C ++访问嵌套类的私有成员 [英] C++ Access private member of nested class

问题描述

标题可能有点误导。我有以下问题：我有一个由树叶和内部节点组成的树。用户应该能够在树中存储任何信息和，树具有一些方法，它们获得一组用户定义的值，并且需要在常量时间（不是摊销）访问相应的树叶。

The title might be a bit misleading. I have the following problem: I have a tree consisting of leaves and internal nodes. The user should be able to store any information in the leaves and the tree has some methods which get a set of user-defined values and need to access the corresponding leaves in constant time (not amortized).

我想出了以下想法，但它不工作，因为不幸的是我无法访问嵌套类的私有成员：用户创建树和也为每个叶子一个 UserElement 的实例，它包含相应叶的user_defined值。一旦像doSomethingWithTheTree（list>）一样的方法被调用并且树被构建，树创建相应的叶并将它保存在私有字段 leaf 中。每当用户想要调用具有对应于其user_defined值的一些叶的方法时，他/她只需通过给出相应的 UserElement 来调用该方法，并且树可以在固定时间检索相应的树叶。

I came up with the following idea but it does not work because unfortunately I cannot access private members of a nested class: The user creates the tree and also for each leaf an instance of UserElement which contains the user_defined value for the corresponding leaf. Once a method like doSomethingWithTheTree(list>) is called and the tree is built, the tree creates the corresponding leaves and saves it in the private field leaf. Whenever the user wants to call a method with some of the leaves corresponding to its user_defined values, he/she just has to call the method by giving the corresponding UserElements and the tree can retrieve the corresponding leaves in constant time.

class Tree {
    public:
        template <typename T>
        class UserElement {
            private:
                T user_value;
                tree_node* leaf; // this has to be private for
                                 // everyone outside the class `Tree`
            public:
                T getInf() {
                    return user_value;
                }
                void setInf(T i) {
                    user_value = i;
                }
        };

        void doSomethingWithTheTree(list<UserElement<T>> elements) {
            ...
            // I want to be able to access elem.leaf for all elements
        }
}

推荐答案

一个嵌套类（在另一个类中声明），而不是（继承自其超类）。

Technically, that's a nested class (declared within another class), not a subclass (which inherits from its superclass).

您可以允许Tree类通过将其作为朋友访问其私有：

You can allow the Tree class to access its privates by making it a friend:

class UserElement {
    friend class Tree;
    // ...
};

或者，为了更好的封装，你可以限制只访问需要它的成员函数，虽然由于需要按正确的顺序声明事物，它有点乱。

or, for better encapsulation, you could restrict access only to the member function(s) that need it, although it gets a bit messy due to the need to declare things in the right order:

class Tree {
public:
    // Declare this so we can declare the function
    template <typename T> class UserElement;

    // Declare this before defining `UserElement` so we can use it
    // in the friend declaration
    template <typename T>
    void doSomethingWithTheTree(list<UserElement<T>> elements) {
        elements.front().leaf;
    }

    template <typename T>
    class UserElement {
        // Finally, we can declare it a friend.
        friend void Tree::doSomethingWithTheTree<T>(list<UserElement<T>>);
        // ...
    };
};

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