GoFデザインパターン - プロキシ編

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

// Subject
class IImageService {
public:
    virtual ~IImageService() = default;
    virtual std::string FetchImage(const std::string& path, const std::string& token) = 0;
};

// RealSubject
class RealImageService : public IImageService {
public:
    std::string FetchImage(const std::string& path, const std::string& token) override {
        (void)token; // 実運用では監査等に使う
        return LoadFromDisk(path);
    }

private:
    std::string LoadFromDisk(const std::string& path) {
        std::cout << "[Real] loading from disk: " << path << "\n";
        return "IMAGE_BYTES(" + path + ")";
    }
};

// Proxy
class ImageServiceProxy : public IImageService {
public:
    std::string FetchImage(const std::string& path, const std::string& token) override {
        if (!IsAllowed(token)) {
            std::cout << "[Proxy] access denied\n";
            return "";
        }

        auto it = cache_.find(path);
        if (it != cache_.end()) {
            std::cout << "[Proxy] cache hit: " << path << "\n";
            return it->second;
        }

        EnsureReal();
        auto bytes = real_->FetchImage(path, token);
        cache_[path] = bytes;
        std::cout << "[Proxy] cache store: " << path << "\n";
        return bytes;
    }

private:
    bool IsAllowed(const std::string& token) const {
        return token == "valid-token";
    }

    void EnsureReal() {
        if (!real_) {
            std::cout << "[Proxy] lazy initialize RealImageService\n";
            real_ = std::make_unique<RealImageService>();
        }
    }

private:
    std::unique_ptr<RealImageService> real_;
    std::unordered_map<std::string, std::string> cache_;
};

int main() {
    std::unique_ptr<IImageService> service = std::make_unique<ImageServiceProxy>();

    // 認可失敗
    auto denied = service->FetchImage("/img/banner.png", "invalid");
    std::cout << "denied size=" << denied.size() << "\n\n";

    // 初回: lazy init + 実体呼び出し + cache store
    auto first = service->FetchImage("/img/banner.png", "valid-token");
    std::cout << "first size=" << first.size() << "\n\n";

    // 2回目: cache hit（実体呼び出しなし）
    auto second = service->FetchImage("/img/banner.png", "valid-token");
    std::cout << "second size=" << second.size() << "\n";

    return 0;
}

// 実行例
// [Proxy] access denied
// denied size=0
//
// [Proxy] lazy initialize RealImageService
// [Real] loading from disk: /img/banner.png
// [Proxy] cache store: /img/banner.png
// first size=28
//
// [Proxy] cache hit: /img/banner.png
// second size=28