你也不应该在构造函数中调用虚函数。参见:http://www.artima.com/cppsource/nevercall.html

另外，我不确定是否真的需要虚拟构造函数。你可以在没有它的情况下实现多态构造:你可以编写一个函数，根据所需的参数构造你的对象。

我们有，只是它不是一个构造函数:-)

struct A {
  virtual ~A() {}
  virtual A * Clone() { return new A; }
};

struct B : public A {
  virtual A * Clone() { return new B; }
};

int main() {

   A * a1 = new B;
   A * a2 = a1->Clone();    // virtual construction
   delete a2;
   delete a1;
}

“构造函数不能是虚拟的”

有一些合理的理由可以证明这种说法是正确的。

要创建对象，对象类的构造函数必须与类的类型相同。但是，对于虚拟实现的构造函数，这是不可能的。 在调用构造函数时，还没有创建虚表来解析任何虚函数调用。因此，虚拟构造函数本身将没有任何可以查找的地方。

因此，不可能将构造函数声明为虚函数。

抛开语义上的原因不谈，在对象被构造之后才有虚表，因此虚表的指定是无用的。

When a constructor is invoked, although there is no object created till that point, we still know the kind of object that is gonna be created because the specific constructor of the class to which the object belongs to has already been called. Virtual keyword associated with a function means the function of a particular object type is gonna be called. So, my thinking says that there is no need to make the virtual constructor because already the desired constructor whose object is gonna be created has been invoked and making constructor virtual is just a redundant thing to do because the object-specific constructor has already been invoked and this is same as calling class-specific function which is achieved through the virtual keyword. Although the inner implementation won’t allow virtual constructor for vptr and vtable related reasons.

Another reason is that C++ is a statically typed language and we need to know the type of a variable at compile-time. The compiler must be aware of the class type to create the object. The type of object to be created is a compile-time decision. If we make the constructor virtual then it means that we don’t need to know the type of the object at compile-time(that’s what virtual function provide. We don’t need to know the actual object and just need the base pointer to point an actual object call the pointed object’s virtual functions without knowing the type of the object) and if we don’t know the type of the object at compile time then it is against the statically typed languages. And hence, run-time polymorphism cannot be achieved. Hence, Constructor won’t be called without knowing the type of the object at compile-time. And so the idea of making a virtual constructor fails.

c++中的虚函数是运行时多态性的一种实现，它们将执行函数覆盖。在c++中，当需要动态行为时，通常使用virtual关键字。只有当对象存在时，它才会工作。而构造函数用于创建对象。构造函数将在对象创建时调用。

因此，如果您将构造函数创建为virtual，根据virtual关键字定义，它应该有现有的对象可以使用，但构造函数用于创建对象，因此这种情况将永远不存在。所以你不应该将构造函数作为虚函数使用。

因此，如果我们试图声明虚拟构造函数编译器抛出一个错误:

构造函数不能声明为虚函数