c++ - C++中的类和方法继承

Question

所以，我以为我明白了，但我没有...这是我的头文件 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

然后我在 shapes.cpp 中将其实现为:

#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 中我做了:

#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();
    }
    ...

我得到这个错误:

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.

我假设错误不言而喻，并查找了类似的问题，我找到了很多答案，但无法找出我代码中的错误...

Answer 1

你声明了 Shape::area 和 Shape::rotate 但你没有定义它们

一种解决方案是像这样更改 shapes.h:

class Shape {
public:
    double h;
    double w;

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

另一种解决方案是将定义添加到 shapes.cpp 中:

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

正如 juanchopanza 所指出的，另一种解决方案是使方法成为纯虚拟的(这可能是最好的):

class Shape {
public:
    double h;
    double w;

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