派生類從兩個相同的派生類

Question

exercise.h是如下

#ifndef EXERCISE_H_ 
#define EXERCISE_H_ 

// ROOT namespace 
namespace root{ 

// USHORT definition 
typedef unsigned short ushort; 

// PI DEFINITION 
const double PI = 3.141592; 

class shape { 
    double height; 
    double width; 
public: 
    shape(double h = 1, double w = 1); 
    virtual ~shape(){} 
    double getHeight() const; 
    double getWidth() const; 
    virtual double area() const = 0; 
}; 

class rectangle : virtual public shape{ 
public: 
    rectangle(double height = 1, double width = 1); 
    double area() const; 
}; 

class triangle : virtual public shape { 
public: 
    triangle(double h = 1, double w = 1); 
    double area() const; 
}; 

class someShape : public rectangle, public triangle{ 
public: 
    someShape(double rh = 1, double rw = 1, double th = 2, double tw = 2); 
    double area() const; 
    double trySomething() const; 
}; 




} // NAMESPACE 

#endif /* EXERCISE_H_ */ 

exercise.cpp是這樣

#include <iostream> 
#include <cmath> 
#include "exercise.h" 
using std::cout; 
using std::cin; 
using std::endl; 
using root::ushort; 

// BEGIN SHAPE CLASS 
root::shape::shape(double h, double w) : height(h), width(w){ 

} 

double root::shape::getHeight() const{ 
    return this->height; 
} 

double root::shape::getWidth() const{ 
    return this->width; 
} 
// END SHAPE CLASS 


// BEGIN RECTANGLE CLASS 
root::rectangle::rectangle(double h, double w) : shape(h,w){ 

} 

double root::rectangle::area() const{ 
    return this->getHeight() * this->getWidth(); 
} 

// END RECTANGLE CLASS 
// BEGIN TRIANGNLE CLASS 
root::triangle::triangle(double h, double w) : shape(h,w){ 

} 

double root::triangle::area() const{ 
    return this->getHeight() * this->getWidth()/2; 
} 
// END TRIANGLE CLASS 



root::someShape::someShape(double rh, double rw, double th, double tw) : rectangle(rh,rw), triangle(th,tw){ 

} 

double root::someShape::area() const { 
    return rectangle::area(); 
} 

double root::someShape::trySomething() const{ 
    return triangle::getHeight() * rectangle::getWidth(); 
} 

和在主

#include <iostream> 
#include "exercise.h" 
using std::cout; 
using std::cin; 
using std::endl; 


int main(){ 
    root::shape *ptrShape; 
    ptrShape = new root::someShape(3,2,4,3); 
    cout << "shape area: " << ptrShape->area() << endl; 

    delete ptrShape; 

} 

當我創建一個someShape對象我只獲得了默認值。雖然我啓動派生類。還有另一件事。事實上，我們分別導出矩形和三角形，並從這些對象中派生出另一個對象。編譯器如何決定使用哪個area（）函數？

Answer 1

這裏的問題是，你有一個虛擬繼承從shape到rectangle和triangle，所以rectangle和triangle構造函數不初始化shape。你someShape構造方法等效於：

root::someShape::someShape(double rh, double rw, double th, double tw) : 
    rectangle(rh,rw), triangle(th,tw), shape() { 

} 

這就是爲什麼你會得到默認值1和1。此外，請注意，由於您具有虛擬繼承，因此無法爲矩形和三角形存儲不同的height和width。你的構造應該是這樣的：

root::someShape::someShape(double h, double w) : 
    rectangle(h, w), triangle(h, w), shape(h, w) { 

} 

或者，如果你不希望有shape一個實例，你應該刪除的rectangle和triangle虛擬繼承。請參閱c++ virtual inheritance。

Answer 2

問題是你正在做虛擬繼承，即矩形和三角形使用virtual關鍵字繼承形狀。因此，在派生類中共享一個形狀類的單個實例。因此，從矩形和三角形調用的構造函數被編譯器完全跳過。

檢查此瞭解更多詳情： http://www.cprogramming.com/tutorial/virtual_inheritance.html https://isocpp.org/wiki/faq/multiple-inheritance