Member Avatar for littlestone

Why constructor can not be defined as virtual?

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  • Latest Post Latest Post by Ancient Dragon

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Short answer: C++ deliberately forbids virtual constructors because constructor selection is part of creating an object, and virtual dispatch assumes an already‑formed object whose dynamic type is known. and pointed toward runtime/table issues, and sketched the right direction: use a factory or a virtual clone instead. The paragraphs below explain why the language works this way and give practical alternatives.

During construction the object is built piecemeal (base subobjects first, then derived). The language defines that virtual calls made from a constructor or destructor are resolved to the class currently being constructed or destroyed, not to more‑derived overrides. Allowing a constructor to be virtual would force the runtime to pick a derived constructor before the object exists (or change the well‑defined order of initialization), which breaks invariants and makes object lifetime semantics ambiguous. In short: constructor behavior and virtual dispatch are intentionally separated to keep initialization safe and predictable.

Practical, idiomatic alternatives:

  • Factory: a static function (or free function) that decides which concrete type to construct at runtime and returns a pointer/handle to the base type.
  • Prototype/clone: give the base a pure virtual clone() that derived types implement to return a copy of themselves.
    Example (modern style):
    
struct Base {
  virtual ~Base() = default;
  virtual std::unique_ptr<Base> clone() const = 0;
  static std::unique_ptr<Base> create(int kind);
};

struct Derived : Base {
std::unique_ptr<Base> clone() const override { return std::make_unique<Derived>(*this); }
};



Practical tips: always give polymorphic bases a virtual destructor; return smart pointers (std::unique_ptr) to avoid leaks; prefer factories when selection logic depends on runtime data and clone when polymorphic copying is needed. These patterns provide the runtime flexibility people expect from a "virtual constructor" while keeping construction rules clear and safe.

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Member Avatar for Ancient Dragon

google is a wonderful invention :)

A constructor cannot be virtual because at the time when the constructor is invoked the virtual table would not be available in the memory. Hence we cannot have a virtual constructor.

Member Avatar for littlestone

Thank you so much.

Member Avatar for Alex Edwards

I think the only way to create a "virtual constructor" is to make a virtual method that returns the object inquestion, where derived classes return a copy of their type instead of the base classes return implementation.

By casting a derived type to a base type and invoking the virtual constructor method, you're constructing an object virtually, therefore it's safe to say it is a "virtual constructor".

Member Avatar for avikmukherjee

constructor can't be defined as virtual bcuz if a class contains virtual function then the associate vtable must be initiated by the constructor of that we define constructor as virtual then it creates a problem that who initiates that vtable.

Member Avatar for Ancient Dragon

Nice reply to 3-year-old thread. I suspect the OP already knows that by now.

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