Re: [問題] pointer的reference(this pointer)

看板C_and_CPP (C/C++)作者scwg ( )時間12年前 (2013/07/02 19:36)推噓2(2推 0噓 6→)

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第一次拿 C++ standard 來回答問題，請多多指教 m(_ _)m 我覺得問題在 28 inner = new Inner(this); 配上 13 Inner(Foo* const& ptr) 首先根據 C++ standard 9.3.2 The this pointer 1 In the body of a non-static (9.3) member function, the keyword this is a prvalue expression whose value is the address of the object for which the function is called. The type of this in a member function of a class X is X*. (prvalue 表示是 rvalue, 但不是 xvalue, 大致上表示不是 movable) 然後 8.5.3 (Initializers) References 5 A reference to type "cv1 T1" is initialized by an expression of type "cv2 T2" as follows: ... - Otherwise, the reference shall be an lvalue reference to a non-volatile const type (i.e., cv1 shall be const), or the reference shall be an rvalue reference. - If the initializer expression - is an xvalue (but not a bit-field), class prvalue, array prvalue or function lvalue and "cv1 T1" is reference-compatible with "cv2 T2", or - has a class type (i.e., T2 is a class type), where T1 is not reference-related to T2, and can be implicitly converted to an xvalue, class prvalue, or function lvalue of type "cv3 T3", where "cv1 T1" is reference-compatible with "cv3 T3" ... - Otherwise, a temporary of type "cv1 T1" is created and initialized from the initializer expression using the rules for a non-reference copy-initialization (8.5). The reference is then bound to the temporary. 因為 this 是 pointer prvalue, 所以只能走最後一條, 產生 temporary. 而 temporary 的生命期離開 28 行後就結束了, 因此之後再用 Inner::outer 就又變成未定義行為了. ※ 引述《QQ29 (我愛阿蓉)》之銘言： : 大家好... : 遇到一個詭異的程式結果 : 我想了好久 : 跟我預期的完全不一樣 : 想請教各位 : 為啥會有這種問題 : 並且想問說這種寫法是不是很危險 : 程式碼很短 : 平台是windows VC : http://codepad.org/xK3IbjUI : 我現在遇到一個case : 我這段程式碼就是在模擬這問題 : 某個inner class有記錄一個外面的pointer : 然後拿這個pointer 一直使用 : 但是外面這pointer 會不定時被delete : 我若是拿這pointer繼續操作 , 就會crash : 於是我天真的想說 : 我乾脆inner class記錄外面pointer的reference(透過this) : 肉眼compile感覺應該可以達到 : 變成說外面只要記得把pointer設定成null : 我裡面只要遇到多判斷if null就do nothing... : 結果就是出現一些我意料之外的情形 : 若是成功, outerInstance 這物件應該會變成null... : 問題疑點都寫在註解... : 麻煩各位跑跑看到底我哪裡觀念有大瑕疵 : 謝謝 -- -----BEGIN GEEK CODE BLOCK----- Version: 3.12 GCS/M d-(+) s:+ a- C++$ UL++B+ P++(++++) L+ E--@ W++ N? o? K? w(++) !O M !V PS++(+++) PE++(+++) !Y PGP t+++ !5 !X R !tv b++ DI++ D+ G e+++>++++ h--* r y+ ------END GEEK CODE BLOCK------ -- ※ 發信站: 批踢踢實業坊(ptt.cc) ◆ From: 128.36.232.45