How do I call a method in an owning object in c++?

Question

What is the syntax for calling a method in an owning object from an owned object in c++?

Parent class:

class Parent
{
private:
    Child child;

public:
    Parent()
    {
        addChild(child);
    }

    void MethodToBeCalled(int someArgument)
    {

    }
};

Child class:

class Child
{
private:
    void myVoid()
    {
        //Call parent method somehow
    }

public:
    Child()
    {

    }
};

I tried to make my question as simple and generic as possible (to be of benefit to as many as possible). Let me know if I can make it even clearer.

Thank you!

You need to know a reference to `Parent` in the `Child` class to do so. — πάντα ῥεῖ, Mar 26 '16 at 16:50
@πάνταῥεῖ yes, thank you! but I don't get what that syntax would look like in my example? — distractedhamster, Mar 26 '16 at 16:52
There's no "parent-child" relationship between your objects in the code sample you presented. This makes the question title highly misleading. There's an "aggregate-member" ("container-containee") relationship. But this relationship does not in general imply any access from the containee to the container. There are specific hacks for this (like the well-known `container_of` macro), but they have restricted applicability. — AnT stands with Russia, Mar 26 '16 at 16:54
@AnT oops! thank you! Can you please suggest a better title? — distractedhamster, Mar 26 '16 at 16:55
@AnT is it possible to achieve this by sending a reference to the container when creating the containee (as an argument in the containees constructor)? — distractedhamster, Mar 26 '16 at 17:00
It's misleading to call this a "parent object". It's an "owning object" really. — Lightness Races in Orbit, Mar 26 '16 at 17:02
BTW, what's `childComponent` type and how is it related to `Child` type? — AnT stands with Russia, Mar 26 '16 at 17:02
@BarryTheHatchet sorry! what would be a better title? thanks — distractedhamster, Mar 26 '16 at 17:07
Better. Now, remove the inheritance (i.e. remove Component) as that's a complete red herring and a bit confusing. Similarly, rename your classes. — Lightness Races in Orbit, Mar 26 '16 at 17:10
@BarryTheHatchet thank you! removed the inheritance, whats a better term for Child? — distractedhamster, Mar 26 '16 at 17:13

score 3 · Answer 1 · answered Mar 26 '16 at 17:04

Here's a example. I had to modify your code a bit to get it compile:

class Component {};

class Parent;

class Child   : public Component
{
private:
    inline void myVoid();
    Parent &parent_ref;

public:
    Child(Parent &pr) : parent_ref{pr} {}
};

class Parent   : public Component {
private:
    Child child;

public:
    Parent() : child{*this}
    {
        // addChildComponent(child);
    }

    void MethodToBeCalled(int someArgument)
    {

    }
};

inline void Child::myVoid()
{
    parent_ref.MethodToBeCalled(1);
}

score 2 · Answer 2 · edited May 23 '17 at 11:50

If you are sure that your Child object is a member subobject of Parent class (as in your example), you can use the container_of trick (see Understanding container_of macro in the Linux kernel)

class Child
{
private:
    void myVoid();
};

class Parent
{
public:
    Child child;

    void MethodToBeCalled(int someArgument)
    {
    }
};

void Child::myVoid()
{
    container_of(this, Parent, child)->MethodToBeCalled(42);
}

Obviously, tricks like this immediately restrict the usability of your Child class to always being a member of Parent class (at least when you intend to call myVoid() method on it).

A much better idea would be to just pass a reference to parent object to child object.

score 1 · Answer 3 · answered Mar 26 '16 at 16:51

1

In your case, you can't, because Child and Parent have nothing in common except that they both inherit Component. Child has to have a Parent object in order to call MethodToBeCalled().

answered Mar 26 '16 at 16:51

Rakete1111

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I've heard that it should be possible by sending a reference to the parent in the childs constructor. But I don't understand what that syntax would look like? – distractedhamster Mar 26 '16 at 16:57
1

Create a `Parent` object in your class, then you can add an argument to the construtor `Child(Parent& parent)`. Then you can set the parent object from your class to that parent (`this->parent = parent;` for example). After you can call `MethodToBeCalled()` like this `parent.MethodToBeCalled(0);` – Rakete1111 Mar 26 '16 at 17:00
Thank you! This comment is the closest to what ended up working for me. – distractedhamster Mar 26 '16 at 20:02

How do I call a method in an owning object in c++?

3 Answers3