C++ 沉思錄課堂練習再解：消息驅動架構實現圖片裝飾與組合的極簡範式

裝飾器基類模板（消除重複）

template
struct DecoraterBase : PictureMsg {
    PictureMsg prev_;
    DecoraterBase(PictureMsg& prev) : prev_(prev) {
        prev_.obj = static_cast(this);
        prev_.proc = &Derived::Handle;
    }
};

邊框裝飾器

struct CFrameDecorater : DecoraterBase {
    using Base = DecoraterBase;
    CFrameDecorater(PictureMsg& p) : Base(p) {}

    static int Handle(void* obj, int msg, void* p1, void* p2) {
        auto self = (CFrameDecorater*)obj;
        auto& p = self->prev_;
        switch (msg) {
            case PM_WIDTH:  return p.GetWidth() + 2;
            case PM_HEIGHT: return p.GetHeight() + 2;
            case PM_ROW: {
                int r = (int)p1; auto& out = *(std::string*)p2;
                if (r == 0 || r == p.GetHeight()+1) {
                    out = "+" + std::string(p.GetWidth(), '-') + "+";
                } else {
                    std::string mid; p.GetRow(r-1, mid);
                    out = "|" + mid + "|";
                }
                return 0;
            }
        }
        return 0;
    }
};

水平拼接裝飾器

struct CHorseDecorater : DecoraterBase {
    PictureMsg right_;
    using Base = DecoraterBase;
    CHorseDecorater(PictureMsg& l, PictureMsg& r) : Base(l), right_(r) {}

    static int Handle(void* obj, int msg, void* p1, void* p2) {
        auto self = (CHorseDecorater*)obj;
        auto& L = self->prev_;
        auto& R = self->right_;
        switch (msg) {
            case PM_WIDTH:  return L.GetWidth() + R.GetWidth();
            case PM_HEIGHT: return std::max(L.GetHeight(), R.GetHeight());
            case PM_ROW: {
                int r = (int)p1; auto& out = *(std::string*)p2;
                std::string l, rstr;
                if (r < L.GetHeight()) L.GetRow(r, l); else l = std::string(L.GetWidth(), ' ');
                if (r < R.GetHeight()) R.GetRow(r, rstr); else rstr = std::string(R.GetWidth(), ' ');
                out = l + rstr;
                return 0;
            }
        }
        return 0;
    }
};

垂直拼接裝飾器（補全）

struct CVerticalDecorater : DecoraterBase {
    PictureMsg bottom_;
    using Base = DecoraterBase;
    CVerticalDecorater(PictureMsg& top, PictureMsg& bot) : Base(top), bottom_(bot) {}

    static int Handle(void* obj, int msg, void* p1, void* p2) {
        auto self = (CVerticalDecorater*)obj;
        auto& T = self->prev_;
        auto& B = self->bottom_;
        switch (msg) {
            case PM_WIDTH:  return std::max(T.GetWidth(), B.GetWidth());
            case PM_HEIGHT: return T.GetHeight() + B.GetHeight();
            case PM_ROW: {
                int r = (int)p1; auto& out = *(std::string*)p2;
                if (r < T.GetHeight()) T.GetRow(r, out);
                else B.GetRow(r - T.GetHeight(), out);
                // 補齊寬度
                int w = std::max(T.GetWidth(), B.GetWidth());
                if ((int)out.size() < w) out.append(w - out.size(), ' ');
                return 0;
            }
        }
        return 0;
    }
};

客戶端演示：任意組合

int main() {
    const char* d[] = {"Paris", "in the", "Spring", "HaHa"};
    CPicture p1(d, 3), p2(d, 4);

    // 疊加多層邊框
    CFrameDecorater f1(p1), f2(p1); // p1 介面現指向 f2 → f1 → p1

    // 水平拼接 + 邊框
    CHorseDecorater h(p1, p2);
    CFrameDecorater fh(p1); // p1 現為 fh → h → (f2→f1→p1) + p2

    // 垂直拼接
    CVerticalDecorater v(p1, p2);

    std::cout << "=== Final p1 ===\n" << p1;
    std::cout << "=== p2 (unmodified) ===\n" << p2;
    return 0;
}

運行時介面劫持鏈圖解

初始：p1.obj → CPicture::Handle

CFrameDecorater f1(p1):
  p1.obj → f1, p1.proc → CFrameDecorater::Handle
  f1.prev_.obj → CPicture::Handle (原 p1 行為)

CFrameDecorater f2(p1):
  p1.obj → f2, p1.proc → CFrameDecorater::Handle
  f2.prev_.obj → f1 (f1 仍指向 CPicture)

CHorseDecorater h(p1, p2):
  p1.obj → h, p1.proc → CHorseDecorater::Handle
  h.prev_.obj → f2 (f2→f1→CPicture)
  h.right_.obj → p2 (CPicture::Handle)

CFrameDecorater fh(p1):
  p1.obj → fh, p1.proc → CFrameDecorater::Handle
  fh.prev_.obj → h (h→f2→f1→CPicture + p2)

為何這不是「破壞封裝」而是「極致封裝」？

1. CPicture 零感知：其成員 `data_`、`rows_`、`width_` 從未被外部直接訪問，僅通過 `PictureMsg` 介面的三個消息查詢。
2. 裝飾器零耦合：`CFrameDecorater` 僅依賴 `PictureMsg` 介面，完全不知 `prev_` 實際是 `CPicture` 還是另一個裝飾器。
3. 單一職責：每個裝飾器只關心「如何修飾寬/高/行」，邏輯內聚度極高。
4. 開放封閉原則：新增裝飾類型（如陰影、旋轉）只需新增類，無需修改現有代碼。

性能分析

• 每行輸出經過 **裝飾層數** 次函數指標調用（內聯極易優化）。
• 無虛函數表查找、無動態派發開銷、無繼承佈局調整。
• 記憶體佔用：每裝飾層僅一個 `PictureMsg`（兩指針大小）+ 成員引用。

結語

這個解法證明：**設計模式的靈魂是「關注點分離與動態組合」，而非「繼承與虛函數」**。消息驅動（或稱攔截器鏈、中介者模式的極簡形式）以最小機制實現了最大靈活性。下次面對「需支援任意組合的裝飾/管道/過濾器」問題時，請先考慮：
struct Interface { void* obj; int(*proc)(void*, int, void*, void*); };
這或許就是你需要的全部框架。