私有继承(has a)

1>访问基类方法　用作用域解析操作符::　　
ep:
valarray<double>::size();
//*************************************************//
2>访问基类对象(对象数据成员)　用强制转换
ep:
const string & Student::name() const
{
   return (const string &) *this;
}
//*************************************************//
3>用using重新定义访问权限
ep:
class Student : private std::string, private std::valarray<double>
{
private:
　　…
public:
    using std::valarray<double>::min;
}

//valarray<double>的min方法　　就可以直接被　student的对象使用

用私有继承来实现has-a

有可以重定义虚函数　　　可以内部使用保护成员　
////////////////////////////////////////////////////////////

 

////////////////////////////////////////////////////////////
////////////////////main.cpp////////////////////////////////
// use_stui.cpp -- using a class with private inheritance
// compile with studenti.cpp
#include <iostream>
#include "studentc.h"
using std::cin;
using std::cout;
using std::endl;

void set(Student & sa, int n);

const int pupils = 3;
const int quizzes = 5;

int main()
{
    Student ada[pupils] =
 {Student(quizzes), Student(quizzes), Student(quizzes)};
 
    int i;
    for (i = 0; i < pupils; i++)
        set(ada[i], quizzes);
    cout << "\nStudent List:\n";
    for (i = 0; i < pupils; ++i)
        cout << ada[i].Name() << endl;
    cout << "\nResults:";
    for (i = 0; i < pupils; i++)
    {
        cout << endl << ada[i];
        cout << "average: " << ada[i].Average() << endl;
    }
    cout << "Done.\n";
    return 0;
}

void set(Student & sa, int n)
{
    cout << "Please enter the student's name: ";
    getline(cin, sa);
    cout << "Please enter " << n << " quiz scores:\n";
    for (int i = 0; i < n; i++)
        cin >> sa[i];
    while (cin.get() != '\n')
        continue;
}

///////////////////////////////////////////////////////////////
/////////////////////////studentc.h////////////////////////////
// studenti.h -- defining a Student class using private inheritance
#ifndef STUDENTC_H_
#define STUDENTC_H_

#include <iostream>
#include <valarray>
#include <string>  
class Student : private std::string, private std::valarray<double>
{  
private:
    typedef std::valarray<double> ArrayDb;
    // private method for scores output
    std::ostream & arr_out(std::ostream & os) const;
public:
    Student() : std::string("Null Student"), ArrayDb() {}
    Student(const std::string & s)
            : std::string(s), ArrayDb() {}
    Student(int n) : std::string("Nully"), ArrayDb(n) {}
    Student(const std::string & s, int n)
            : std::string(s), ArrayDb(n) {}
    Student(const std::string & s, const ArrayDb & a)
            : std::string(s), ArrayDb(a) {}
    Student(const char * str, const double * pd, int n)
            : std::string(str), ArrayDb(pd, n) {}
    ~Student() {}
    double Average() const;
    double & operator[](int i);
    double operator[](int i) const;
    const std::string & Name() const;
// friends
    // input
    friend std::istream & operator>>(std::istream & is,
                                     Student & stu);  // 1 word
    friend std::istream & getline(std::istream & is,
                                  Student & stu);     // 1 line
    // output
    friend std::ostream & operator<<(std::ostream & os,
                                     const Student & stu);
};

#endif

////////////////////////////////////////////////////////////
//////////////studentc.cpp//////////////////////////////////
// studenti.cpp -- Student class using private inheritance
#include "studentc.h"
using std::ostream;
using std::endl;
using std::istream;
using std::string;
using namespace std;

// public methods
double Student::Average() const
{
    if (ArrayDb::size() > 0)
        return ArrayDb::sum()/ArrayDb::size(); 
    else
        return 0;
}

const string & Student::Name() const
{
    return (const string &) *this;
}

double & Student::operator[](int i)
{
    return ArrayDb::operator[](i);         // use ArrayDb::operator[]()
}

double Student::operator[](int i) const
{
    return ArrayDb::operator[](i);
}

// private method
ostream & Student::arr_out(ostream & os) const
{
    int i;
    int lim = ArrayDb::size();
    if (lim > 0)
    {
        for (i = 0; i < lim; i++)
        {
            os << ArrayDb::operator[](i) << " ";
            if (i % 5 == 4)
                os << endl;
        }
        if (i % 5 != 0)
            os << endl;
    }
    else
        os << " empty array ";
    return os;
}

// friends
// use String version of operator>>()
istream & operator>>(istream & is, Student & stu)
{
    is >> (string &)stu;
    return is;
}

// use string friend getline(ostream &, const string &)
istream & getline(istream & is, Student & stu)
{
    getline(is, (string &)stu);
    return is;
}

// use string version of operator<<()
ostream & operator<<(ostream & os, const Student & stu)
{
    os << "Scores for " << (const string &) stu  << ":\n";
    stu.arr_out(os);  // use private method for scores
    return os;
}