在C ++中定义虚拟get和set函数是否被认为是一种好习惯?

问题描述

例如，如果我有一个简单的2级类层次结构，例如:

If I have a simple 2 level class hierarchy as, for instance, this one:

// level 1
class Spare_Part{
private:
    string name;
    double price;
public:
    Spare_Part();
    string getName() { return name; }
    double getPrice() { return price; }
    virtual int getQuantity() { return -1; }; // may also define it as pure virtual
};
//level 2
class On_hand : public Spare_Part{
private:
    int quantity;
    string location;
public:
    On_hand();
    int getQuantity(){ return quantity; }
};

我想使用指向基类"Spare_part"的指针访问类"On_hand"中的成员"quantity"，因此我将"getQuantity"功能设为虚拟的.该代码运行良好，但是，我感到我不应该具有get/set函数(即使是虚拟函数)来访问在层次结构下某个位置定义的成员.是否真的认为这是一种不良做法，应该通过例如重新设计类来避免这种做法?整个层次结构稍微复杂一些.除了现有"类别外，还有通过合同供应商提供的零件类别.前提是我无法知道供应商可以提供多少零件，这就是为什么数量"不包括在基类中.

I want to have access to the member 'quantity' in the class 'On_hand' using a pointer to the base class 'Spare_part', so I made the 'getQuantity' function a virtual one. The code works fine, however, I have a feeling that I shouldn't have a get/set function (even though a virtual one) to access a member that is defined somewhere down the hierarchy. Is this really considered a bad practice that should be avoided by, for example, redesign of the classes?The whole hierarchy is a little bit more complex. Alongside the 'On_hand' class there is class for parts available through contracted suppliers. The assumption is that I wouldn't be able to know how many parts are available through the suppliers and that is why 'quantity' is not included in the base class.

推荐答案

在这种情况下，这是不好的做法；如果在 Spare_Part 上调用 getQuantity 没有意义，则不应编译.它绝对不应该只是返回标志值.

In this case, yes, this is bad practice; if it doesn't make sense to call getQuantity on a Spare_Part, it shouldn't compile. It should definitely not just return a flag value.

可能需要重新设计您的课程.应该使用多态来为 is-a 关系建模，但您的用途主要是将更多数据标记到该类上.您应该改为使用组合，也许制作一个 Part_Store 类，其中包含一个 Spare_Part ，数量和位置.

A redesign of your classes is probably necessary. Polymorphism should be used to model an is-a relationship, but your use is mostly to tag more data onto the class. You should use composition instead, perhaps making a Part_Store class which contains a Spare_Part, the quantity and location.

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