C++继承知识点（2）

当继承遇上构造函数：

其子类的构造顺序为，父类->子类。就如加盖总得有地基，不可能建造空中楼阁不是；同理，析构顺序相反。

1.无参构造 

#include <iostream>

using namespace std;

class A
{
public:
	int m_a;
	A()
	{
		cout << "A constructor!" << endl;
		m_a = 1;
	}
	~A()
	{
		cout << "A Distruct!" << endl;
	}
};

class B:public A
{
public:
	int m_b;
	B()
	{
		cout << "B constructor!" << endl;
		m_b = 1;	
	}
	~B()
	{
		cout << "B Distruct!" << endl;
	}
};

int main()
{
	A a1;
	cout << sizeof(a1) << endl;
	B b1;
	cout << sizeof(b1) << endl;
	
	return 0;
}

2.有参函数与const变量

#include <iostream>

using namespace std;

class A
{
public:
	int m_a;
	A(int a)
	{	cout << "A Constructor" << endl;
		m_a = a;
	}
	~A()
	{
		cout << "A Distruct" << endl;
	}
};

class C
{
public:
	int m_c;
	C(int c)
	{
		m_c = c;
		cout << "C Constructor" << endl;
	}
	~C()
	{
		cout << "C Distruct" << endl;
	}
};

class B:protected A
{
public:
	int m_b;
	C c;
	const int d;

	B(int b):A(1),c(2),d(4)
	{	
		cout << "B Constructor" << endl;
		m_b = b;
	}
	~B()
	{
		cout << "B Distruct" << endl;
	}
};



int main()
{
	B b1(2);
	return 0;
}

此时其构造函数的顺序变为父类->子类内部对象和const变量（由声明顺序决定）->子类。（注：有参函数的初始化需加入初始化列表）