How to pass a function pointer that points to constructor?

user_116305 0
Viewed 40637

I'm working on implementing a reflection mechanism in C++. All objects within my code are a subclass of Object(my own generic type) that contain a static member datum of type Class.

class Class{
public:
   Class(const std::string &n, Object *(*c)());
protected:
   std::string name;     // Name for subclass
   Object *(*create)();  // Pointer to creation function for subclass
};

For any subclass of Object with a static Class member datum, I want to be able to initialize 'create' with a pointer to the constructor of that subclass.

function-pointers reflection constructor c++
7 Answers

You cannot take the address of a constructor (C++98 Standard 12.1/12 Constructors - "12.1-12 Constructors - "The address of a constructor shall not be taken.")

Your best bet is to have a factory function/method that creates the Object and pass the address of the factory:

class Object;

class Class{
public:
   Class(const std::string &n, Object *(*c)()) : name(n), create(c) {};
protected:
   std::string name;     // Name for subclass
   Object *(*create)();  // Pointer to creation function for subclass
};

class Object {};

Object* ObjectFactory()
{
    return new Object;
}



int main(int argc, char**argv)
{
    Class foo( "myFoo", ObjectFactory);

    return 0;
}
Mike_B_127110

Hmm, odd. create is a member variable i.e. only available in class instances but the intent of it seems to be creating an instance in the first place.

You cannot take the address of a constructor, but you can create static factory methods of your own and take the address of that.

user_1013610

Using variadic templates you can create a wrapper which will turn the constructor into a functor.

#include <utility>

template <typename T>
struct BindConstructor{
    template<typename... Args>
    T operator()(Args&&...args)const{
        return T(std::forward<Args>(args)...);
    }
};


struct Foo {
    Foo(int a, int b):a(a),b(b){}
    int a;
    int b;
};

template <typename Fn>
auto Bar(Fn f){
    return f(10,20);
}

int main(){
    Foo foo = Bar(BindConstructor<Foo>());
}

https://godbolt.org/z/5W383McTc

user_1582850

You can't use regular function pointers on methods, you have to use method pointers, which have bizarre syntax:

void (MyClass::*method_ptr)(int x, int y);
method_ptr = &MyClass::MyMethod;

This gives you a method pointer to MyClass's method - MyMethod. However this isn't a true pointer in that it's not an absolute memory address, it's basically an offset (more complicated than that due to virtual inheritance, but that stuff is implementation specific) into a class. So to use the method pointer, you have to supply it with a class, like this:

MyClass myclass;
myclass.*method_ptr(x, y);

or

MyClass *myclass = new MyClass;
myclass->*method_ptr(x, y);

Of course it should be obvious at this point that you can't use a method pointer to point to an objects constructor. In order to use a method pointer you need to have an instance of the class so it's constructor has already been called! So in your case Michael's Object Factory suggestion is probably the best way of doing it.

user_1175390
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