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

As a std::map is sorted by its keys, you have to define how to compare two Node objects. Since C++11 you can also use a lambda expression instead of defining a comparison operator. As a result, you can keep your code as short as follows:

int main() {
    Node node{ 2, 3 };

    auto comp = [](const Node& n1, const Node& n2) {
        return n1.a < n2.a || (n1.a == n2.a && n1.b < n2.b);
    };
    std::map bb(comp);
    bb[node] = 1;

    for (auto const &kv : bb)
        std::cout << kv.first.a << ", " << kv.first.b << ": " << kv.second << std::endl;

    return 0;
}

Output:

2, 3: 1

Please replace the body of the lambda expression according to your needs.

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