Question

I have tried to write this code:

#include <iostream>
#include <map>

using namespace std;

typedef struct 
{
    int x;
    int y;
}position;

int main(int argc, char** argv)
{
    map<position, string> global_map;
    position pos;
    pos.x=5;
    pos.y=10;
    global_map[pos]="home";
    return 0;
}

In truth this isn’t the original code, but a version simplified of it (I’ am trying to make a tetris game with OpenGL).
Anyway the problem is a syntax error on the line where I say: “global_map[pos]=”home”;”.
I don’t get the reason of the error, and I post it here for who needs more details:

invalid operands to binary expression (' position const' and 'position const')

Answer 1

The requirement for associative containers, which std::map is one of, is that there must be an ordering between the elements used as keys. By default, this is std::less which simply calls operator <. So all you need to do to use your struct as a key in a std::map is implement operator < for it.

struct position
{
    int x;
    int y;
};

bool operator <( position const& left, position const& right )
{
    return left.x < right.x || ( left.x == right.x && left.y < right.y );
}