为什么调用基本构造函数而不是带参数的构造函数（虚拟继承）？

Question

#include <iostream>

using namespace std;

class Point
{
    int x,y;
public:
    Point()
    {
        x=0;
        y=0;
    }
    Point(int x, int y)
    {
        this->x=x;
        this->y=y;
    }
    Point(Point &p)
    {
        x=p.x;
        y=p.x;
    }
    friend class Square;
};

class Square
{
    Point _point;
    int side;
public:
    Square() {cout<<"Square.\n";}
    Square(Point &p, int side_val): _point(p), side(side_val)
    {
        cout<<"Square constructor that should be used.\n";
    }
};

class Rectangle: public virtual Square
{
    int side2;
public:
    Rectangle() {}
    Rectangle(Point &p, int side_1, int side_2): Square(p,side_1), side2(side_2) {}
};

class Rhombus: public virtual Square
{
    Point opposite_point;
public:
    Rhombus() {cout<<"Rhombus. \n";}
    Rhombus(Point &p, Point &q, int side_1): Square(p, side_1), opposite_point(q)
    {
        cout<<"Rhombus constructor that should be used \n";
    }
};

class Paralellogram: public Rectangle, public Rhombus
{
public:
    Paralellogram(Point &p, Point &q, int side_1, int side_2): Rhombus(p,q,side_1),Rectangle(p,side_1,side_2)
    {
        cout<<"Parallelogram constructor that should be used\n";
    }
};

int main()
{
    Point down_left(3,5);
    Point up_right(2,6);
    int side_1=5;
    int side_2=7;
    Paralellogram par(down_left,up_right,side_1,side_2);
}

我得到的输出是：

Square
Rhombus constructor that should be used
Paralellogram constructor that should be used

我想要做的是实例化一个平行图，它结合了来自菱形和矩形的变量（它应该有一个_point、opposite_point、side、side2），我不想要他们加倍因此我使用虚拟继承。但是我打算使用的 Square 的构造函数永远不会被调用，即使是一次，而是调用基本构造函数。

我该怎么办？放弃虚拟继承？

Answer 1

在虚继承中，虚基是根据最派生的class构造的。

class Paralellogram: public Rectangle, public Rhombus
{
public:
    Paralellogram(Point &p, Point &q, int side_1, int side_2) :
        Square(), // You have implicitly that
        Rectangle(p,side_1,side_2),
        Rhombus(p,q,side_1)
    {
        cout<<"Parallelogram constructor that should be used\n";
    }
};

你可能想要

class Paralellogram: public Rectangle, public Rhombus
{
public:
    Paralellogram(Point &p, Point &q, int side_1, int side_2) :
        Square(p, side_1),
        Rectangle(p,side_1,side_2),
        Rhombus(p,q,side_1)
    {
        cout<<"Parallelogram constructor that should be used\n";
    }
};

Answer 2

如 Understanding virtual base classes and constructor calls 中所述：

There is always just one constructor call, and always of the actual, concrete class that you instantiate. It is your responsibility to endow each derived class with a constructor which calls the base classes' constructors if and as necessary, as you did in [Rectangle and Rhombus]'s constructor[s].

问问自己：如果 Rectangle 和 Rhombus 调用不同意如何初始化 Square 的唯一（自虚拟）实例怎么办？我们应该听谁的？答案是两者都不是。当前 class，此处 Parallelogram，始终负责初始化虚拟基地。并且由于这里没有指定构造函数，所以认为Parallelogram调用了Square.

的默认构造函数

为什么调用基本构造函数而不是带参数的构造函数（虚拟继承）？

Why does the base constructor get called instead of the one with parameters (virtual inheritance)?

c++

oop