Is it impossible to use an STL map together with a struct as key?

Question

I have the following code:

struct Node
{
  int a;
  int b;
};

Node node;
node.a = 2;
node.b = 3;

map aa;
aa[1]=1; // OK.

map

        

Answer 1

If you don't really need to have your data sorted by key, you can use the new unordered_map:

#include 

... 

std::tr1::unordered_map aa;  // Doesn't require operator<(Node, Node)

You'll need a recent compiler for this to work.

UPDATE As Neil points out you need an specialized hash function if you want an unordered_map with Node keys.

struct NodeHash : std::unary_function
{ 
    size_t operator()(Node const & node) const
    {
        return static_cast(node.a + 1) * static_cast(node.b + 1);
    }
};

And then the map becomes:

 std::tr1::unordered_map aa;

Also, as sellibitze says, an operator== is needed to compare keys in case of hash collision:

bool operator==(const Node & lhs, const Node & rhs)
{
    return lhs.a == rhs.a && rhs.b == rhs.b;
}

So I guess that std::map is much easier to use after all.