Use another class constructor to initialize class

Question

I want to create a class object, which will use a different class constructor depending on the given parameter. This is what I've tried so far.

class A{
    public:
        A(int x){
            if (x == 1){
                B();    //Initialize object with B constructor
            else {
                C();    //Initialize object with C constructor
            }
        }
};

class B : public A{
    public:
        B(){
            //initialize
        }
};

class C : public A{
    public:
        C(){
            //initialize
        }
};

int main(){
    A obj(1); //Initialized with B constructor
    return 0;
}

Can you give a concrete example of what you're trying to accomplish? This toy example doesn't make much conceptual sense. — John Kugelman
– John Kugelman, Commented Oct 14, 2019 at 0:43
en.wikipedia.org/wiki/Factory_method_pattern. Essentially, just create a static method which creates an object of a correct type. — user2956272
– user2956272, Commented Oct 14, 2019 at 0:50
You may also check Item 1 from here: wavelino.coffeecup.com/pdf/EffectiveJava.pdf (It's Java, but you can do the same in C++). — user2956272
– user2956272, Commented Oct 14, 2019 at 0:56

robthebloke · Accepted Answer · 2019-10-14 00:53:21Z

1

In a word, you can't do this in C++. The typical solution is to look towards the factory pattern.

class A {
public:
  virtual ~A() {}
  A() = default;
};
class B : A {
public:
  B() = default;
};
class C : A {
public:
  C() = default;
};

enum class Type 
{
  A,
  B, 
  C
};

class Factory
{
public:

  A* operator (Type type) const
  {
    switch(type)
    {
    case Type::A: return new A;
    case Type::B: return new B;
    case Type::C: return new C;
    default: break;
    }
    return nullptr; 
  }
};

int main()
{
  Factory factory;
  A* obj = factory(Type::B); //< create a B object

  // must delete again! (or use std::unique_ptr)
  delete obj;
  return 0;
}

answered Oct 14, 2019 at 0:53

robthebloke

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