介紹
Day2介紹了單例模式(Singleton pattern)，其缺點為獲取單例的程式需知道該單例的具體(concrete)，例如獲取單例時會寫:
YearLogger* yearLogger = YearLogger::GetInstance();
這裡用戶程式碼需先知道具體的YearLogger類，此讓用戶程式碼與YearLogger類形成了耦合。
Game Programming Pattern一書對此提供了一種寫法，概念為把目標單例多加一層抽象，獲取該單例的程式不須知道該單例的具體，僅需用該抽象層來操作單例。

C++範例

#include <iostream>
#include <string>

class Logger
{
public:
    virtual ~Logger() {}
    virtual void Log() const = 0;
};

class YearLogger : public Logger
{
public:
    virtual void Log() const
    {
        std::cout << "2023" << std::endl;
    }
};

class Locator
{
public:
    static void Provide(Logger *service)
    {
        m_logger = service;
    }

    static Logger *GetLogger()
    {
        return m_logger;
    }

private:
    static Logger *m_logger;
};

Logger* Locator::m_logger;

int main()
{
    // 創建yearLogger實例
    YearLogger *yearLogger = new YearLogger();
    
    // 將yearLogger實例存入locator
    Locator::Provide(yearLogger);

    // 獲取logger實例
    Logger *pLogger = Locator::GetLogger();

    // 用logger印訊息
    pLogger->Log();

    delete yearLogger;

    return 0;
}

Outpu:

2023

Ref:
https://gameprogrammingpatterns.com/service-locator.html