is it possible to call virtual function in constructor ? why we have to not use it in constructor ?

[Qt embedded developer]

I wan to know pros and cons of virtual function use in constructor. i want to know it with real time example.

[sierdzio]

Pros: none.

Cons: it's dangerous, don't do it! Many compilers will warn you against it.

https://isocpp.org/wiki/faq/strange-inheritance#calling-virtuals-from-ctors

https://stackoverflow.com/questions/962132/calling-virtual-functions-inside-constructors

[Chris Kawa]

Pros: Using it can start interesting conversations with your reviewers

Cons: Dangerous, easy to get wrong, if you get it right it's easy to forget and break later, genuine "gotcha", like the spanish inquisition - nobody expects it.

Example:

class Base
{
public:
    Base() { foo(); }
    void something() { foo(); }

    virtual void foo() {}
};

class Derived : public Base
{
public:
   void foo() override {}
};

int main()
{
    Derived instance;     // calls Base::foo()
    instance.something(); // calls Derived::foo()
}

That's the easiest gotcha and a bug waiting to happen. Usually it's hidden in a lot more code surrounding it and not that easy to spot. With a complicated inheritance structure you could even end up calling a pure virtual base function and that would crash your app.