STL雜記(4)

1.模板執行個體化(instantiated)的兩個層次，聲明和使用時的定義檢查，參數依賴內容只做聲明檢查

// details/lazy.cpp  template <typename T> class Safe { }; template <int N> class Danger {   public:     typedef char Block[N];  // would fail for N<=0  }; template <typename T, int N> class Tricky {   public:     virtual ~Tricky() {     }     void no_body_here(Safe<T> = 3);     void inclass() {         Danger<N> no_boom_yet;     }     // void error() { Danger<0> boom; }      // void unsafe(T (*p)[N]);      T operator->();     // virtual Safe<T> suspect();      struct Nested {         Danger<N> pfew;     };     union {  // anonymous union         int align;        Safe<T> anonymous;     }; }; int main() {    Tricky<int, 0> ok; }