我应该如何编写一个复制构造函数来初始化另一个 object？

Question

Say, as a simplified example, I have a class object House which also has an object Kitchen.

Here is the header file:

class Kitchen {
   private:
       int width;
       int height;
       int length;
   public:
       Kitchen(int width, height, length); // default constructor
};

class House {
   private:
       int houseId;
       Kitchen newKitchen;
   public:
       House(Kitchen newKitchen, int houseId); // default constructor
       House& operator=(House const& other); // copy assignment
       House(House const& other); // copy constructor
       ~House(); // destructor
};

Copying houseId works fine within the copy assignment function. But I'm getting an error referring to House::House(House const& other) { *this = other; } House::House(House const& other) { *this = other; } as the following:

error: constructor for 'House' must explicitly initialize the member 'newKitchen' which does not have a default constructor

which I'm not sure about as I thought my declaration of a default constructor covered this?

Answer 1

Firstly, your "default constructor" in Kitchen is not a default constructor, it is a user-defined constructor. It should initialize the members, and also I would re-enable copy and move behavior following the rule of five .

class Kitchen {
   private:
       int width;
       int height;
       int length;
   public:
       // Use member initialization list
       Kitchen(int _width, int _height, int _length) : width(_width), height(_height), length(_length) {}

       // Rule of 5
       Kitchen(Kitchen const&) = default;
       Kitchen& operator=(Kitchen const&) = default;
       Kitchen(Kitchen&&) = default;
       Kitchen& operator=(Kitchen&&) = default;
};

Then later your House can use this user-defined constructor in a similar way

House(Kitchen _newKitchen, int _houseId) : houseId(_houseId), newKitchen(_newKitchen) {}

Note that your Kitchen could simply be a POD aggregate type to save the trouble

class Kitchen
{
public:
    int width;
    int height;
    int length;
}

This will follow the "rule of zero" and will be default constructable, aggregate initializable, copyable, and moveable. The same would then follow for your House class as well.