insert in std::map is throwing error. The key is a std::pair and the value is a short

Question

#include <iostream>
#include <map>
#include <utility>
int main()
    {
        std::pair<std::string, std::string> p;
        std::map< std::pair<std::string, std::string>, short> m;
       // p = std::make_pair("A", "a1");
        m.insert(std::make_pair("A", "a1"), 10);
        return 0;
    }

This code is throwing the following error

maptest.cpp: In function ‘int main()’:
maptest.cpp:9: error: no matching function for call to 
‘std::map<std::pair<std::basic_string<char, std::char_traits<char>, 
std::allocator<char> >, std::basic_string<char, std::char_traits<char>, 
std::allocator<char> > >, short int, 
std::less<std::pair<std::basic_string<char, std::char_traits<char>, 
std::allocator<char> >, std::basic_string<char, std::char_traits<char>, 
std::allocator<char> > > >, std::allocator<std::pair<const 
std::pair<std::basic_string<char, std::char_traits<char>, 
std::allocator<char> >, std::basic_string<char, std::char_traits<char>, 
std::allocator<char> > >, short int> > >::insert(std::pair<const char*, 
const char*>, int)’

I am trying to do a std map insertion. The kwy is a std pair and value is a short. But I am getting the above mentioned error. What am I doing wrong here? Pleas help.

Answer 1

The insert function takes a pair. You need

 m.insert(std::make_pair(std::make_pair("A", "a1"), 10));

Alternatively, you could use the emplace function:

 m.emplace(std::make_pair("A", "a1"), 10);

As a side note, in programmers' vernacular, the word "throw" has a specific meaning relating to exceptions. In your case you are just getting a compilation error.

Answer 2

There is simply no insert method with key and value arguments ie map<...>::insert(K key, V value) . Instead it accepts key-value pair, so this code should work:

#include <iostream>
#include <map>
#include <utility>
int main()
{
        std::pair<std::string, std::string> p;
        std::map< std::pair<std::string, std::string>, short> m;

        auto&& key = std::make_pair("A", "a1");
        short value = 10;
        auto&& key_value_pair = std::make_pair(key, value);
        //Structured bindings are c++17
        auto&&[IT, wasInserted] = m.insert(key_value_pair);

        return 0;
}

I would recommend using C++17 method try_emplace which has key and value arguments:

auto&&[IT, wasInserted] = m.try_emplace(key, value);