c++ - 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

这个问题已经有答案了:

Calling a virtual base class's overloaded constructor (4 个回答)

已关闭 去年。

#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

我想做的是实例化一个平行四边形，它组合了菱形和矩形的变量(它应该有一个_point、opposite_point、side、side2)，我不希望它们加倍，因此我正在使用虚拟继承。但是我打算使用的 Square 构造函数永远不会被调用，甚至一次，而是调用基本构造函数。

我该怎么办？放弃虚拟继承？

Answer 1

在虚拟继承中，虚拟基是根据最底层的派生类构造的。

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";
    }
};