引入中介變數提升程式碼可讀性：重構實戰案例

重構前：巢狀存取難閱讀

if (TIMER_STARTED == _handle->state_) {
    timer_handle_t next;

    if (g_timer_next == _handle) {
        g_timer_next = (timer_handle_t) dll_next(&g_bucket_firing->dll_, &_handle->node_);
    }
    next = (timer_handle_t) dll_next(&g_buckets[_handle->bucket_index_].dll_, &_handle->node_);
    if (0 != next) {
        next->round_ += _handle->round_;
    }
    dll_remove(&g_buckets[_handle->bucket_index_].dll_, &_handle->node_);
    if (g_buckets[_handle->bucket_index_].reentrance_ > 0) {
        g_bucket_firing->level_++;
    }
}

重構後：語意清晰、減少重複運算

if (TIMER_STARTED == _handle->state_) {
    timer_handle_t next;
    bucket_t *p_bucket = &g_buckets[_handle->bucket_index_];  // 中介指標

    if (g_timer_next == _handle) {
        g_timer_next = (timer_handle_t) dll_next(&g_bucket_firing->dll_, &_handle->node_);
    }
    next = (timer_handle_t) dll_next(&p_bucket->dll_, &_handle->node_);
    if (0 != next) {
        next->round_ += _handle->round_;
    }
    dll_remove(&p_bucket->dll_, &_handle->node_);
    if (p_bucket->reentrance_ > 0) {
        g_bucket_firing->level_++;
    }
}

改進效益

• 可讀性：p_bucket 語意明確，取代冗長陣列索引
• 效能：減少重複計算 _handle->bucket_index_ 及陣列索引
• 維護性：若桶結構變更，只需修改一處
• 除錯：中介變數可在除錯器直接觀察

通用原則

1. 複雜表達式超過 2 層巢狀 → 提取中介變數
2. 重複子表達式出現 ≥ 2 次 → 提取中介變數
3. 命名要表達「業務含義」而非「資料結構路徑」
4. 配合 const 防止意外修改

延伸：Clang-Tidy 自動重構

現代工具可自動偵測並建議提取：

// 啟用檢查
clang-tidy -checks='readability-*', 'performance-*' ...