创建型模式——工厂方法模式

工厂方法设计模式适用于以下情况：

• 当一个类不知道它所必须创建的对象的类的时候；

• 当一个类希望由它的子类来指定它所创建的对象的时候；

• 当类将创建对象的职责委托给多个帮助子类中的某一个；

下面是一个工厂方法模式的示例程序：

cpp 复制代码

#include <iostream>
#include <memory>
#include <string>

// --- Step 1: Abstract Product ---
// Defines the common interface for all objects the factory can create.
class Document {
public:
    virtual ~Document() = default; // Virtual destructor is crucial for polymorphism
    virtual void Read() const = 0;
};

// --- Step 2: Concrete Products ---
// Provide specific implementations of the Product interface.
class PdfDocument : public Document {
public:
    void Read() const override {
        std::cout << "Reading content from a PDF document.\n";
    }
};

class WordDocument : public Document {
public:
    void Read() const override {
        std::cout << "Reading content from a Word WordDocument.\n";
    }
};

// --- Step 3: Creator (Abstract Factory Class) ---
// Declares the factory method that returns a Product object.
class DocumentApplication {
public:
    virtual ~DocumentApplication() = default;

    // The core Factory Method
    virtual std::unique_ptr<Document> CreateDocument() const = 0;

    // The Creator often contains business logic relying on the Product
    void OpenDocument() const {
        // Call the factory method to create a Product object
        std::unique_ptr<Document> doc = CreateDocument();
        // Use the product without knowing its concrete class
        doc->Read();
    }
};

// --- Step 4: Concrete Creators ---
// Override the factory method to change the resulting product's type.
class PdfApplication : public DocumentApplication {
public:
    std::unique_ptr<Document> CreateDocument() const override {
        return std::make_unique<PdfDocument>();
    }
};

class WordApplication : public DocumentApplication {
public:
    std::unique_ptr<Document> CreateDocument() const override {
        return std::make_unique<WordDocument>();
    }
};

// --- Client Code ---
int main() {
    // Client interacts with creators and products via their abstract interfaces
    std::unique_ptr<DocumentApplication> app;

    std::cout << "Client: Launching PDF Application...\n";
    app = std::make_unique<PdfApplication>();
    app->OpenDocument();

    std::cout << "\nClient: Launching Word Application...\n";
    app = std::make_unique<WordApplication>();
    app->OpenDocument();

    return 0;
}

程序运行结果如下：

shell 复制代码

$ g++ -o main main.cpp
$ ./main
Client: Launching PDF Application...
Reading content from a PDF document.

Client: Launching Word Application...
Reading content from a Word WordDocument.