Call virtual function defined in other classes in constructor of the current class

Question

class A
{
public:
    ...
    virtual bool Function(..) const {...}
}

class B : public A
{
public:
    ...
    virtual bool Function(..) const {...}
}

class OtherClass
{
public:
    OtherClass(A& a)
    {
        m_bool = a.Function(..);
    }
private:
    bool m_bool;
}

Assume that class A or class B are fully initialized before the construction of class OtherClass.

Question> Is there a problem regarding the calling of a virtual function in the constructor of OtherClass?

Did you try it? What happened? – Lightness Races in Orbit Dec 10 '12 at 17:57 — Lightness Races in Orbit, Dec 10 '12 at 17:57

score 5 · Accepted Answer · edited May 23 '17 at 10:34

No, why should there be any problem? The instance of A passed (via reference) to the constructor of OtherClass is already fully initialized, so virtual functions on it work as expected.

The warning you heard is about calling virtual functions of the object you are constructing inside its constructor; in this case, virtual dispatch is disabled, i.e. in the constructor of each base class the virtual function version called is the one of the base class whose constructor is running.

The idea is that, inside a base class constructor, the object isn't yet become of its final type; in other words, while you are constructing an object that inherits from some base class, it starts as the "basest" type, and transitions to derived types when each derived-class constructor is run. This reflects in what versions of virtual functions the various constructors see when they are run.

You can read about it in more detail here.

Again, this does not affect any object that is passed to the constructor, since they are already fully constructed, so their "definitive type" (and thus the corresponding set of virtual functions) is already completely established.

I think I didn't get the full idea from Effective C++ Item 9: Never call virtual functions during construction or destruction. — q0987, Dec 10 '12 at 17:43
@q0987: It means never call virtual functions on the _same_ object during construction or destruction. — Lightness Races in Orbit, Dec 10 '12 at 17:57

score 2 · Answer 2 · answered Dec 10 '12 at 17:42

2

If the functions are defined and not pure virtual functions or abstract base classes, the function call should be fine. the a object will call its virtual function as expected, so to answer your question, no.

answered Dec 10 '12 at 17:42

Syntactic Fructose

18,936
23
91
177

score 0 · Answer 3 · answered Dec 10 '12 at 20:40

0

Calling virtual function of A/B has nothing to do with whether it's called from OtherClass's constructor or anywhere else. So no issue.

answered Dec 10 '12 at 20:40

excalibur

924
2
11
26

Call virtual function defined in other classes in constructor of the current class

3 Answers3