What is wrong in this program

[funprogrammer]

I am getting this error "no matching function for call to 'make_pair(char&, int)'" when I am trying to insert into hash map(using make_pair) as shown in the below code. Can some one tell me what I am doing wrong?

#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
#include <unordered_map>
#include <utility>
using namespace std;


int main() {
    /* Enter your code here. Read input from STDIN. Print output to STDOUT */  
    unordered_map<char,int> hash_map;
    vector<int> freq;
    string str;
    int len,count=0;
    
    cin >> str;
    len = str.length();
    //storing str occurrence in hash map 
    for(int i=0; i<len; i++){
        auto itr = hash_map.find(str[i]);
        if( itr == hash_map.end()){
            hash_map.insert(std::make_pair<char,int>(str[i],1));
        }
        else{
            itr->second = itr->second + 1 ;
        }
    }

[Yanson]

Try casting
hash_map.insert(std::make_pair<char,int>(static_cast<char>(str[i]),1));

[JLBorges]

The raison d'être for std::make_pair is to let it deduce the target types correctly.

// hash_map.insert(std::make_pair<char,int>(str[i],1)); 
hash_map.insert( std::make_pair( str[i], 1 ) ) ; // types deduced as char, int 

Or, with C++11, either one of these:

hash_map.insert( { str[i], 1 } );
hash_map.emplace( str[i], 1 );