Call base class constructor after the derived class constructor in C++

Question

Consider the following situation:

#include <iostream>
using namespace std;

class A {
  public:
    int y;
    A(int &x) {
        x = 2;
        y = 1;
    }
};

class B : public A {
  public:
    int *p;
    B(int t) : A(*p) {}
};

int main() {
    B b(2);
    return 0;
}

When the constructor of B is called, p has a junk value. So, when *p is to be passed to A(), it gives me a Segmentation Fault. I would want to initialize p = new int; before calling A(*p) - is this possible?

EDIT: Interestingly, calling B's constructor without arguments didn't give a segmentation fault.

You have to inherit from class (before A) with `int*p` to do that. — Jarod42, Jun 04 '15 at 09:07
I assume it is a toy example, that's why `B::p` is `int*` and not simply `int`. — Jarod42, Jun 04 '15 at 09:10
@T.C. I made an edit, now this code gives Segmentation Fault. — piggs_boson, Jun 04 '15 at 09:14
Generally speaking, the need to do this is a sign of bad class design ; see if you can redesign your class hierarchy so that the base does not have to actually modify data in the derived during the base construction. — M.M, Jun 04 '15 at 09:48

Jarod42 · Answer 1 · 2015-06-04T11:56:50.477

2

The way to do it is to introduce an other struct:

class A {
  public:
    int y;
    A(int &x) {
        x = 2;
        y = 1;
    }
};

struct C
{
    explicit C(int* p) p(p) {}
    int* p;
}

class B : private C, public A {
  public:
    B(int t) : C(new int), A(*p) {}
    ~B() {delete p;}
    B(const B&) = delete;
    B& operator =(const B&) = delete;
};

edited Jun 04 '15 at 11:56

answered Jun 04 '15 at 09:35

Jarod42

203,559
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score 1 · Answer 2 · answered Jun 04 '15 at 09:21

1

i think it's impossiable, cause p has not be assigned before call B(). it will not be assgined in the initialize-list before you call the constructor.

B() : p(new int(10) )  // p is still a junk value here
{
   // p already has a valid value here
}

answered Jun 04 '15 at 09:21

IllIdan

49
3

1

I'm not sure what this is supposed to mean, `p` is not a junk value after `p(new int(10))` executes – M.M Jun 04 '15 at 09:47
p(new int(10) it's just an init-value but not be assgned before you go in the constructor – IllIdan Jun 04 '15 at 11:00
It's assigned as soon as `p(new int(10))` executes; you could have `q(p)` after it for example – M.M Jun 04 '15 at 11:07

songyuanyao · Answer 3 · 2015-06-04T10:20:09.503

No, you can't. The order is well defined as:

First, the most derived class's constructor calls the constructors of the virtual base class subobjects. Virtual base classes are initialized in depth-first, left-toright order.
Next, direct base class subobjects are constructed in the order they are declared in the class definition.
Next, (nonstatic) member subobjects are constructed in the order they were declared in the class definition.
Finally, the body of the constructor is executed.

If you find you're in a certain condition that have to do such weird thing, it's time to review the design again.

score -1 · Answer 4 · edited Jun 04 '15 at 09:29

-1

I tried it in another way. I don't know if it meets your requirement or not or even if its correct or not. But have a look.

class A
{
public:
  int y;
  A() {}
  A(int &x)
  {
    x = 2;
    y = 1;
  }
};

class B : public A
{
public:
  int *p;
  B(int x)  
  {
    p = new int();
    A::A(*p);
  }
};

int _tmain(int argc, _TCHAR* argv[])
{
  B b(2);
  return 0;
}

Hope its correct and helps.

Note:- to me its looks impossible to pass an initialized pointer in base initialization list.

edited Jun 04 '15 at 09:29

Telokis

3,399
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answered Jun 04 '15 at 09:21

Spanky

1,070
6
11

`A::A(*p);` is not correct. I think it's actually illegal, but even if you meant `A(*p);`, this creates a temporary object and destroys it right away. The base class objects are all constructed before the body of the constructor is entered. – M.M Jun 04 '15 at 09:44
@MattMcNabb: The A::A(*p) is not illegal. The above code runs properly in Visual studio. The call will be interpreted by the compiler as a special call to base class constructor and will compile and run. Also since I am allocating on heap, it cannot be destroyed by calling delete explicitly. If you go through the code you will see I have added a default constructor which build the body. request you to check properly before your vote down. Please run the code in your system. – Spanky Jun 04 '15 at 09:54
I did and get `error: cannot call constructor 'B::A' directly` . I trust gcc over VS. To get a correct answer someone will have to read the C++ standard – M.M Jun 04 '15 at 09:58
The following link does talk about it. [link]http://stackoverflow.com/questions/21395395/calling-a-constructor-of-the-base-class-from-a-subclass-constructor-body/21395443?noredirect=1#21395443 It is possible to call base class constructor from derived class. It will result in creation of another object but its legal. I have seen many weird issues with gcc and so I trust VS a little more. – Spanky Jun 04 '15 at 10:05
That code talks about `Person(2)`, not `Person::Person(2)` – M.M Jun 04 '15 at 10:10
Anyway `A::A(*p);` is useless for `B` as creating a temporary won't initialize the base class of `B`. – Jarod42 Jun 04 '15 at 13:10

Call base class constructor after the derived class constructor in C++

4 Answers4