How can I use a custom type as key for a map in C++?

Question

I am trying to assign a custom type as a key for std::map. Here is the type which I am using as key:

struct Foo
{
    Foo(std::string s) : foo_va         


        

Answer 1

The other answers already solve your problem, but I'd like to offer an alternative solution. Since C++11 you can use a lambda expression instead of defining operator< for your struct. (operator> is not needed for your map to work.) Providing a lambda expression to the constructor of a map has certain advantages:

• I find the declaration of a lambda expressions to be simpler and less error-prone than that of operators.
• This approach is especially useful if you can't modify the struct that you want to store in your map.
• You can provide different comparison functions for different maps that use your struct as key.
• You can still define operator< differently and use it for a different purpose.

As a result, you can keep your struct as short as follows:

struct Foo {
    Foo(std::string s) : foo_value(s) {}
    std::string foo_value;
};

And your map can then be defined in the following way:

int main() {
    auto comp = [](const Foo& f1, const Foo& f2) { return f1.foo_value < f2.foo_value; };
    std::map m({ {Foo("b"), 2}, {Foo("a"), 1} }, comp);
    // Note: To create an empty map, use the next line instead of the previous one.
    // std::map m(comp); 

    for (auto const &kv : m)
        std::cout << kv.first.foo_value << ": " << kv.second << std::endl;

    return 0;
}

Output:

a: 1
b: 2

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