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c++ - 关于C++设计模式的问题，纯虚方法调用

问题描述

在尝试来自https://en.wikipedia.org/wiki/Bridge_pattern的代码时，我遇到了虚拟方法问题。

错误消息是：

纯虚方法称为
终止调用没有活动异常
中止（核心转储）

代码是用g++ -o bridge bridge.cpp -std=c++11

为什么drawing_api_.DrawCircle()在中调用虚函数DrawingAPI？

#include <iostream>
#include <string>
#include <vector>


class DrawingAPI {
  public:
    virtual ~DrawingAPI() = default;
    virtual std::string DrawCircle(float x, float y, float radius) const = 0;
};

class DrawingAPI01 : public DrawingAPI {
  public:
    std::string DrawCircle(float x, float y, float radius) const override {
      return "API01.circle at " + std::to_string(x) + ":" + std::to_string(y) +
        " - radius: " + std::to_string(radius); 
    }
};

class DrawingAPI02 : public DrawingAPI {
  public:
    std::string DrawCircle(float x, float y, float radius) const override {
      return "API02.circle at " + std::to_string(x) + ":" + std::to_string(y) +
        " - radius: " + std::to_string(radius); 
    }
};

class Shape {
  public:
    Shape(const DrawingAPI& drawing_api) : drawing_api_(drawing_api) {}
    virtual ~Shape() = default;

    virtual std::string Draw() const = 0;
    virtual float ResizeByPercentage(const float percent) = 0;

  protected:
    const DrawingAPI& drawing_api_;
};

class CircleShape: public Shape {
  public:    
    CircleShape(float x, float y, float radius, const DrawingAPI& drawing_api)
      : Shape(drawing_api), x_(x), y_(y), radius_(radius) {}

    std::string Draw() const override {
        return drawing_api_.DrawCircle(x_, y_, radius_);
    }

    float ResizeByPercentage(const float percent) override {
      return radius_ *= (1.0f + percent/100.0f);
    }
  
  private:
    float x_, y_, radius_;
};

int main(int argc, char** argv) {
  std::vector<CircleShape> shapes {
    CircleShape{1.0f, 2.0f, 3.0f, DrawingAPI01{}},
    CircleShape{5.0f, 7.0f, 11.0f, DrawingAPI02{}}
  }; 

  for (auto& shape: shapes) {
    shape.ResizeByPercentage(2.5);
    std::cout << shape.Draw() << std::endl;
  }

  return 0;
}

标签： c++design-patterns

解决方案

发布的代码有问题——特别是，Shape该类持有一个对绘图 API 对象的引用：

class Shape {
[...]

protected:
   const DrawingAPI& drawing_api_;
};

...但是它引用的对象是一个临时对象，一旦CircleShape内部的构造函数main()返回，就会被销毁。

避免错误的一种方法是声明DrawingAPI01and对象，使其生命周期比引用它们DrawingAPI02的对象的生命周期更长，例如，更改为如下所示：CircleShapemain()

int main(int argc, char** argv) {
   DrawingAPI01 api01;   // declaring these up here keeps them valid
   DrawingAPI01 api02;   // until the end of main()

   std::vector<CircleShape> shapes {
      CircleShape{1.0f, 2.0f, 3.0f, api01},
      CircleShape{5.0f, 7.0f, 11.0f, api02}
   };

   for (auto& shape: shapes) {
      shape.ResizeByPercentage(2.5);
      std::cout << shape.Draw() << std::endl;
   }

   return 0;
}

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