C++ 設計模式：建造者模式完整解析與 Director 控制下分步建構實作

建造者模式將複雜物件建構與其表示分離，使相同建構過程可建立不同表示。本文以「建造小人」六步驟（頭、身、左手、右手、左腳、右腳）為例，詳解 Director 控制流程、Builder 介面、具體建造者與 C++ 實作。

核心角色

• Product（產品）：複雜物件（如 Man），含多個組成部分。
• Builder（抽象建造者）：宣告建構各部件介面（BuildHead 等）與 GetResult()。
• ConcreteBuilder：實作具體建構邏輯，維護半成品狀態。
• Director（指揮者）：定義建構步驟順序，協調 Builder 完成產品。

UML 類別圖與時序圖

C++ 完整實作

#include <iostream>
using namespace std;

enum ManType { kFatMan, kThinMan, kNormal };

class Man {
    ManType m_type;
public:
    void SetHead(ManType t) { m_type = t; }
    void SetBody(ManType t) { m_type = t; }
    void SetLeftHand(ManType t) { m_type = t; }
    void SetRightHand(ManType t) { m_type = t; }
    void SetLeftFoot(ManType t) { m_type = t; }
    void SetRightFoot(ManType t) { m_type = t; }
    void ShowMan() {
        switch (m_type) {
            case kFatMan: cout << "I'm a fat man" << endl; break;
            case kThinMan: cout << "I'm a thin man" << endl; break;
            default: cout << "I'm a normal man" << endl;
        }
    }
};

class Builder {
public:
    virtual ~Builder() {}
    virtual void BuildHead() = 0;
    virtual void BuildBody() = 0;
    virtual void BuildLeftHand() = 0;
    virtual void BuildRightHand() = 0;
    virtual void BuildLeftFoot() = 0;
    virtual void BuildRightFoot() = 0;
    virtual Man* GetMan() = 0;
};

class FatManBuilder : public Builder {
    Man *m_man;
public:
    FatManBuilder() : m_man(new Man) {}
    ~FatManBuilder() { delete m_man; }
    void BuildHead() override { m_man->SetHead(kFatMan); }
    void BuildBody() override { m_man->SetBody(kFatMan); }
    void BuildLeftHand() override { m_man->SetLeftHand(kFatMan); }
    void BuildRightHand() override { m_man->SetRightHand(kFatMan); }
    void BuildLeftFoot() override { m_man->SetLeftFoot(kFatMan); }
    void BuildRightFoot() override { m_man->SetRightFoot(kFatMan); }
    Man* GetMan() override { return m_man; }
};

class ThinManBuilder : public Builder {
    Man *m_man;
public:
    ThinManBuilder() : m_man(new Man) {}
    ~ThinManBuilder() { delete m_man; }
    void BuildHead() override { m_man->SetHead(kThinMan); }
    void BuildBody() override { m_man->SetBody(kThinMan); }
    void BuildLeftHand() override { m_man->SetLeftHand(kThinMan); }
    void BuildRightHand() override { m_man->SetRightHand(kThinMan); }
    void BuildLeftFoot() override { m_man->SetLeftFoot(kThinMan); }
    void BuildRightFoot() override { m_man->SetRightFoot(kThinMan); }
    Man* GetMan() override { return m_man; }
};

class Director {
    Builder *m_builder;
public:
    Director(Builder *b) : m_builder(b) {}
    void CreateMan() {
        m_builder->BuildHead();
        m_builder->BuildBody();
        m_builder->BuildLeftHand();
        m_builder->BuildRightHand();
        m_builder->BuildLeftFoot();
        m_builder->BuildRightFoot();
    }
};

int main() {
    Builder *builder = new FatManBuilder();
    Director director(builder);
    director.CreateMan();
    Man *man = builder->GetMan();
    man->ShowMan();  // I'm a fat man
    delete man;
    delete builder;
}

建造者 vs 抽象工廠

現代 C++ 改良建議

• 使用 std::unique_ptr<Man> 管理產品生命週期，GetMan() 回傳 unique_ptr（移動語意）。
• Builder 介面改為 virtual unique_ptr<Man> Build() 內部封裝所有步驟，簡化 Director。
• 流式介面：builder->BuildHead().BuildBody()... 回傳 *this，支援鏈式呼叫。

總結

建造者模式將「建構演算法」（Director）與「具體表示」（ConcreteBuilder）分離，支援同一流程產出不同產品。適合建構過程固定、部件組合變化大的複雜物件場景。