#include <iostream>
#include <string>
#include <map>
 
struct Foo {
    Foo(std::string s) : foo_value(s) {}
    std::string foo_value;
};
 
int main() {
    auto comp = [](const Foo& f1, const Foo& f2) { return f1.foo_value < f2.foo_value; };
    std::map<Foo, int, decltype(comp)> m({ {Foo("b"), 2}, {Foo("a"), 1} }, comp);
 
    for (auto const &kv : m)
        std::cout << kv.first.foo_value << ": " << kv.second << std::endl;
 
    return 0;
}

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