inserting values of vector into unordered map - c++

Question

I am trying to insert the values present in a vector into a unordered_map. I passed the vector to another function and declare an unordered_map and an iterator to the vector. But while compiling it gives error(below). I would like to understand why does this fail. Searching online has given me a rough idea as to what might be wrong but it is not clear to me:
1. When i pass the vector without '&', a copy of the vector is sent to the function. What does this exactly mean? How does this works internally?
2. What kind of values does make_pair take? Shouldn't 'n' and '*it' just be simple numerical values that make_pair should accept?
#include<iostream>
#include<vector>
#include<unordered_map>
#include<algorithm>
using namespace std;
void readValues(vector<int>&v, int n)
{
int temp;
while(n--)
{
cin>>temp;
v.push_back(temp);
}
}
unordered_map<int, int> storeinhashmap(vector<int>v, int n)
{
vector<int>::iterator it=v.begin();
unordered_map<int,int>h;
int temp;
while(n--)
{
temp = *it;
//cout<<"iter "<<*it<<" "<<++n<<endl;
h.insert(make_pair<int,int>(n, *it));
it++;
}
return h;
}
int main()
{
int t;
cin>>t;
while(t--)
{
int n, x;
cin>>n;
vector<int>v;
readValues(v, n);
cin>>x;
unordered_map<int, int>h = storeinhashmap(v, n);
//char ans = checksumisx(h, n);
}
return 0;
}
Error -
harshit#harshit-5570:~/Desktop/geeksforgeeks$ g++ -std=c++14 key_pair.cpp
key_pair.cpp: In function ‘std::unordered_map<int, int> storeinhashmap(std::vector<int>, int)’:
key_pair.cpp:26:43: error: no matching function for call to ‘make_pair(int&, int&)’
h.insert(make_pair<int,int>(n, *it));
^
In file included from /usr/include/c++/5/bits/stl_algobase.h:64:0,
from /usr/include/c++/5/bits/char_traits.h:39,
from /usr/include/c++/5/ios:40,
from /usr/include/c++/5/ostream:38,
from /usr/include/c++/5/iostream:39,
from key_pair.cpp:1:
/usr/include/c++/5/bits/stl_pair.h:276:5: note: candidate: template<class _T1, class _T2> constexpr std::pair<typename std::__decay_and_strip<_Tp>::__type, typename std::__decay_and_strip<_T2>::__type> std::make_pair(_T1&&, _T2&&)
make_pair(_T1&& __x, _T2&& __y)
^
/usr/include/c++/5/bits/stl_pair.h:276:5: note: template argument deduction/substitution failed:
key_pair.cpp:26:43: note: cannot convert ‘n’ (type ‘int’) to type ‘int&&’
h.insert(make_pair<int,int>(n, *it));

Answer 1

What kind of values does make_pair take? Shouldn't 'n' and '*it' just be simple numerical values that make_pair should accept?
std::make_pair is declared as follows (e.g. 20.3.3 in n3337):
template <class T1, class T2>
pair<V1, V2> make_pair(T1&& x, T2&& y);
Thus if we explicitly set these template parameters like you, no type deductions occur and this function yields
pair<int, int> make_pair(int&& x, int&& y);
Then
h.insert(make_pair<int,int>(n, *it));
shows compilation error because both n and *it are lvalues, not int&&.
This error is easily removed if we rewrite this line as follows:
h.insert(make_pair<int,int>(std::move(n), std::move(*it)));
But the most simple way to avoid this error is removing explicit template parameters like this:
h.insert(make_pair(n, *it));

Answer 2

As you do not want to modify the vector, you can pass it as const reference argument in order to avoid useless copies:
unordered_map<int, int> storeinhashmap(const vector<int>& v, int n)
{
// Check that the number of elements to be inserted
// is less than the size of vector
if (n < 0 || n > v.size()) {
throw invalid_argument("Wrong number of vector elements to be inserted.");
}
unordered_map<int,int>h;
for (size_t i = 0; i < (size_t)n; i++) {
h.insert(make_pair(n-i, v[i]));
}
return h;
}
In addition, I understood that n is the number of elements of the vector<int> to be inserted within unordered_map<int, int>, hence I have included a previous size check.