設計模式（C++）——工廠模式

工廠模式

工廠模式提供了一種創建對象的方式。它將創建的過程隱藏了起來，調用者只負責獲取，不關心創建的細節。

// Shape.h
#pragma once
// 抽象類
class BaseShape
{
public:
    BaseShape(int x, int y) : x_(x), y_(y);
    void draw() = 0;
private:
    int x_, y_;
}

//ShapeImpl.h
#pragma once
#include "Shape.h"

#include <iostream>

class Circle final : public BaseShape
{
public:
    Circle(int x, int y, int r) : BaseShape(x, y), r_(r) {};
    virtual void draw() override
    {
        std::cout << "This is a circle" << std::endl;
    }
private:
    int r_;
}

class Square final : public BaseShape
{
public:
    Square(int x, int y, int l) : BaseShape(x, y), l_(l) {};
    virtual void draw() override
    {
        std::cout << "This is a square" << std::endl;
    }
private:
    int l_;
}
class Rectangle final : public BaseShape
{
public:
    Rectangle (int x, int y, int l, int w) : BaseShape(x, y), l_(l), w_(w) {};
    virtual void draw() override
    {
        std::cout << "This is a rectangle " << std::endl;
    }
private:
    int l_, w_;
}

// ShapeFactory.h
#pragma once
#include "Shape.h"

enum class ShapeType
{
    CIRCLE,
    SQUARE,
    RECTANGLE
}

//工廠類
class ShapeFactory
{
public:
    BaseShape* getShape(ShapeType t, int x, int y);
}

// ShapeFactory.cpp
#include "ShapeFactory.h"
#include "ShapeImpl.h"
BaseShape* ShapeFactory::getShape(ShapeType t, int x, int y)
{
    switch(t)
    {
    case ShapeType::CIRCLE:
    {
        return new Circle(x, y, 1);
    }
    case ShapeType::SQUARE:
    {
        return new Square(x, y, 1);
    }
    case ShapeType::RECTANGLE:
    {
        return new Rectanngle(x, y, 2, 3);
    }
    default:
    {
        return nullptr;
    }
}

//main.cpp
#include "ShapeFactory.h"

int main(void)
{
    ShapeFactory f;
    BaseShape* ptr = f.getShape(ShapeType::CIRCLE, 0, 0);
    ptr->draw();
    delete ptr;
    // print: This is a circle

    ptr = f.getShape(ShapeType::SQUARE, 0, 0);
    ptr->draw();
    delete ptr;
    // print: This is a square
    
    Shape* ptr = f.getShape(ShapeType::RECTANGLE, 0, 0);
    ptr->draw();
    delete ptr;
    // print: This is a rectangle
    
    return 0;
}

可以看到，當外界調用時，它只看到了兩個類：BaseShape和ShapeFactory。對於ShapeFactory如何構造BaseShape，構造的圓半徑是多少，構造的矩形長寬是多少並不關心。此時就可以使用工廠模式。
該類和建造者類有一定的相似，具體我會在建造者類裏說明。