c++primer中文版第4版中英對照三

1、CPrimer中文版中文版(第4版)(中英對照中英對照)三5.12.5.12.TypeTypeConversionsConversions5.12.5.12.類型轉(zhuǎn)換類型轉(zhuǎn)換Thetypeoftheoper(s)determinewhetheranexpressionislegaliftheexpressionislegaldeterminesthemeaningoftheexpression.HoweverinCsometypesar

2、erelatedtooneanother.Whentwotypesarerelatedwecanuseanobjectvalueofonetypewhereanoperoftherelatedtypeisexpected.Twotypesarerelatedifthereisaconversionconversionbetweenthem.表達(dá)式是否合法取決于操作數(shù)的類型，而且合法的表達(dá)式其含義也由其操作數(shù)類型決定。但是，在C中，某些類

3、型之間存在相關(guān)的依賴關(guān)系。若兩種類型相關(guān)，則可在需要某種類型的操作數(shù)位置上，使用該類型的相關(guān)類型對象或值。如果兩個類型之間可以相互轉(zhuǎn)換轉(zhuǎn)換，則稱這兩個類型相關(guān)。Asanexampleconsider考慮下列例子：intival=0ival=3.5413typicallycompileswithawarningwhichassigns6toival.ival的值為6。Theoperstotheadditionoperatarevalues

4、oftwodifferenttypes:3.541isaliteraloftypedouble3isaliteraloftypeint.RatherthanattempttoaddvaluesofthetwodifferenttypesCdefinesasetofconversionstotransfmtheoperstoacommontypebefeperfmingthearithmetic.Theseconversionsareca

5、rriedoutautomaticallybythecompilerwithoutprogrammerinterventionsometimeswithoutprogrammerknowledge.Fthatreasontheyarereferredtoasimplicittypeconversions.首先做加法操作，其操作數(shù)是兩個不同類型的值：3.541是double型的字面值常量，而3則是int型的字面值常量。C并不是把兩個不同類

6、型的值直接加在一起，而是提供了一組轉(zhuǎn)換規(guī)則，以便在執(zhí)行算術(shù)操作之前，將兩個操作數(shù)轉(zhuǎn)換為同一種數(shù)據(jù)類型。這些轉(zhuǎn)換規(guī)則由編譯器自動執(zhí)行，無需程序員介入——有時甚至不需要程序員了解。因此，它們也被稱為隱式類型轉(zhuǎn)換。Thebuiltinconversionsamongthearithmetictypesaredefinedtopreserveprecisionifpossible.Mostoftenifanexpressionhasbothin

7、tegralfloatingpointvaluestheintegerisconvertedtofloatingpoint.Inthisadditiontheintegervalue3isconvertedtodouble.Floatingpointadditionisperfmedtheresult6.541isoftypedouble.C定義了算術(shù)類型之間的內(nèi)置轉(zhuǎn)換以盡可能防止精度損失。通常，如果表達(dá)式的操作數(shù)分別為整型和浮點(diǎn)型，則

8、整型的操作數(shù)被轉(zhuǎn)換為浮點(diǎn)型。本例中，整數(shù)3被轉(zhuǎn)換為double類型，然后執(zhí)行浮點(diǎn)類型的加法操作，得double類型的結(jié)果6.541。Thenextstepistoassignthatdoublevaluetoivalwhichisanint.Inthecaseofassignmentthetypeofthelefthoperdominatesbecauseitisnotpossibletochangethetypeoftheobject

9、onthelefthside.Whentheleftrighthtypesofanassignmentdifferthe在第六章中學(xué)習(xí)）。?Anexpressionusedtoinitializeassigntoavariableisconvertedtothetypeofthevariable:用一表達(dá)式初始化某個變量，或?qū)⒁槐磉_(dá)式賦值給某個變量，則該表達(dá)式被轉(zhuǎn)換為該變量的類型：intival=3.143.14convertedtoi

10、ntintipip=0theint0convertedtoanullpointeroftypeintInadditionaswellseeinChapter7implicitconversionsalsooccurduringfunctioncalls.另外，在函數(shù)調(diào)用中也可能發(fā)生隱式類型轉(zhuǎn)換，我們將在第七章學(xué)習(xí)這方面的內(nèi)容。5.12.2.5.12.2.TheTheArithmeticArithmeticConversionsConve

11、rsions5.12.2.5.12.2.算術(shù)轉(zhuǎn)換算術(shù)轉(zhuǎn)換Thelanguagedefinesasetofconversionsamongthebuiltintypes.Amongthesethemostcommonarethearithmeticarithmeticconversionsconversionswhichensurethatthetwoopersofabinaryoperatsuchasanarithmeticlogica

12、loperatareconvertedtoacommontypebefetheoperatisevaluated.Thatcommontypeisalsotheresulttypeoftheexpression.C語言為內(nèi)置類型提供了一組轉(zhuǎn)換規(guī)則，其中最常用的是算術(shù)轉(zhuǎn)換算術(shù)轉(zhuǎn)換。算術(shù)轉(zhuǎn)換保證在執(zhí)行操作之前，將二元操作符（如算術(shù)或邏輯操作符）的兩個操作數(shù)轉(zhuǎn)換為同一類型，并使表達(dá)式的值也具有相同的類型。Therulesdefineahier

13、archyoftypeconversionsinwhichopersareconvertedtothewidesttypeintheexpression.Theconversionrulesaredefinedsoastopreservetheprecisionofthevaluesinvolvedinamultitypeexpression.Fexampleifoneoperisoftypelongdoublethentheother

14、isconvertedtotypelongdoubleregardlessofwhatthesecondtypeis.算術(shù)轉(zhuǎn)換規(guī)則定義了一個類型轉(zhuǎn)換層次，該層次規(guī)定了操作數(shù)應(yīng)按什么次序轉(zhuǎn)換為表達(dá)式中最寬的類型。在包含多種類型的表達(dá)式中，轉(zhuǎn)換規(guī)則要確保計(jì)算值的精度。例如，如果一個操作數(shù)的類型是longdouble，則無論另一個操作數(shù)是什么類型，都將被轉(zhuǎn)換為longdouble。Thesimplestkindsofconversionare

15、integralintegralpromotionspromotions.Eachoftheintegraltypesthataresmallerthanintsignedunsignedshtunsignedshtispromotedtointifallpossiblevaluesofthattypefitinanint.Otherwisethevalueispromotedtounsignedint.Whenboolvaluesar

16、epromotedtointafalsevaluepromotestozerotruetoone.最簡單的轉(zhuǎn)換為整型提升整型提升：對于所有比int小的整型，包括、signed、unsigned、sht和unsignedsht，如果該類型的所有可能的值都能包容在int內(nèi)，它們就會被提升為int型，否則，它們將被提升為unsignedint。如果將bool值提升為int，則false轉(zhuǎn)換為0，而true則轉(zhuǎn)換為1。ConversionsCo

17、nversionsbetweenbetweenSignedSignedUnsignedUnsignedTypesTypes有符號與無符號類型之間的轉(zhuǎn)換有符號與無符號類型之間的轉(zhuǎn)換Whenanunsignedvalueisinvolvedinanexpressiontheconversionrulesaredefinedtopreservethevalueoftheopers.Conversionsinvolvingunsignedope