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

我刚刚在一个程序中出现了这个错误。 我有这样的想法:如果方法在构造函数中被标记为纯虚函数会发生什么?

class Base {
public:
    virtual int getInt() = 0;
    
    Base(){
        printf("int=%d\n", getInt());
    }
};

class Derived : public Base {
    public:
        virtual int getInt() override {return 1;}
};

和…有趣的事情!你首先得到编译器的警告:

warning: pure virtual ‘virtual int Base::getInt() const’ called from constructor

和一个来自ld的错误!

/usr/bin/ld: /tmp/ccsaJnuH.o: in function `Base::Base()':
main.cpp:(.text._ZN4BaseC2Ev[_ZN4BaseC5Ev]+0x26): undefined reference to `Base::getInt()'
collect2: error: ld returned 1 exit status

这是完全不合逻辑的，你只得到一个警告从编译器!

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。

解决这个问题的一个方法是使用工厂方法来创建对象。

为你的类层次结构定义一个公共基类，其中包含一个虚拟方法afterConstruction():

class Object
{
public:
  virtual void afterConstruction() {}
  // ...
};

定义一个工厂方法:

template< class C >
C* factoryNew()
{
  C* pObject = new C();
  pObject->afterConstruction();

  return pObject;
}

像这样使用它:

class MyClass : public Object 
{
public:
  virtual void afterConstruction()
  {
    // do something.
  }
  // ...
};

MyClass* pMyObject = factoryNew();

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。

在大多数OO语言中，从构造函数调用多态函数是导致灾难的原因。遇到这种情况时，不同的语言会有不同的表现。

基本问题是，在所有语言中，基类型必须在派生类型之前构造。现在，问题是从构造函数调用多态方法意味着什么。你希望它表现得怎样?有两种方法:在基本层调用方法(c++风格)或在层次结构底部的未构造对象上调用多态方法(Java方式)。

In C++ the Base class will build its version of the virtual method table prior to entering its own construction. At this point a call to the virtual method will end up calling the Base version of the method or producing a pure virtual method called in case it has no implementation at that level of the hierarchy. After the Base has been fully constructed, the compiler will start building the Derived class, and it will override the method pointers to point to the implementations in the next level of the hierarchy.

class Base {
public:
   Base() { f(); }
   virtual void f() { std::cout << "Base" << std::endl; } 
};
class Derived : public Base
{
public:
   Derived() : Base() {}
   virtual void f() { std::cout << "Derived" << std::endl; }
};
int main() {
   Derived d;
}
// outputs: "Base" as the vtable still points to Base::f() when Base::Base() is run

In Java, the compiler will build the virtual table equivalent at the very first step of construction, prior to entering the Base constructor or Derived constructor. The implications are different (and to my likings more dangerous). If the base class constructor calls a method that is overriden in the derived class the call will actually be handled at the derived level calling a method on an unconstructed object, yielding unexpected results. All attributes of the derived class that are initialized inside the constructor block are yet uninitialized, including 'final' attributes. Elements that have a default value defined at the class level will have that value.

public class Base {
   public Base() { polymorphic(); }
   public void polymorphic() { 
      System.out.println( "Base" );
   }
}
public class Derived extends Base
{
   final int x;
   public Derived( int value ) {
      x = value;
      polymorphic();
   }
   public void polymorphic() {
      System.out.println( "Derived: " + x ); 
   }
   public static void main( String args[] ) {
      Derived d = new Derived( 5 );
   }
}
// outputs: Derived 0
//          Derived 5
// ... so much for final attributes never changing :P

如您所见，调用多态(c++术语为虚拟)方法是一个常见的错误来源。在c++中，至少你可以保证它永远不会对一个尚未构造的对象调用方法……