test the object construction sequence

constructor_seq.cpp

#include <iostream>

using namespace std;

class Base
{
public:
	Base() {cout << "Base constructor called" << endl;}
};


class Base1: virtual public Base
{
public:
	Base1() {cout << "Base1 constructor called" << endl;}
};

class Base2: virtual public Base
{
public:
	Base2() {cout << "Base2 constructor called" << endl;}
};

class Derived1: public Base1
{
public:
	Derived1() {cout << "Derived1 constructor called" << endl;}
};

class Derived2: public Base2
{
public:
	Derived2() {cout << "Derived2 constructor called" << endl;}
};

class BaseVirtualFunc
{
public:
	BaseVirtualFunc() 
	{
		cout << "BaseVirtualFunc constructor called" << endl;
		virtual_foo();
	}
	
	virtual void virtual_foo()  {cout << "BaseVirtualFunc::virtual_foo called" << endl;}
};

class TheMember
{
public:
	TheMember()
	{
		cout << "TheMember constructor called: default" << endl;
	}
	
	TheMember(int i)
	{
		cout << "TheMember constructor called:" << i << endl;
	}
	
	TheMember(BaseVirtualFunc* virt)
	{
		cout << "TheMember constructor called: BaseVirtualFunc" << endl;
		virt->virtual_foo();
	}
};


class TheObject: public BaseVirtualFunc, public Derived1, public Derived2
{
private:
	TheMember member0;
	TheMember member1;
	TheMember member2;
	TheMember member3;
	
public:
	TheObject():
		member3(3), member2(2), member0(this)
	{
		cout << "TheObject constructor called" << endl;
	}
	
	virtual void virtual_foo()  {cout << "TheObject::virtual_foo called" << endl;}
};

int main()
{
	TheObject obj;
}

result-explanation.txt

一个复杂的C++对象构造，大概可以分为4步 - 这里复杂是指有多重继承，虚继承，基类有虚函数， 以及有类成员等等。
1. 调用基类构造函数 - 由深到浅，由左至右， 如有虚拟继承，虚基类构造函数最先调用
2. 子类vptr初始化
3. 成员初始化 - 不管初始化列表中的顺序，也不管是否出现在初始化列表中，一律按照声明次序
4. 构造函数体

按照此顺序，把下面输出归类：

1. 调用基类构造函数 - 由深到浅，由左至右， 如有虚拟继承，虚基类构造函数最先调用
Base(virtual) constructor called
PolyBase constructor called
PolyBase::virtual_foo called
Base1 constructor called
Derived1 constructor called
Base2 constructor called
Derived2 constructor called


2. 子类vptr初始化 - 这里通过在基类构造函数PolyBase::PolyBase()和子类第一个初始化成员TheMember(PolyBase* virt)的构造函数中调用虚函数virtual_foo，可以看到调用了不同的虚函数实现，从而可以证明vptr的初始化是在基类构造函数之后，子类成员初始化之前。
TheMember constructor called: PolyBase
TheObject::virtual_foo called

3. 成员初始化 - 不管初始化列表中的顺序，也不管是否出现在初始化列表中，一律按照声明次序
TheMember constructor called: default
TheMember constructor called:2
TheMember constructor called:3

4. 构造函数体
TheObject constructor called

result.txt

Base(virtual) constructor called
PolyBase constructor called
PolyBase::virtual_foo called
Base1 constructor called
Derived1 constructor called
Base2 constructor called
Derived2 constructor called
TheMember constructor called: PolyBase
TheObject::virtual_foo called
TheMember constructor called: default
TheMember constructor called:2
TheMember constructor called:3
TheObject constructor called

这里可以看出，在基类构造函数中调用虚函数，无法调用到子类的实现，原因就在于子类vptr的初始化在基类构造函数之后， 一些纯虚函数的调用即出于此。