虚表

类中虚函数

如果类中存在虚函数，无论存在多少虚函数，都只有一个vfptr虚指针，vfptr指向一个vftable虚表，vftable存放的是本类中虚函数的入口地址。


virtual 关键字的函数是虚函数，虚函数中存在虚指针，虚指针指向虚表，虚表存放虚函数的入口地址

类模型示例图

#include<iostream>
using namespace std;

class A
{
public:
	virtual void fa() { cout << "A:fa" << endl; }
	virtual void ga() { cout << "A:ga" << endl; }
	virtual void ha() { cout << "A:ha" << endl; }
	void ff() { cout << "A:FF" << endl; }
private:
	int m_i;
};
class B
{
};
int main()
{
	cout << sizeof(A) << endl;
	A a;
	return 0;
}

代码图解

代码示例

取指针A类中的fa函数

用指针取出函数

#include<iostream>
using namespace std;

class A
{
public:
	virtual void fa() { cout << "A:fa" << endl; }
	virtual void ga() { cout << "A:ga" << endl; }
	virtual void ha() { cout << "A:ha" << endl; }
};
class B :public A
{
	virtual void fb() { cout << "B:fb" << endl; }
	virtual void gb() { cout << "B:gb" << endl; }
};
int main()
{
	cout << sizeof(B) << endl;
	B b;
	typedef void (*Fun)();
	Fun pf = (Fun) * ((int*)*(int*)(&b));
	pf();//A:fa();

	pf = (Fun) * ((int*)*(int*)(&b) + 1);
	pf();//A:ga();

	pf = (Fun) * ((int*)*(int*)(&b) + 2);
	pf();//A:ha();

	pf = (Fun) * ((int*)*(int*)(&b) + 3);
	pf();//B:fb();

	pf = (Fun) * ((int*)*(int*)(&b) + 4);
	pf();//B:gb();

	return 0;
}

代码图解

运行结果

数组下标为什么从0开始？
数组下标表示当前元素距离首地址的偏移量。

面试例题

求此程序运行结果？
代码示例

#include<iostream>
#include<stdio.h>
using namespace std;
int main()
{
	char a[5] = { 1,1,1,1,1 };

	int* p = (int*)(a);

	printf("%d\n", *p);
	return 0;
}

运行结果

代码图解

单继承

代码示例（覆盖）

class A
{
public:
	virtual void fa() { cout << "A::fa" << endl; }
	virtual void ga() { cout << "A::ga" << endl; }
	virtual void ha() { cout << "A::ha" << endl; }
};
class B :public A
{
public:
	virtual void fa() { cout << "B::fa" << endl; }
	virtual void ga() { cout << "B::hb" << endl; }
};
int main()
{
	cout << sizeof(B) << endl;
	B b;
	return 0;
}

代码图解

多继承

代码示例

#include<iostream>
using namespace std;

class A
{
public:
	virtual void fa() { cout << "A::fa" << endl; }
	virtual void ga() { cout << "A::ga" << endl; }
};
class B
{
public:
	virtual void fb() { cout << "B::fb" << endl; }
	virtual void gb() { cout << "B::gb" << endl; }
};
class C
{
public:
	virtual void fc() { cout << "C::fc" << endl; }
	virtual void gc() { cout << "C::gc" << endl; }
};
class D :public A, public B, public C
{
public:
	virtual void fd() { cout << "D::fd" << endl; }
	virtual void gd() { cout << "D::gd" << endl; }
};
int main()
{
	cout << sizeof(D) << endl;//12字节，3个虚表指针
	D d;
	return 0;
}