C++使用内部类实现一种Builder模式（属性内容不对外暴露，不需管理指针）

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假设我们有一个生产车的工厂，每辆车有如下属性：系列、颜色、载人数

头文件如下：

#ifndef TEST_H
#define TEST_H#include <string>
#include <memory>using namespace std;class Car
{
public:class Builder{public:Builder(const Builder &builderArg);Builder(const Builder &&builderArg);Builder& operator=(const Builder &builderArg);Builder& operator=(const Builder &&builderArg);Builder(const string &series);Builder& setColor(const string &color);    Builder& setSize(const int size);                    Car build();private:shared_ptr<void> carBuildData;friend class Car;};void drive();Car() = default;~Car() = default;Car(const Builder &builder);Car(const Car &carArg);Car(const Car &&carArg);Car& operator=(const Car &carArg);Car& operator=(const Car &&carArg);private:shared_ptr<void> carData;
};#endif // !TEST_H

用shared_ptr<void>的目的在于希望内部数据隐藏，对外不体现。

实现如下：

#include "test.h"
#include <iostream>
#include <functional>struct CarData_t{string series;          //  系列string color = "RED";   //  颜色int size = 5;           //  大小（载人数）CarData_t(const string &seriesArg) : series(seriesArg) {}
};Car::Builder::Builder(const Car::Builder &builderArg)
{carBuildData = builderArg.carBuildData;
}Car::Builder::Builder(const Car::Builder &&builderArg)
{carBuildData = builderArg.carBuildData;
}Car::Builder& Car::Builder::operator=(const Car::Builder &builderArg)
{carBuildData = builderArg.carBuildData;return *this;
}Car::Builder& Car::Builder::operator=(const Car::Builder &&builderArg)
{carBuildData = builderArg.carBuildData;return *this;
}Car::Car(const Car &carArg)
{carData = carArg.carData;
}Car::Car(const Car &&carArg)
{carData = carArg.carData;
}Car& Car::operator=(const Car &carArg)
{carData = carArg.carData;return *this;
}Car& Car::operator=(const Car &&carArg)
{carData = carArg.carData;return *this;
}Car::Builder::Builder(const string &series)
{CarData_t testdata(series);carBuildData = make_shared<CarData_t>(testdata);
}Car Car::Builder::build()
{Car test(*this);// 这里直接返回， 函数返回值Car未加&，会触发移动构造函数，如果返回值为Car&，会触发拷贝构造函数return test;
}//  减少重复造轮子，每个set函数都要判断指针是否为空，用回调可以避免重复判断
void operateTestDataArg(void* testDataVoidPtr, function<void(CarData_t*)> callback)
{if (testDataVoidPtr == nullptr) { return; }auto testDataPtr = reinterpret_cast<CarData_t*>(testDataVoidPtr);if (testDataPtr == nullptr) {return;}callback(testDataPtr);
}void Car::drive()
{operateTestDataArg(carData.get(), [&](CarData_t* testDataPtr) {cout << "series : " << testDataPtr->series << "    ";cout << "color : " << testDataPtr->color << "    ";cout << "size : " << testDataPtr->size << endl;});
}Car::Car(const Builder &builder)
{operateTestDataArg(builder.carBuildData.get(), [&](CarData_t* buildDataPtr) {carData = make_shared<CarData_t>(*buildDataPtr);});
}Car::Builder& Car::Builder::setColor(const string &color)
{operateTestDataArg(carBuildData.get(), [&](CarData_t* buildDataPtr) {buildDataPtr->color = color;});return *this;
}Car::Builder& Car::Builder::setSize(const int size)
{operateTestDataArg(carBuildData.get(), [&](CarData_t* buildDataPtr) {buildDataPtr->size = size;});return *this;
}

下面来使用：

int main()
{auto benz = Car::Builder("Benz").build();cout << "1 : Benz              "; benz.drive();//  设置benz为公司用车，由于实现了赋值运算符函数，使用了shared_ptr, benz析构后CompanyCar也能继续使用，CompanyCar和benz中的carData指向同一片空间，对于shared_ptr来说，指向该空间的指针均析构了后该空间才会自动释放auto CompanyCar = benz;cout << "2 : CompanyCar        "; CompanyCar.drive();auto HondaBaseBuilder = Car::Builder("Honda");Car HondaDefault(HondaBaseBuilder);cout << "3 : HondaDefault      "; HondaDefault.drive();auto HondaBlue = HondaBaseBuilder.setColor("Blue").setSize(5).build();cout << "4 : HondaBlue         "; HondaBlue.drive();auto HondaSmallBlack = HondaBaseBuilder.setColor("Black").setSize(2).build();cout << "5 : HondaSmallBlack   "; HondaSmallBlack.drive();//  ps：HondaBaseBuilder属性一直在变 此时初始化car已经不是默认值了，可以自己加入reset函数扩展 或者重新设置属性，此处功能为保证为Honda系列，不用重复Builder("Honda")Car HondaCurrent(HondaBaseBuilder);cout << "5 : HondaCurrent      "; HondaCurrent.drive();return 0;
}

运行结果：

1 : Benz              series : Benz     color : RED    size : 5
2 : CompanyCar        series : Benz     color : RED    size : 5
3 : HondaDefault      series : Honda    color : RED    size : 5
4 : HondaBlue         series : Honda    color : Blue   size : 5
5 : HondaSmallBlack   series : Honda    color : Black  size : 2
5 : HondaCurrent      series : Honda    color : Black  size : 2