談到 Design Patterns,有興趣的人可以看看這一系列對 Erich Gamma 的訪談:

Erich Gamma 談到如何使用 DP 以及經過這十年他想法的一些改變。DP 固然是 programmer 設計程式時的一大利器,然而水可載舟亦可覆舟,誤用 DP 往往會造成過度的設計以及沒有必要的彈性。例如下面這個我很久以前看過的一個實際例子:

class Toy
{};

class Barbie : public Toy
{};

class Lego : public Toy
{};

class ToyFactory
{
public:
    static ToyFactory* Instance();
    virtual Toy* CreateToy(LPCTSTR id);
protected:
    ToyFactory();
    virtual ~ToyFactory();
private:
    static ToyFactory* m_pToyFactory;
};

ToyFactory * ToyFactory::m_pToyFactory = NULL;

ToyFactory::ToyFactory()
{}

ToyFactory::~ToyFactory()
{}

ToyFactory * ToyFactory::Instance()
{
    if (!m_pToyFactory)
        m_pToyFactory = new ToyFactory;
    return m_pToyFactory;
}

Toy* ToyFactory::CreateToy(LPCTSTR id)
{
    if (strcmp(id, "barbie") == 0)
        return new Barbie;
    else if (strcmp(id, "lego") == 0)
        return new Lego;

    return NULL;
}

int main()
{
    ToyFactory::Instance()->CreateToy("barbie");
    return 0;
}

明明一個 global function 可以做到的事卻硬要用 Factory Method 及 Singleton 這兩個 pattern 來包裝,這已經是為了 DP 而 DP。

To add flexibility, you really have to be able to justify it by a requirement. If you don't have a requirement up front, then I wouldn't put a hook for flexibility in my system up front.

在使用任何 Design Patterns 之前,你該想想 Erich Gamma 的這番建議。