类-友元函数

友元函数再造

• 分为友元类和友元函数，关键字为friend。具体特性见代码实例

#ifndef SALAS_DATE
#define SALAS_DATE
#include <iostream>
#include <string>

class Screen;
class Window_mgr
{
public:
    using ScreenIndex = std::vector<Screen>::size_type;
    void clear(ScreenIndex i);
    Window_mgr() = default;
    //////////////////////////////////////////////////////////////////////////
    //1>f:
owworkcomec++primerclasstest.cppclass.h(37): error C2027: 使用了未定义类型“Window_mgr”
    //1 > f:
owworkcomec++primerclasstest.cppclass.h(36) : 参见“Window_mgr”的声明
    //1 > f:
owworkcomec++primerclasstest.cppclass.h(37) : error C2061 : 语法错误 : 标识符“ScreenIndex”
    //1 > Class.cpp                                                       //错误竟然无法避免；我竟无言以对需要放在同一文件里面
    //////////////////////////////////////////////////////////////////////////
private:
    std::vector<Screen>screens{};// Screen(24, 80, ' ')在这里不能放在这里会有语法错误，静态成员初始化须在构造函数里面初始化

};
class Person
{
    private:
        mutable int a;
//      static constexpr double data_db = 1;
        std::string Pername;    
        std::string PerAddr;
    public: 
        Person() = default;
        Person(const std::string setname, const std::string setaddr) :Pername(setname), PerAddr(setaddr) {}
        std::string getname() const { return Pername; }
        std::string getAddr() const { return  this ->PerAddr; }//隐式this显式替换
        inline std::string setPerName(std::string setname) { Pername = setname; }
        inline void showPerDate() { std::cout << Pername << PerAddr; }
        friend int readline(const std::string name);
        friend std::istream &read(std::istream&, Person&);
        friend std::ostream &write(std::ostream&, Person&);//友元类和友元函数会破坏类的封装性;
};
    int readline(const std::string name);
    std::istream &read(std::istream&, Person&);
    std::ostream &write(std::ostream&, Person&);
class Perfriend
{
private:
    int age;
    int height;
    int money;
public:
    Perfriend(int Age, int Height, int Mon) :age(Age), money(age) {}//虽然不会有什么警告信息出现但是还是会出现一些错误型的错误

};


class Screen
{
    friend class Window_mgr;
    friend void Window_mgr::clear(ScreenIndex i);
    public:

        using pos = std::string::size_type;//类型别名
        typedef std::string::size_type pos;
        Screen&set(char);
        Screen&set(pos, pos, char);
        Screen() = default;
        Screen(pos ht, pos wd, char c) :height(ht) ,weight(wd), contents(ht *wd, c) {}//读取光标处的字符
        char get()const { return contents[cursor]; }    //隐式内联
        inline char get(pos ht, pos wd)const;           //显式内联
        Screen &move(pos r, pos s);                     //能在之后被设为内联函数
        Screen &display(std::ostream &os) 
        {
            do_display(os);
            return *this;
        }
        const Screen &display(std::ostream &os)const
        {
            do_display(os);
            return *this;
        }
    private:
        pos cursor = 0;
        pos height = 0, weight = 0;
        std::string contents;
        void do_display(std::ostream &os)const
        {
            os << contents;
        }
};
typedef double Money;
class Accoount
{
public:
    Money blance() { return bal;}
private:
    //typedef double Money;//错误，不能重新定义Money。虽然在vs2013中编译通过，但是韩式不够严谨的代码。
    Money bal;
}; 
//int weight;
//class Glass
//{
//public:
//  typedef std::string :: size_type poos;
//  void setheight(poos);
//  poos weight = 0;
//
//
//};
//Glass::poos verify(Glass::poos);
//void Glass::setheight(poos var)
//{
//  //var 参数
//  //
//  this->weight = verify(var);
//};



//////////////////////////////////////////////////////////////////////////
//委托构造函数
//////////////////////////////////////////////////////////////////////////
class Sales_date
{
public:
    Sales_date(std::string s,unsigned cnt,double pries):
        bookNo(s), unit_sold(cnt), revenue(cnt*pries) { }
    Sales_date() :Sales_date(" ", 0, 0);
    Sales_date(std::string s) :Sales_date(s, 0, 0);
    Sales_date(std::istream &is) :Sales_date() { read(is, *this); }//委托构造函数，在已有构造函数的基础上使用此构造函数，自己本身也算作一个构造函数u
private:
    std::string bookNo;
    unsigned unit_sold;
    double revenue;
};
#endif

-.测试用例

#include "Class.h"
#include <iostream>
using namespace std;
inline Screen &Screen::move(pos r, pos s)
{
    pos row = r*weight;
    cursor = row + s;
    return *this;

}
char Screen::get(pos r, pos c)const
{
    pos row = r*weight;
    return contents[cursor + c];
}




int main(int argc, char **argv)
{
    Screen myscreen(5, 5, ' ');
    Screen::pos ht = 24, wd = 80;
    Screen scr(ht, wd, ' ');
    Screen *p = &scr;
    char c = scr.get();
    c = p->get();
    myscreen.move(4, 0).get();
    system("pause");
}