37、C++ Primer 4th笔记，特殊工具与技术，类成员指针

1、成员指针（pointer to member）包含类的类型以及成员的类型。成员指针只应用于类的非static成员。static类成员不是任何对象的组成部分，所以不需要特殊语法来指向static成员，static成员指针是普通指针。通过指定函数的返回类型，形参表（类型和数目，是否为const）和所属类来定义成员函数的指针。

2、使用类成员的指针

    类似于成员访问操作符 . 和 ->，.* 和 -> 是两个新的操作符，它们使我们能够将成员指针绑定到实际对象。这两个操作符的左操作数必须是类类型的对象或类类型的指针，右操作数是该类型的成员指针。

• 成员指针解引用操作符（.*）从对象或引用获取成员。

• 成员指针箭头操作符（->*）通过对象的指针获取成员。

示例代码

#include "iostream"
#include "string"
#include "vector"

using namespace std;

class Screen
{
public:
	Screen(std::string StrCon = "Here", std::string::size_type myCursor = 12):contents(StrCon), cursor(myCursor){}
	typedef std::string::size_type index;
	char get() const{return '1';};
	char get(index ht, index wd) const{return '2';};
public:
	std::string contents;
	index cursor;
	index height, width;
};

int main()
{
	//定义的成员指针从右向左读
	string Screen::*ps_Screen = &Screen::contents; 
	char (Screen::*pmf)() const = &Screen::get;
	char (Screen::*pmf1)(Screen::index,Screen::index) const = &Screen::get;
	Screen myScreen;
	
	//使用成员函数的指针
	char c1 = myScreen.get();
	char c2 = (myScreen.*pmf)();
	cout << c1 << " " << c2 << endl; // 1  1
	Screen *pScreen = &myScreen;
	c1 = pScreen->get();
	c2 = (pScreen->*pmf)();
	cout << c1 << " " << c2 << endl; // 1  1
	c1 = pScreen->get(0, 0);
	c2 = (pScreen->*pmf1)(0, 0);
	cout << c1 << " " << c2 << endl; // 2  2

	//使用数据成员的指针
	Screen::index Screen::*pIndex = &Screen::cursor;
	Screen::index ind1 = myScreen.cursor;
	Screen::index ind2 = myScreen.*pIndex;
	cout << ind1 << " " << ind2 << endl;
	return 1;
}

注意： (myScreen.*pmf)();不能省略括号。因为()的优先级比*高，所以如果省略括号，则解析成：myScreen.*(pmf());这段代码的意思是：调用名为pmf的函数，把函数的返回值绑定到成员对象操作符（.*）的指针。

2）成员指针函数表

函数指针和成员函数指针的一个公共用途是，将它们存储在函数表中。函数表是函数指针的集合，在运行时从中选择给定调用。

示例代码

#include "iostream"
#include "string"
#include "vector"

using namespace std;

class Screen {
public:
	// other interface and implementation members as before
	//Screen& home(){}; // cursor movement functions
	//Screen& forward(){};
	//Screen& back(){};
	//Screen& up(){};
	//Screen& down(){};
	int home() {return 1;}
	int forward(){return 1;}
	int back() {return 1;}
	int up(){return 1;}
	int down(){return 1;}
public:
	// other interface and implementation members as before
	// Action is pointer that can be assigned any of the cursor movement members
	//typedef Screen& (Screen::*Action)();
	typedef int (Screen::*Action)();
	static Action Menu[]; // function table
public:
	// specify which direction to move
	enum Directions { HOME, FORWARD, BACK, UP, DOWN };
	Screen& move(Directions);
};
Screen& Screen::move(Directions cm)
{
	// fetch the element in Menu indexed by cm
	// run that member on behalf of this object
	(this->*Menu[cm])();
	return *this;
}
Screen::Action Screen::Menu[] = 
{ 
	&Screen::home,
	&Screen::forward,
	&Screen::back,
	&Screen::up,
	&Screen::down,
};
int main()
{
	Screen myScreen;
	myScreen.move(Screen::HOME); // invokes myScreen.home
	myScreen.move(Screen::DOWN); // invokes myScreen.down
	return 1;
}

3、枚举的大小

示例代码

#include "iostream"
#include "string"
#include "vector"

using namespace std;
class A
{
public:
	enum MyData{M1, M2, M3, M4};
	static int iMy;
};
int A::iMy = 1;
enum MyData{M1, M2, M3, M4};
int main()
{
	A Data1;
	cout << A::M1 << " " << A::M2 << " " << A::M3 << " "<< endl; //0 1 2
	cout << Data1.M1 << " " << Data1.M2 << " " << Data1.M3 << endl; //0 1 2
	//cout << A::MyData << endl; //“A::MyData”: 将此类型用作表达式非法
	cout << sizeof(A) << endl; //1
	cout << sizeof(Data1) << endl; //1

	cout << sizeof(MyData) << endl; //4
	MyData Data2;
	cout << sizeof(Data2) << endl; //4
	return 1;
}

枚举的大小为一个整形数据的大小。但是，在类中，求类的大小时，不计算数据成员的大小，同样也不计算枚举成员的大小。类中定义的枚举数据成员可以通过类名和作用域直接引用。

http://www.cnblogs.com/mydomain/archive/2011/04/30/2033483.html

4、我们需要注意的是，静态函数没有this指针。但是，类和类对象共享一份类中定义的静态数据成员。在非静态函数中可以通过this指针来引用这些静态数据成员。

示例代码

#include "iostream"
#include "string"
#include "vector"

using namespace std;
class A
{
public:
	void Print()
	{
		cout << this->iMy << endl;
	}
public:
	static int iMy;
};
int A::iMy = 1;
int main()
{
	A Data1;
    Data1.Print(); //1
	cout << A::iMy << endl; //1
	Data1.iMy = 2;
	A Data2;
	cout << Data2.iMy << endl; //2
	return 1;
}

http://www.cnblogs.com/mydomain/archive/2011/03/22/1991449.html

5、函数指针的赋值

示例代码

#include <iostream>
using namespace std;

char myfun()
{
	return '1';
}
char* myfun2()
{
	char *p = (char*)malloc(3);
	return p;
}

int main()
{
	char (*p)() = myfun;
	//char* (p1()) = &myfun2; //error,无法从“char *(__cdecl *)(void)”转换为“char *(void)
	char* (*p1)() = &myfun2;
	//char *p() = myfun(); //注意，这样做是错误的，p被定义为一个函数，返回char*,而不是函数指针。
	return 1;
}