• 一个完整的使用成员函数指针的例子


    Screen.h

    #ifndef SCREEN_H
    #define SCREEN_H
    
    #include <string>
    
    class Screen {
    public:
        typedef std::string::size_type pos;
    
        // Action is a type that can point to a member function of Screen
        // that returns a reference to a Screen and takes no arguments
        using Action = Screen&(Screen::*)();
    
        // constructor: build screen of given size containing all blanks
        Screen(pos ht = 0, pos wd = 0): contents(ht * wd, ' '), cursor(0), 
                                        height(ht), width(wd) { }
        friend int main();
        // data is a static member that returns a pointer to member
        static const std::string Screen::*data() 
         { return &Screen::contents; }
    
        char get_cursor() const { return contents[cursor]; }
        char get() const { return contents[cursor]; }
        inline char get(pos ht, pos wd) const;
    private:
        std::string contents;
        pos cursor;
        pos height, width;
    
    public:
        // cursor movement functions 
        // beware: these functions don't check whether the operation is valid
        Screen& home() { cursor = 0; return *this; }
        Screen& forward() { ++cursor; return *this; }
        Screen& back() { --cursor; return *this; }
        Screen& up() { cursor += height; return *this; }
        Screen& down() {cursor -= height; return *this; }
    
        // specify which direction to move; enum see XREF(enums)
        enum Directions { HOME, FORWARD, BACK, UP, DOWN };
        Screen& move(Directions);
    private:
        static Action Menu[];      // function table
    };
    
    char Screen::get(pos r, pos c) const // declared as inline in the class
    {
        pos row = r * width;      // compute row location
        return contents[row + c]; // return character at the given column
    }
    
    inline
    Screen& Screen::move(Directions cm)
    {
        // run the element indexed by cm on this object
        return (this->*Menu[cm])(); // Menu[cm] points to a member function
    }
    
    #endif

    Screen.cpp

    #include "Screen.h"
    
    Screen::Action Screen::Menu[] = { &Screen::home,
                                      &Screen::forward,
                                      &Screen::back,
                                      &Screen::up,
                                      &Screen::down,
                                    };

    useScreen.cpp

    #include "Screen.h"
    
    #include <functional>
    using std::function; 
    
    #include <iostream>
    using std::cout; using std::endl;
    
    #include <string>
    using std::string;
    
    struct X {
        int foo(int i) { cout << "foo(" << i << ")" << endl; return i; }
    };
    
    void xfcn()
    {
        function<int (X*, int)> f;
        f = &X::foo;        // pointer to member
    
        X x;
        int v = f(&x, 5);
        cout << "v = " << v << endl;
    }
    
    int main ()
    {
        // pdata can point to a string member of a const (or nonconst) Screen
        const string Screen::*pdata;  // uninitialized
        pdata = &Screen::contents;    // points to the contents member
        auto pdata2 = &Screen::contents; // equivalent declaration
    
        // data() returns a pointer to the contents member of class Screen
        const string Screen::*pdata3 = Screen::data();
        
        // Screen objects
        Screen myScreen, *pScreen = &myScreen;
        const Screen cScreen, *pcScreen = &cScreen;
    
        // .* dereferences pdata to fetch the contents member 
        // from the object myScreen
        auto str = myScreen.*pdata;  // s is a string
        auto cstr = cScreen.*pdata;  // c is a const string
        
        // ->* dereferences pdata to fetch contents 
        // from the object to which pScreen points
        str = pScreen->*pdata;  
        
        // pmf is a pointer that can point to a Screen member function 
        // that takes no arguments, returns a char, and is const
        // that returns a char and takes no arguments
        auto pmf = &Screen::get_cursor;
        char (Screen::*pmf2)() const = &Screen::get; // same type as pmf
        
        pmf = &Screen::get; // which version of get deduced from type of pmf
        pmf2 = &Screen::get_cursor;
        
        Screen s;
        char c1 = s.get_cursor(); // gets character at the cursor directly
        char c2 = (s.*pmf2)();    // calls get_cursor indirectly through pmf2
        
        // call the function to which pmf points 
        // on the object to which pScreen points
        c1 = (pScreen->*pmf)();  
        
        // pmf3 points to the two-parameter version of get
        char (Screen::*pmf3)(Screen::pos, Screen::pos) const = &Screen::get;
        c1 = myScreen.get(0,0);     // call two-parameter version of get 
        c2 = (myScreen.*pmf3)(0,0); // equivalent call to get
        
        // Op is a type that can point to a member function of Screen 
        // that returns a char and takes two pos arguments
        using Op = char (Screen::*)(Screen::pos, Screen::pos) const;
        // equivalent declaration of Op using a typedef
        typedef char (Screen::*Op)(Screen::pos, Screen::pos) const;
    
        Op get = &Screen::get; // get points to the get member of Screen
        
        myScreen.move(Screen::HOME);  // invokes myScreen.home
        myScreen.move(Screen::DOWN);  // invokes myScreen.down
    
        // bind an object of type function to a pointer to member
        function<char (const Screen*)> f = &Screen::get_cursor;
    
        return 0;
    }
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  • 原文地址:https://www.cnblogs.com/wuchanming/p/3759370.html
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