如何確保？標(biāo)準(zhǔn)確保。因?yàn)檫@是標(biāo)準(zhǔn)規(guī)定的。
以下摘自C++11Standard(draftN3690)
15.2Constructorsanddestructors[except.ctor]
不光保證了被析構(gòu)，還規(guī)定了析構(gòu)的順序。
1)Ascontrolpassesfromthepointwhereanexceptionisthrowntoahandler,destructorsareinvokedforallautomaticobjectsconstructedsincethetryblockwasentered.Theautomaticobjectsaredestroyedinthereverseorderofthecompletionoftheirconstruction.
比較特殊一點(diǎn)的，如果異常發(fā)生在構(gòu)造或析構(gòu)的時候，其子對象也能確保被正確的析構(gòu)。而該對象本身呢？構(gòu)造的時候它還不存在呢，所以無須擔(dān)心。析構(gòu)的情況會在后面說明。
2)Anobjectofanystoragedurationwhoseinitializationordestructionisterminatedbyanexceptionwillhavedestructorsexecutedforallofitsfullyconstructedsubobjects(excludingthevariantmembersofaunion-likeclass),thatis,forsubobjectsforwhichtheprincipalconstructor(12.6.2)hascompletedexecutionandthedestructorhasnotyetbegunexecution.Similarly,ifthenon-delegatingconstructorforanobjecthascompletedexecutionandadelegatingconstructorforthatobjectexitswithanexception,theobject’sdestructorwillbeinvoked.Iftheobjectwasallocatedinanew-expression,thematchingdeallocationfunction(3.7.4.2,5.3.4,12.5),ifany,iscalledtofreethestorageoccupiedbytheobject.
這整個過程被稱為"stackunwinding"，翻譯過來叫:堆棧輾轉(zhuǎn)開解。
3)Theprocessofcallingdestructorsforautomaticobjectsconstructedonthepathfromatryblocktothepointwhereanexceptionisthrowniscalled“stackunwinding.”Ifadestructorcalledduringstackunwindingexitswithanexception,std::terminateiscalled(15.5.1).[Note:Sodestructorsshouldgenerallycatchexceptionsandnotletthempropagateoutofthedestructor.—endnote]
注意看那個“Note”，標(biāo)準(zhǔn)對編譯器做了要求，對于析構(gòu)函數(shù)來說，是要對自身負(fù)責(zé)的。
正是因?yàn)閟tackunwinding的保證作為基礎(chǔ)，才有了我們所熟知的RAII技術(shù)。
另外可以注意到，如果在stackunwinding期間拋出異常呢？就只能調(diào)用std::terminate了:
15.5.1Thestd::terminate()function[except.terminate]
2)Insuchcases,std::terminate()iscalled(18.8.3).Inthesituationwherenomatchinghandlerisfound,itisimplementation-de?nedwhetherornotthestackisunwoundbeforestd::terminate()iscalled.Inthesituationwherethesearchforahandler(15.3)encounterstheoutermostblockofafunctionwithanoexcept-speci?cationthatdoesnotallowtheexception(15.4),itisimplementation-de?nedwhetherthestackisunwound,unwoundpartially,ornotunwoundatallbeforestd::terminate()iscalled.Inallothersituations,thestackshallnotbeunwoundbeforestd::terminate()iscalled.Animplementationisnotpermittedto?nishstackunwindingprematurelybasedonadeterminationthattheunwindprocesswilleventuallycauseacalltostd::terminate()
stackunwinding的過程并不保證做完，但最終肯定是要調(diào)用std::terminate來終止。
                            

在拋出異常后調(diào)用棧內(nèi)存對象的析構(gòu)函數(shù)，在C++標(biāo)準(zhǔn)里有規(guī)定。析構(gòu)函數(shù)本來就不是顯式調(diào)用的，編譯器和運(yùn)行環(huán)境自然知道什么時候應(yīng)該調(diào)用析構(gòu)函數(shù)，只要它們是按照C++標(biāo)準(zhǔn)實(shí)現(xiàn)的。至于如何實(shí)現(xiàn)，我想也不難吧，只需要給初始化過的對象做個標(biāo)記，處理異常的時候逐個調(diào)用它們的析構(gòu)函數(shù)不就好了。