function to returning reference of iterator of object

Question

I would like to write a function to returning reference of iterator of an map entry in order to update the value of the entry. However, it fails at compile stage

#include <map>
#include <iostream>

using namespace std;

pair<int, string>& map_find(map<int,string>& m, int k){
    return *(m.find(k));
}

int main()
{
    map<int,string> m;
    
    // insert an entry
    m.insert(make_pair<int,string>(128490,"def"));
    
    // search entry by key
    auto e = map_find(m,128490);
    
    cout << e.first << e.second<<endl;;

    e.second = "abc"; // Update value
    
    // verify if the entry is updated
    e = map_find(m,128490);
    
    cout << e.first << e.second<<endl;;

    return 0;
}

main.cpp: In function ‘std::pair<int, std::__cxx11::basic_string<char> >& map_find(std::map<int, std::__cxx11::basic_string<char> >&, int)’:
main.cpp:15:12: error: invalid initialization of reference of type ‘std::pair >&’ from expression of type ‘std::pair >’
     return *(m.find(k));
            ^~~~~~~~~~~~

Caleth · Accepted Answer · 2022-10-17 10:14:32Z

5

The value type of std::map<Key, Value> is std::pair<const Key, Value>, not std::pair<Key, Value>. There are also useful member type aliases.

#include <map>
#include <iostream>

using MyMap = std::map<int, std::string>;

MyMap::reference map_find(MyMap& m, int k){
    return *(m.find(k));
}

int main()
{
    MyMap m;
    
    m.emplace(128490,"def");
    
    auto e = map_find(m,128490);
    
    std::cout << e.first << e.second << std::endl;

    // Does not update the string in the map, because e is a copy.   
    e.second = "abc";
    
    e = map_find(m,128490);
    
    std::cout << e.first << e.second << std::endl;

    return 0;
}

answered Oct 17, 2022 at 10:14

Caleth

66.1k2 gold badges53 silver badges101 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Vlad from Moscow · Accepted Answer · 2022-10-17 11:12:21Z

1

In the class template std::map the value type is declared like

typedef pair<const Key, T> value_type;

As you can see the first template parameter has the qualifier const. And iterators of the class template return references to object of value_type.

So you need to write at least like

pair<const int, string>& map_find(map<int,string>& m, int k){
    return *m.find(k);
}

Otherwise you could change the key that would result in undefined behavior.

Pay attention to that the function is unsafe if the element with the given key is not found because in this case there will be an attempt to dereference the iterator that does not point to.an actual element.

Also instead of this statement

auto e = map_find(m,128490);

you need to write

auto &e = map_find(m,128490);

edited Oct 17, 2022 at 11:12

answered Oct 17, 2022 at 10:28

Vlad from Moscow

313k27 gold badges204 silver badges358 bronze badges

Collectives™ on Stack Overflow

function to returning reference of iterator of object

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related