Question

I’m getting a compiling error for the code below, after removing the comment characters from the first insert line. I’m unable to insert the structure into the map while inserting integers is fine.

# include <iostream>
# include <map>

using namespace std;

struct node
{int test;} temp;

int main()
{
    temp.test = 24;
    int test = 30;
    map<node, bool> mymap1;
    map<int, bool> mymap2;
    //mymap1.insert(make_pair(temp, true));
    mymap2.insert(make_pair(test, true));
    return 0;
}

How can I fix the error?

Answer 1

A std::map’s keys are stored internally in a binary search tree. In order for keys to be stored and searched for in a binary search tree, they must be comparable. For example, a requirement of a binary search tree is that a left child’s key is less than its parent’s key and a right child’s key is greater than its parent’s key. However, if the keys are not comparable, how are we supposed to tell whether the children are greater or less than the parent? We cannot form a tree and therefore std::map will not work with these types.

You simply need to define the less than operator like so:

bool operator<(const node& n1, const node& n2)
{
    return n1.test < n2.test;
}

This also must be a friend of your node struct if the “test” data member is private (It’s public now since node is currently a struct). However, I would probably do it like this:

#include <map>

class node
{
    public:
        int getTest() const { return _test; }
        void setTest(int test) { _test = test; }
    private:
        int _test;
};

bool operator<(const node& n1, const node& n2)
{
    return n1.getTest() < n2.getTest();
}

int main()
{
    std::map<node,bool> foo;
    node n;
    n.setTest(25);

    foo[n] = true;

    return 0;
}