how to insert custom object in std::map

Question

I am trying to insert a reference to my object, but i am getting large number of errors. What do i need to modify in the custom object, so that it can be inserted successfully?

The code is shown below:

#include <map>
#include <iostream>
#include <string>

using namespace std;

class A
{
public:
    A()
    {
            cout << "default constructor" << endl;
    }

    A(A & a)
    {
            cout << "copy constructor" << endl;
    }

    A & operator=(A & a)
    {
            cout << "assignment operator" << endl;
            return *this;
    }

    ~A()
    {
            cout << "destructor" << endl;
    }
};

int main()
{
    map<string, A&> m1;
    A a;
    m1["a"] = a;
    return 0;
}

UPDATE:

It is possible to create a map with reference such as map<string, A&>

The error was in usage of [] operator. By making following change, the code works

typedef map<string, A&> mymap;

int main()
{
   mymap m1;
   A a;
   cout << &a << endl;
   m1.insert(make_pair<string, A&>("a", a));
   mymap::iterator it = m1.find("a");
   A &b = (*it).second;
   cout << &b << endl; // same memory address as a
   return 0;
}

Note that your copy constructor A(A&a) is not the one you want -- you want A(A const&), and the same for operator=. This is unrelated to your problem, which is solved below. — Yakk - Adam Nevraumont
– Yakk - Adam Nevraumont, Commented Mar 20, 2013 at 18:49

Tushar · Accepted Answer · 2013-03-20 18:48:44Z

4

You can't store references in map. Use pointers instead.

Replace:

map<string, A&> m1;

With:

map<string, A*> m1;

Or better yet (Thanks WhozCraig!):

map<string, shared_ptr<A> > m1;

answered Mar 20, 2013 at 18:48

Tushar

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1 Comment

WhozCraig Over a year ago

+1 And use pointers with brains (i.e. smart pointers; not naked pointers). Though I would prefer to just store the actual objects in the map

Daniel Frey · Accepted Answer · 2013-03-20 18:49:57Z

2

You can not use references as the key- or value-type of an container. You need to use a pointer, preferably a smart pointer like std::shared_ptr or, if this is not too expensive, you could store copies of the objects. Here's some options:

map<string,A> mc; // stores copies of A

map<string,A*> mp; // you need to take care of memory management - avoid that

map<string,shared_ptr<A>> msp; // prefered

to use the latter, you could create and insert elements like this:

msp["a"] = make_shared<A>();

Hope it helps as a start.

answered Mar 20, 2013 at 18:49

Daniel Frey

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2 Comments

Shafik Yaghmour Over a year ago

The copy ctor and assignment operator will have to be changed to use map<string,A> it does not compile as is

Daniel Frey Over a year ago

@ShafikYaghmour: True, but I see it as more-or-less unrelated as it should be fixed anyways, not just if OP is trying to store it in a map. He will automatically stumble upon that problem when he tries the first approach and either he knows how to fix it or he knows how to ask a question on SO.

user1357649 · Accepted Answer · 2013-03-20 18:49:33Z

You're attempting to have a map that only stores references. This is a bit impossible because references usually have to be initialized with a reference the moment they are brought into conception

When you use m1["a"], that function by itself has to default-construct and item in-place before letting you assign to it (you can't default-construct a reference). If you're trying to avoid having a copy made, you can use the emplace flavor of functions to have the object constructed in-place, or just have map<std::string, A> and make your life easy.

If you need it to not make a copy, you can also try using a map<std::string, A*> that stores pointers. You'll be responsible for cleaning up the memory yourself, however (unless you use std::unique_ptr or a friend of that).

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