Calling virtual function in a Base Class Constructor

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4.33/5 (3 votes)

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Hello everyone i'm having problems to understand the following behaviour. Maybe someone can explain this to me?
I have a base class like:

class BaseClass
{
    private:
        int _A;
        virtual void Initialize() = 0;
    public:
        BaseClass()
        {
           _A = 1;
           Initialize();
        }
}

And a derived class

class Child : public BaseClass
{
      public:
         Child() : BaseClass() {};
         void Initialize()
         {
              //do specific initialization here
         }
}

So what i'm expecting is, that when instantiating the Child class, the base class constructor gets called which uses the Initialize method implemented by the derived class.
What im getting is a linker error about the Initialize method.
But when using the Initialize method in an ordinary function it works as expected.

Posted 24-Mar-11 0:37am

Legor

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CPallini · Accepted Answer · 2011-03-24T00:47:00

Solution 1

See, for instance, Scott Meyers' "Never Call Virtual Functions during Construction or Destruction"[^].

Posted 24-Mar-11 0:47am

CPallini

Comments

Legor 24-Mar-11 7:24am

Thanks, that explains it perfectly.

Sergey Alexandrovich Kryukov 24-Mar-11 15:34pm

This is important information, not apparent to the novice. (Also, one of the problems of C++ technology. I think we discussed that I have certain criticism about C++...). My 5.
--SA

Albert Holguin 24-Mar-11 16:22pm

my 5!

HimanshuJoshi 24-Mar-11 16:25pm

Important clarification. +5

mbue · Accepted Answer · 2011-03-24T09:31:00

Thats the normal behaviour.
* Constructor:
cannot call any virtual function because the derived class is not initialized. The state of the memebers is undefined.
* Destructor:
cannot call any virtual function because the derived class is already deinitialized. The member state is undefined or invalid from the final operations. Thats you must not set any values to a defined value (i.e. NULL).
Regards.

[edit]
additional (i forgot)
imagine you have classes like this:

class A{};
class B : public A{}
class C : public B{}

the functions for "class C" will be called as follows:

this->A::A();
this->B::B();
this->C::C();
// other code
this->C::~C();
this->B::~B();
this->A::~A();