C ++中的類和方法繼承

Question

所以，我以為我明白了，但是我沒有...這是我的頭文件shapes.h ：

#ifndef __shapes__
#define __shapes__

class Shape {

public:
    double h;
    double w;

    virtual double area(void);

    virtual void rotate(void);
};

class Rectangle : public Shape {
public:
    Rectangle(double h, double w);

    double area(void);

    void rotate(void);

private:
    double h;
    double w;
};

#endif

然后在shapes.cpp其實現為：

#include "shapes.h"
#include <cmath>
#include <math.h>

/*
 * Rectangle methods
 */
Rectangle::Rectangle(double height, double width) {
    this->h = height;
    this->w = width;
}

double Rectangle::area() {
    return this->h * this->w;
}

void Rectangle::rotate() {
    double temp = this->h;

    this->h = this->w;
    this->w = temp;
}

在我的main.cpp我做了：

#include <vector>
#include "shapes.h"

using namespace std;

int main(void){

    vector<Shape *> shapes;

    Rectangle u(2,5);
    shapes.push_back(&u);
    Rectangle v(3, 4);
    shapes.push_back(&v);

    double area = 0;
    for(Shape * p : shapes){
        area += p->area();
    }
    ...

我得到這個錯誤：

Undefined symbols for architecture x86_64:
    "typeinfo for Shape", referenced from:
      typeinfo for Rectangle in shapes-12a86a.o
  "vtable for Shape", referenced from:
      Shape::Shape() in shapes-12a86a.o
  NOTE: a missing vtable usually means the first non-inline virtual member function has no definition.

我以為該錯誤說明了一切，並尋找了類似的問題，我找到了很多答案，但無法在我的代碼中找出錯誤...

Answer 1

您聲明了Shape::area和Shape::rotate但未定義它們

一種解決方案是更改shape.h，如下所示：

class Shape {
public:
    double h;
    double w;

    virtual double area(void) { return 0; }
    virtual void rotate(void) {}
};

另一種解決方案是改為將定義添加到shapes.cpp：

double Shape::area() { return 0; }
void Shape::rotate() {}

正如juanchopanza所指出的，另一種解決方案是使方法成為純虛擬方法（這可能是最好的）：

class Shape {
public:
    double h;
    double w;

    virtual double area(void) = 0;
    virtual void rotate(void) = 0;
};