Question

访问引用的类实例的私有成员的适当方法是什么？例如，从initialNumberOfColumns的实例访问Cookie？然后在另一个类成员函数中使用该值，例如在引用访问者检索Game::Game()类的成员之后Cookie。

first.h：

class Cookie {
    public:
        Cookie();
        ~Cookie();
        void takeABite (int column, int row);
        int getNumberOfRows();
        int getNumberOfRows(int colNum);
        int getNumberOfColumns();
        int getNumberOfColumns(int rowNum);
        void display (std::ostream& output);

    private:
        int initialNumberOfRows;
        int numberOfRows;
        int numberOfColumns
        int* cookie;
};

second.h：

class Game {
    public:
        Game();
        bool gameEnded();
        bool biteIsLegal (int column, int row);
        Cookie& getCookie();

    private:
        Cookie cookie;
};

second.cpp是我遇到困难的地方。我知道我需要使用Cookie& Game::getCookie()，但我不知道如何返回Cookie类的私有成员，以便可以在下面的成员函数Game()中访问它们。：

Cookie& Game::getCookie() {
    return //not sure how to access;
}

Game::Game() {
    initialNumberOfRows = numberOfRows = 4 + rand() % (MAXROWS - 4);
    numberOfColumns = numberOfRows;

    while (numberOfColumns == numberOfRows) {
        numberOfColumns = 4 + rand() % (MAXROWS - 4);
    }

    cout << "The cookie has " << numberOfRows << " rows of " 
         << numberOfColumns << " columns" << endl;

    for (int row = 0; row < numberOfRows; ++row) {
        cookie[row] = numberOfColumns;
    }
}

Answer 1

Game类有一个cookie成员。这就是Game::getCookie()方法应返回的内容：

Cookie& Game::getCookie() {
    return cookie;
}

现在，在Game构造函数内，您可以直接访问cookie成员，因此您无需使用getCookie()来访问它。并且Cookie类具有读取您尝试使用的大多数值的公共方法，但它不提供对设置这些成员值的任何访问权限，或完全访问其私人 initialNumberOfRows成员。

您在Game构造函数中尝试执行的操作应该在Cookie构造函数中完成：

Cookie::Cookie() {
    initialNumberOfRows = numberOfRows = 4 + rand() % (MAXROWS - 4);

    numberOfColumns = numberOfRows;
    while (numberOfColumns == numberOfRows) {
        numberOfColumns = 4 + rand() % (MAXROWS - 4);
    }

    cookie = new int[numberofRows];

    for (int row = 0; row < numberOfRows; ++row) {
        cookie[row] = numberOfColumns;
    }
}

然后Game构造函数可以根据需要记录值：

Game::Game() {
    cout << "The cookie has " << cookie.getNumberOfRows() << " rows of " 
         << cookie.getNumberOfColumns() << " columns" << endl;
}

现在，正如所说，Cookie类违反了Rule of Three。它需要实现复制构造函数和复制赋值运算符以确保其int *cookie字段的完整性：

class Cookie {
public:
    Cookie();
    Cookie(const Cookie &src);
    ~Cookie();

    Cookie& operator=(const Cookie &rhs);
    void swap(Cookie &other);

    ...

private:
    int initialNumberOfRows;
    int numberOfRows;
    int numberOfColumns
    int* cookie;
};

#include <algorithm>

Cookie::Cookie() {
    initialNumberOfRows = numberOfRows = 4 + rand() % (MAXROWS - 4);

    numberOfColumns = numberOfRows;
    while (numberOfColumns == numberOfRows) {
        numberOfColumns = 4 + rand() % (MAXROWS - 4);
    }

    cookie = new int[numberOfRows];
    std::fill(cookie, cookie + numberOfRows, numberOfColumns);
}

Cookie::Cookie(const Cookie &src) :
    initialNumberOfRows(src.initialNumberOfRows),
    numberOfRows(src.numberOfRows),
    numberOfColumns(src.numberOfColumns),
    cookie(new int[numberOfRows])
{
    std::copy(src.cookie, src.cookie + numberOfRows, cookie);
}

Cookie::~Cookie()
{
    delete[] cookie;
}

Cookie& Cookie::operator=(const Cookie &rhs)
{
    if (this != &rhs)
        Cookie(rhs).swap(*this);
    return *this;
}

void Cookie::swap(Cookie &other)
{
    std::swap(initialNumberOfRows, other.initialNumberOfRows);
    std::swap(numberOfRows, other.numberOfRows);
    std::swap(numberOfColumns, other.numberOfColumns);
    std::swap(cookie, other.cookie);
}

否则，请将int *cookie成员改为std::vector，让编译器和STL为您处理内存管理的辛苦工作：

#include <vector>

class Cookie {
public:
    Cookie();

    ...

private:
    int initialNumberOfRows;
    int numberOfRows;
    int numberOfColumns
    std::vector<int> cookie;
};

Cookie::Cookie() {
    initialNumberOfRows = numberOfRows = 4 + rand() % (MAXROWS - 4);

    numberOfColumns = numberOfRows;
    while (numberOfColumns == numberOfRows) {
        numberOfColumns = 4 + rand() % (MAXROWS - 4);
    }

    cookie.resize(numberOfRows);
    std::fill(cookie.begin(), cookie.end(), numberOfColumns);
}

通过引用访问器访问私有成员

1 个答案: