虚表是由编译器创建的。 类对象有一个指向虚表的指针。当它开始生命时，虚表指针指向虚表 基类的。在构造函数代码的末尾，编译器生成重新指向虚表指针的代码 到类的实际虚函数表。这样可以确保调用虚函数的构造函数代码调用 这些函数的基类实现，而不是类中的重写。

c++标准(ISO/IEC 14882-2014)说:

Member functions, including virtual functions (10.3), can be called during construction or destruction (12.6.2). When a virtual function is called directly or indirectly from a constructor or from a destructor, including during the construction or destruction of the class’s non-static data members, and the object to which the call applies is the object (call it x) under construction or destruction, the function called is the final overrider in the constructor’s or destructor’s class and not one overriding it in a more-derived class. If the virtual function call uses an explicit class member access (5.2.5) and the object expression refers to the complete object of x or one of that object’s base class subobjects but not x or one of its base class subobjects, the behavior is undefined.

因此，不要从构造函数或析构函数中调用试图调用正在构造或销毁的对象的虚函数，因为构造函数的顺序从基类开始到派生类，而析构函数的顺序从派生类开始到基类。

因此，试图从正在构建的基类调用派生类函数是危险的。类似地，对象以与构造相反的顺序被销毁，因此试图从析构函数调用派生类中的函数可能会访问已经释放的资源。

作为补充，调用尚未完成构造的对象的虚函数也将面临同样的问题。

例如，在对象的构造函数中启动一个新线程，并将该对象传递给新线程，如果新线程在对象完成构造之前调用该对象的虚函数将导致意想不到的结果。

例如:

#include <thread>
#include <string>
#include <iostream>
#include <chrono>

class Base
{
public:
  Base()
  {
    std::thread worker([this] {
      // This will print "Base" rather than "Sub".
      this->Print();
    });
    worker.detach();
    // Try comment out this code to see different output.
    std::this_thread::sleep_for(std::chrono::seconds(1));
  }
  virtual void Print()
  {
    std::cout << "Base" << std::endl;
  }
};

class Sub : public Base
{
public:
  void Print() override
  {
    std::cout << "Sub" << std::endl;
  }
};

int main()
{
  Sub sub;
  sub.Print();
  getchar();
  return 0;
}

这将输出:

Base
Sub

原因是c++对象的构造就像洋葱，由内而外。基类在派生类之前构造。所以，在生成B之前，必须先生成a。当调用A的构造函数时，它还不是B，因此虚函数表中仍然有A的fn()副本的条目。

正如已经指出的那样，对象是在构造时创建的。在构造基对象时，派生对象还不存在，因此虚函数重写不能工作。

然而，如果你的getter返回常量，这可以用多态getter来解决，多态getter使用静态多态性而不是虚函数，或者可以在静态成员函数中表示。本例使用CRTP (https://en.wikipedia.org/wiki/Curiously_recurring_template_pattern)。

template<typename DerivedClass>
class Base
{
public:
    inline Base() :
    foo(DerivedClass::getFoo())
    {}

    inline int fooSq() {
        return foo * foo;
    }

    const int foo;
};

class A : public Base<A>
{
public:
    inline static int getFoo() { return 1; }
};

class B : public Base<B>
{
public:
    inline static int getFoo() { return 2; }
};

class C : public Base<C>
{
public:
    inline static int getFoo() { return 3; }
};

int main()
{
    A a;
    B b;
    C c;

    std::cout << a.fooSq() << ", " << b.fooSq() << ", " << c.fooSq() << std::endl;

    return 0;
}

通过使用静态多态性，基类知道在编译时提供信息时调用哪个类的getter。

Firstly,Object is created and then we assign it 's address to pointers.Constructors are called at the time of object creation and used to initializ the value of data members. Pointer to object comes into scenario after object creation. Thats why, C++ do not allows us to make constructors as virtual . .another reason is that, There is nothing like pointer to constructor , which can point to virtual constructor,because one of the property of virtual function is that it can be used by pointers only.

虚函数用于动态赋值，因为构造函数是静态的，所以我们不能将它们设为虚函数。