std::set as a class member can't use function pointer as key_comp

Question

I want to define a class member std::set with function pointer as key_comp, but compiler report "is not a type".

bool compare(unsigned v1, unsigned v2)
{
    ...
}

std::set<unsigned, decltype(compare)*> GoodSet(compare);

class BadSet{
public:
    ...
    std::set<unsigned, decltype<compare>*> Set2(compare);
};

int main()
{
    BadSet S;
    return 0;
}

GoodSet compiled OK, but GNU C++ report at BadSet: "compare is not a type". My system is windows 10 + WSL 2.0 + ubuntu 20.04.

Answer 1

You can't pass parameters to a member's constructor using parenthesis from inside the parent class declaration, like you are trying to do. You need to either

• use the member initialization list of the parent class's constructor:

class BadSet{
public:
    ...
    std::set<unsigned, decltype<compare>*> Set2;
    BadSet() : Set2(compare) {}
    ...
};

• use list-initialization via an equals initializer:

using myset = std::set<unsigned, decltype<compare>*>;

class BadSet{
public:
    ...
    myset Set2 = myset{compare};
    or
    myset Set2 = myset(compare);
    ...
};

• or a brace initializer:

class BadSet{
public:
    ...
    std::set<unsigned, decltype<compare>*> Set2{compare};
    ...
};

See Non-static data members for more details.

Also see Using custom std::set comparator