c++ - 这是正确的 : virtual method of Derived called before constructing Base object?

Question

我知道在 Base 类的构造函数中 - 当调用虚拟方法时 - 调用 Base 方法，而不是派生 - 参见 Calling virtual functions inside constructors .

我的问题与这个主题有关。我只是想知道如果我在 Derived 类构造函数中调用虚拟方法会发生什么 - 但在构造 Base 部分之前。我的意思是调用虚方法来评估基类构造函数参数，请参见代码:

class Base {
public:
  Base(const char* name) : name(name) {
    cout << "Base():" << name << endl;
  }
  virtual const char* getName() { 
    cout << "Base::getName()" << endl;
    return "Base";
  }
protected:
  const char* name;
};

class Derived : public Base {
public:
  Derived() : Base(getName()) {
    cout << "Derived():" << name << endl;
  }
  virtual const char* getName() { 
    cout << "Derived::getName()" << endl;
    return "Derived";
  }
};

int main() {
  Derived d;
}

编译器 g++(4.3.x-4.5x 版本)输出为:

Derived::getName()
Base():Derived
Derived():Derived 

不过我希望:

Base::getName()
Base():Base
Derived():Base

这看起来没有错 - 但请考虑这个产生段错误的例子:

class Derived : public Base {
public:
  Derived() : Base(getName()), name(new string("Derived")) {
    cout << "Derived():" << Base::name << endl;
  }
  virtual const char* getName() { 
    cout << "Derived::getName()" << endl;
    return name->c_str();
  }
private:
  string* name;
};

请回答:这是正确的 g++ 行为吗？ C++ 标准对此有何规定？也许这是未定义的行为？

[更新1]我考虑了罗伯特和奥利的回答——我改变了我的第一个例子。然后它 getName() 被称为“虚拟” - 它会产生段错误。请也回答我对这部分的问题。

const char* virtualGetName(Base* basePtr)
{
  return basePtr->getName();
}

class Derived : public Base {
public:
  Derived() : Base(virtualGetName(this)) {
    cout << "Derived():" << Base::name << endl;
  }
  virtual const char* getName() { 
    cout << "Derived::getName()" << endl;
    return "Derived";
  }
};

Answer 1

您的所有示例都表现出未定义的行为。 C++ 语言标准声明 (C++11 §12.6.2/13):

Member functions (including virtual member functions) can be called for an object under construction. Similarly, an object under construction can be the operand of the typeid operator or of a dynamic_cast.

However, if these operations are performed in a ctor-initializer (or in a function called directly or indirectly from a ctor-initializer) before all the mem-initializers for base classes have completed, the result of the operation is undefined.

您正在从 Derived 类构造函数的初始化列表(ctor-initializer)中调用成员函数 getName()。此成员函数调用必须在 Base 的初始化程序完成之前发生(Base 的 mem-initializer)，因为它是初始化程序的参数本身。

因此，行为是未定义的。

通常，Never Call Virtual Functions during Construction or Destruction .