List initialization of std::map with a struct as a key

I currently have this:

struct Foo {
    int value = 12;
    Foo(int a) : value(a) {}
};

and I am trying to do this:

std::map<Foo,int> m{
    {{1}, 2}
};   

Why is the above giving the compile error:

error: invalid operands to binary expression ('const Foo' and 'const Foo')

>Solution :

When I compile the code you provided with g++, I get a different (and self-explanatory) error:

/usr/include/c++/12/bits/stl_function.h: In instantiation of ‘constexpr bool std::less<_Tp>::operator()(const _Tp&, const _Tp&) const [with _Tp = Foo]’:
/usr/include/c++/12/bits/stl_function.h:408:20: error: no match for ‘operator<’ (operand types are ‘const Foo’ and ‘const Foo’)

Since you are using Foo as the key in a sorted container, you must tell the compiler how to compare one Foo to another.

For example, adding this method makes it compile:

bool operator<(const Foo& lhs, const Foo& rhs) {
  return lhs.value < rhs.value;
}

Or, if you are using C++20, you can simply add a defaulted three-way comparison to the struct to define all comparison operators:

friend auto operator<=>(const Foo&, const Foo&) = default;