【C++设计模式】第四篇：建造者模式（Builder）

注意：复现代码时，确保 VS2022 使用 C++17/20 标准以支持现代特性。

分步骤构造复杂对象，实现灵活装配

1. 模式定义与用途

核心目标：将复杂对象的构建过程分离，使得同样的构建步骤可以创建不同的表示形式。
常见场景：

创建包含多个组件的复杂对象（如游戏角色、文档格式）
需要逐步构造对象，并支持不同配置选项
避免构造函数参数爆炸（尤其是可选参数众多时）

2. 模式结构解析

在这里插入图片描述

Builder：定义构造步骤的抽象接口（如 buildHead(), buildBody()）
ConcreteBuilder：实现具体构造逻辑，提供获取结果的接口
Director：控制构造流程（可选，可让客户端直接操作Builder）
Product：最终构造的复杂对象

3. 现代 C++ 实现示例：游戏角色构造

3.1 基础实现

#include <iostream>
#include <memory>
#include <string>// 产品：游戏角色
class GameCharacter {
public:void setHead(const std::string& head) { head_ = head; }void setBody(const std::string& body) { body_ = body; }void setWeapon(const std::string& weapon) { weapon_ = weapon; }void describe() const {std::cout << "Character: " << head_ << ", " << body_ << ", wielding " << weapon_ << "\n";}private:std::string head_;std::string body_;std::string weapon_;
};// 抽象建造者
class CharacterBuilder {
public:virtual ~CharacterBuilder() = default;virtual void buildHead() = 0;virtual void buildBody() = 0;virtual void buildWeapon() = 0;virtual std::unique_ptr<GameCharacter> getResult() = 0;
};// 具体建造者：骑士
class KnightBuilder : public CharacterBuilder {
public:KnightBuilder() { character_ = std::make_unique<GameCharacter>(); }void buildHead() override { character_->setHead("Steel Helmet"); }void buildBody() override { character_->setBody("Plate Armor"); }void buildWeapon() override { character_->setWeapon("Longsword"); }std::unique_ptr<GameCharacter> getResult() override { return std::move(character_); }private:std::unique_ptr<GameCharacter> character_;
};// 具体建造者：法师
class MageBuilder : public CharacterBuilder {
public:MageBuilder() { character_ = std::make_unique<GameCharacter>(); }void buildHead() override { character_->setHead("Pointed Hat"); }void buildBody() override { character_->setBody("Robe"); }void buildWeapon() override { character_->setWeapon("Staff"); }std::unique_ptr<GameCharacter> getResult() override { return std::move(character_); }private:std::unique_ptr<GameCharacter> character_;
};// 指挥者（可选）
class CharacterDirector {
public:std::unique_ptr<GameCharacter> createCharacter(CharacterBuilder& builder) {builder.buildHead();builder.buildBody();builder.buildWeapon();return builder.getResult();}
};// 客户端代码
int main() {KnightBuilder knightBuilder;MageBuilder mageBuilder;CharacterDirector director;auto knight = director.createCharacter(knightBuilder);auto mage = director.createCharacter(mageBuilder);knight->describe(); // Character: Steel Helmet, Plate Armor, wielding Longswordmage->describe();   // Character: Pointed Hat, Robe, wielding Staffreturn 0;
}

代码解析：

将角色构造分解为独立步骤，新增角色类型只需添加新的 ConcreteBuilder
使用 std::unique_ptr 明确所有权转移，防止资源泄漏

3.2 支持链式调用的增强实现

// 流畅接口（Fluent Interface）改进
class AdvancedCharacterBuilder {
public:AdvancedCharacterBuilder& withHead(const std::string& head) {head_ = head;return *this;}AdvancedCharacterBuilder& withBody(const std::string& body) {body_ = body;return *this;}AdvancedCharacterBuilder& withWeapon(const std::string& weapon) {weapon_ = weapon;return *this;}std::unique_ptr<GameCharacter> build() {auto character = std::make_unique<GameCharacter>();character->setHead(head_);character->setBody(body_);character->setWeapon(weapon_);return character;}private:std::string head_;std::string body_;std::string weapon_;
};// 客户端使用
void createCustomCharacter() {auto builder = AdvancedCharacterBuilder();auto character = builder.withHead("Hood").withBody("Leather Armor").withWeapon("Dagger").build();character->describe();
}

代码解析：

通过返回 this 指针实现链式调用，提升代码可读性
支持可选参数和任意顺序设置属性

4. 应用场景示例：HTTP请求构造

class HttpRequest {
public:void setMethod(const std::string& method) { method_ = method; }void setUrl(const std::string& url) { url_ = url; }void addHeader(const std::string& key, const std::string& value) {headers_[key] = value;}void setBody(const std::string& body) { body_ = body; }void send() const {std::cout << "Sending " << method_ << " " << url_ << " with body: " << body_ << "\n";}private:std::string method_;std::string url_;std::map<std::string, std::string> headers_;std::string body_;
};class HttpRequestBuilder {
public:HttpRequestBuilder() : request_(std::make_unique<HttpRequest>()) {}HttpRequestBuilder& method(const std::string& method) {request_->setMethod(method);return *this;}HttpRequestBuilder& url(const std::string& url) {request_->setUrl(url);return *this;}HttpRequestBuilder& header(const std::string& key, const std::string& value) {request_->addHeader(key, value);return *this;}HttpRequestBuilder& body(const std::string& body) {request_->setBody(body);return *this;}std::unique_ptr<HttpRequest> build() {return std::move(request_);}private:std::unique_ptr<HttpRequest> request_;
};// 使用示例
void sendPostRequest() {auto request = HttpRequestBuilder().method("POST").url("https://api.example.com/data").header("Content-Type", "application/json").body(R"({"key": "value"})").build();request->send();
}

5. 优缺点分析

优点	缺点
分步骤构造复杂对象，代码清晰	需定义多个Builder类，增加代码量
支持不同配置和构造顺序	对简单对象可能过度设计
隔离复杂构造逻辑与业务代码	需维护Builder与Product的同步