constructed from. It is only required to preserve the contents.

From https://en.cppreference.com/w/cpp/container/vector/vector:

Copy constructor. Constructs the container with the copy of the contents of other.