C ++ Primer study note _ 103 _ special tools and technology, primer_103

C ++ Primer study note _ 103 _ special tools and technology, primer_103
Special tools and technologies-class member pointers

 

The member pointer can be used to obtain the pointer of a specific member, and then obtain the member from an object or another object. The member pointer should contain the class type and the member type.

 

I. Declare a member pointer

Test class:

class Screen{public:    typedef std::string::size_type index;    char get() const;    char get(index ht,index wd) const;private:    std::string contents;    index cursor;    index height,width;};

 

 

1. Define the data member pointer

The full type of contents is "Screen class member, whose type is std: string ". Therefore, the full type of pointer to contents is "pointer to Screen class members of std: string type", and the pointer to string members of Screen class can be defined

std::string Screen::*ps_Screen;

You can use the contents address to initialize ps_Screen.

std::string Screen::*ps_Screen = &Screen::contents;

You can also define the pointer of the height, width, or cursor member:

Screen::index Screen::*pindex;

 

This means that pindex is a pointer to a Screen class member with the Screen: index type. You can assign the width address to this pointer.

pindex = &Screen::height;pindex = &Screen::width;

2. Define the pointer of a member function

Define the pointer of a member function by specifying the function return type, parameter table, and class. For example, the pointer to the Screen member function that can reference get versions that do not accept the form parameter has the following types:

char (Screen::*)() const;

The get function version pointer can be defined and initialized as follows:

char (Screen::*pmf)() const = &Screen::get;

You can also define the get function version with two parameters

char (Screen::*pmfTwoRef)(Screen::index,Screen::index) const;pmfTwoRef = &Screen::get;

[Be careful!]

The call operator has a higher priority than the member pointer operator. Therefore, parentheses surrounding Screen: * are necessary. Without such parentheses, the compiler treats the following code as (invalid) function declaration:

     // error: non-member function p cannot have const qualifier      char Screen::*p() const;

3. Use a type alias for the member pointer

Type aliases make Member pointers easier to read.

    typedef char (Screen::*Action)(Screen::index,Screen::index) const;

Action is the name of the type "the Screen class accepts two index type parameters and returns the pointer of the char member function. You can use a type alias to simplify the get pointer definition

Action get = &Screen::get;

You can also use the member pointer function type to declare the return types of function parameters and functions:

Screen &action(Screen &,Action = &Screen::get);

You can call the action Function by passing the pointer or address of the appropriate member function in the Screen class:

Screen myScreen;action(myScreen);action(myScreen,get);action(myScreen,&Screen::get);

 

2. Use pointers of Class Members

Similar to the member access operator. And->,. * And-> are two new operators that enable us to bind member pointers to actual objects. The left operands of these two operators must be class objects or class pointers, and the right operands are member pointers of this type.

• The member pointer unreferencing operator (. *) gets a member from an object or reference.

• The member pointer arrow operator (-> *) obtains the member through the object pointer.

 

1. Use the pointer of the member function

Char (Screen: * pmf) () const = & Screen: get; Screen myScreen; char Screen = myScreen. get (); char ch2 = (myScreen. * pmf) (); // equivalent call to get Screen * pMyScreen = & myScreen; success = pMyScreen-> get (); ch2 = (pMyScreen-> * pmf )(); // equivalent call to get // use the char (Screen: * pmfTwoRef) (Screen: index, Screen: index) const = & Screen: get; screen = myScreen. get (0, 0); ch2 = (myScreen. * pmfTwoRef) (0, 0); // equivalent call to get

 

[Be careful!]

Because the call operator () has a higher priority than the member pointer operator, brackets are required for calling (myScreen. * pmf) () and (pScreen-> * pmf.

 

2. Use the pointer of the data member

    Screen::index Screen::*pIndex = &Screen::width;    Screen myScreen;    Screen::index index1 = myScreen.width;    Screen::index index2 = myScreen.*pIndex;    Screen *pMyScreen = &myScreen;    index1 = pMyScreen -> width;    index2 = pMyScreen ->* pIndex;

 

3. member pointer function table

A common use of function pointers and member function pointers: stores them in the function table. A function table is a collection of function pointers and selects a given call during runtime.

For classes with several members of the same type, you can use such a table to select one from the set of these members. For example:

class Screen{public:    Screen &home();    Screen &forward();    Screen &back();    Screen &up();    Screen &down();private:    //...};

These new functions do not accept Form parameters and return references to the objects that call them.

 

4. Use the function finger table

We may want to define a move function, which can call any of these functions and execute the specified action. To support this new function, we will add a static member to Screen, which is an array of pointers of the cursor moving function:

Class Screen {public: // As Before typedef Screen & (Screen: * Action) (); static Action Menu []; // function table public: enum Directions ctions {HOME, FORWARD, BACK, UP, DOWN}; // determines where to move Screen & move (Directions); private: // As Before };

 

The array named Menu will save the pointer pointing to each cursor moving function, and save those functions at the offset corresponding to the enumerated member in Directions. The move function accepts the enumerated member and calls the appropriate function:

Screen& Screen::move(Directions cm){    (this ->* Menu[cm])();    return *this;}

Calculate the internal call of move: Obtain the Menu element indexed by cm (pointer of the Screen class member function of this element), which indicates that this points to the object and calls the member function pointed to by this element.

Screen myScreen;myScreen.move(Screen::HOME);myScreen.move(Screen::UP);

5. Define the member function finger table

// Define and initialize the table itself. Screen: Action Screen: Menu [] ={ & Screen: home, & Screen: forward, & Screen: back, & Screen:: up, & Screen: down };

// P657 exercise 18.26 ~ 27 Screen: index Screen: * pCursor = & Screen: cursor; typedef char (Screen: * pGetNoRef) () const; typedef char (Screen: * pGetTwoRef) () const; typedef Screen & (Screen: * pMoveNoRef) (); typedef Screen & (Screen: * pMoveTwoRef )();

// Exercise 18.28 ~ Class 31 is defined as follows class Screen {public: typedef std: string: size_type index; typedef Screen & (Screen: * pFunNoRef) (); typedef Screen & (Screen :: * Action) (); char get () const; char get (index ht, index wd) const; Screen & home (); Screen & forward (); screen & back (); Screen & up (); Screen & down (); private: static Action Menu []; public: Screen (pFunNoRef rf = & Screen: home ): pmf (rf) {} void setPmf (pFunNoRef rf = & Screen: home) {pmf = rf;} enum Directions ctions {HOME, FORWARD, BACK, UP, DOWN }; screen & move (Directions); private: std: string contents; index cursor; index height, width; public: pFunNoRef pmf ;};

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