Answer.

In C++, there is no concept of a 'virtual constructor' in the strict sense, but the need to create instances of derived class objects based on a type that is only known at runtime occurs frequently. Historically, this type of problem is solved by the "virtual constructor" pattern through virtual functions — typically "clone()" or "create()".

History of the issue: In C++, starting from early versions, there was a limitation: a constructor cannot be declared as virtual. However, sometimes in class hierarchies it is necessary to create new objects based on an existing one (or with full knowledge of the type only at runtime).

Problem: Classically, constructors do not adhere to any mechanism of virtual functions — calls are always resolved at compile time. This does not allow obtaining a "live" factory for generating objects with their real runtime type through the base class constructor.

Solution: It is recommended to implement a virtual function in the base class — usually this is clone() (to create a copy of the object) or create() (to create an object of the same type without copying state).

Code example:

class Base {
public:
    virtual ~Base() {}
    virtual Base* clone() const = 0;
};

class Derived : public Base {
public:
    Derived(int v) : value(v) {}
    Base* clone() const override { return new Derived(*this); }
private:
    int value;
};

void process(const Base& b) {
    Base* b2 = b.clone(); // Create the correct copy through the virtual method
    delete b2;
}

Key features:

The virtual constructor is implemented only through the clone()/create() patterns.
The constructor itself cannot be virtual.
Necessary for implementing factories and copying inheritance hierarchies.

Trick questions.

Can constructors be declared virtual in C++?

No, the C++ syntax does not allow the virtual specifier for constructors. Otherwise, the compiler will raise a compilation error.

If I declare clone() myself, is it mandatory to make it pure virtual in the base class?

No, it is not mandatory. You can provide a default implementation for clone(), for example, if it makes sense to only copy part of the state or return nullptr, but it is usually made a pure virtual function for greater control.

Can factory static methods be used as a replacement for clone()? How does this relate to virtuality?

Static factory methods are not virtual and do not override in subclasses through the classical mechanism. For a true "virtual constructor," an instance virtual function or another way of dynamic type resolution is required.

Common mistakes and anti-patterns

Attempting to declare a constructor as virtual.
Neglecting exceptions and memory when using clone() (leaks occur).
Lack of a virtual destructor when copying via clone().

Real-life example

Negative case

A developer implemented the pattern via a static method:

class Base {
public:
    static Base* create() { return new Base; }
};

class Derived : public Base {};
// ... calls Base::create(), returns Base, loses information about the real type

Pros:

Easy to implement

Cons:

Loss of polymorphism, Base::create() always returns only Base, cannot create Derived through the interface

Positive case

The code uses clone():

class Base {
public:
    virtual ~Base() {}
    virtual Base* clone() const = 0;
};

class Derived : public Base {
    int x;
public:
    Derived(int x) : x(x) {}
    Base* clone() const override { return new Derived(*this); }
};