C++字符串完全指南 第二部分-字符串封装类(String Wrapper Classes)

1.简介
因为C类型的字符串容易出错和难于管理，更不用提黑客经常利用这个进行缓冲区溢出攻击。现在存在很多字符的封装类。不幸的是，哪个类使用在哪种情况下不是很清楚，也不要把C类型的字符串直接转化成封装类。
这篇文章将覆盖字符串封装类包括Win32API,MFC,STL,WTL,和theVisualC++runtimelibrary。我将描述每个类的使用方法。怎么去构造对象，怎么从一种类型转化成另一个类。
为了更好的理解这篇文章，你必须理解不同字符的类型和编码，我在第一部分已经进行了介绍。

Rule#1ofstringclasses
直接转化是一个错误的想法，除非有明确的文档说明了这个情况。对一个字符进行转化对这个字符来说不会做任何改变，不要这么写：
voidSomeFunc(LPCWSTRwidestr);
main()
{
SomeFunc((LPCWSTR)"C://foo.txt");//WRONG!
}
这肯定会100%的失败，这个编译会通过，因为转化编译器不会进行类型检查，但是编译并不意味着代码正确。
在接下来的例子中，我将指出转化在什么情况下是没有问题的。

C-stylestringsandtypedefs
就像我第一部分所说的那样，WindowsAPIs是预先定义好的的，TCHARs，它是在编译的时候根据你是否定义了_MBCS或

_UNICODE

宏来选择是MBCS或Unicode字符。你可以通过第一部分得到TCHAR的全面描述，我将列出所有的字符类型定义。

Type	Meaning
WCHAR	Unicodecharacter( wchar_t )
TCHAR	MBCSorUnicodecharacter,dependingonpreprocessorsettings
LPSTR	stringof char ( char* )
LPCSTR	constantstringof char ( constchar* )
LPWSTR	stringof WCHAR ( WCHAR* )
LPCWSTR	constantstringof WCHAR ( constWCHAR* )
LPTSTR	stringof TCHAR ( TCHAR* )
LPCTSTR	constantstringof TCHAR ( constTCHAR* )

Herearethe

OLECHAR

-relatedtypedefsyouwillsee:

Type	Meaning
OLECHAR	Unicodecharacter( wchar_t )
LPOLESTR	stringof OLECHAR ( OLECHAR* )
LPCOLESTR	constantstringof OLECHAR ( constOLECHAR* )

TherearealsotwomacrosusedaroundstringandcharacterliteralssothatthesamecodecanbeusedforbothMBCSandUnicodebuilds:

Type	Meaning
_T(x)	Prepends L totheliteralinUnicodebuilds.
OLESTR(x)	Prepends L totheliteraltomakeitan LPCOLESTR .

Therearealsovariantson

_T

thatyoumightencounterindocumentationorsamplecode.Therearefourequivalentmacros--

TEXT

,

_TEXT

,

__TEXT

,and

__T

--thatalldothesamething.

StringsinCOM-BSTRandVARIANT

许多自动化和其它的COM接口用BSTR来代替strings,但是BSTR有一些缺陷，在这里我将介绍BSTR.
BSTR是Pascal类型(wherethelengthisstoredexplicitlyalongwiththedata)的字符串和C类型wherethestringlengthmustbecalculatedbylookingforaterminatingzerocharacter).的字符串的混合体。BSTR是一个Unicode类型的字符串，string预留了他它的长度，它也是以‘0’字符结尾的，这里有一个例子anexampleof"Bob"asa

BSTR

:

06000000	4200	6F00	6200	0000
--length--	B	o	b	EOS

注意字符串的长度（length）存储在前面。COM库知道传输多少个数据(Asasidenote,a

BSTR

canholdanyarbitraryblockofdata,notjustcharacters,andcancontainembeddedzerocharacters.However,forthepurposesofthisarticle,Iwillnotconsidersuchcases.)
A

BSTR

variableinC++isactuallyapointertothefirstcharacterofthestring.Infact,thetype

BSTR

isdefinedthisway:

typedefOLECHAR*BSTR;

Thisisveryunfortunate,becauseinrealitya

BSTR

isnotthesameasaUnicodestring.Thattypedefdefeatstype-checkingandallowsyoutofreelymix

LPOLESTR

sand

BSTR

s.Passinga

BSTR

toafunctionexpectinga

LPCOLESTR

(or

LPCWSTR

)issafe,howeverthereverseisnot.Therefore,it'simportanttobeawareoftheexacttypeofstringthatafunctionexpects,andpassthecorrecttypeofstring.
有许多APIs是用来操作BSTRs，但是最重要的两个函数是为BSTR分配和销毁空间，他们是SysAllocString()和

SysFreeString()

.SysAllocString()把一个Unicode字符串拷贝到一个BSTR变量中，

SysFreeString()是释放一个BSTR开辟的空间。

BSTRbstr=NULL;
bstr=SysAllocString(L"HiBob!");
if(NULL==bstr)
//outofmemoryerror
//Usebstrhere...
SysFreeString(bstr);
很自然的，BSTR封装类接管了内存管理。
另一个自动化的接口是VARIANT。它是用来在无类型的语言，比如JScript、VBScript传送数据的。一个
VARIANT可以包含许多的类型，比如long和IDispatch*。当一个VARIANT包含一个字符串，它就是一个BSTR.
对VARIANT我后面会讲的更详细。

Stringwrapperclasses
NowthatI'vecoveredthevarioustypesofstrings,I'lldemonstratethewrapper
classes.Foreachone,I'llshowhowtoconstructanobjectandhowtoconvertitt
oaC-stylestringpointer.TheC-stylepointerisoftennecessaryforanAPIcall,
ortoconstructanobjectofadifferentstringclass.Iwillnotcoverother
operatorstheclassesprovide,suchassortingorcomparison.
Onceagain,donotblindlycastobjectsunlessyouunderstandexactlywhatthe
resultingcodewilldo.
ClassesprovidedbytheCRT
_bstr_t
[code]_bstr_t

isacompletewrapperarounda

BSTR

,andinfactithidestheunderlying

BSTR

.Itprovidesvariousconstructors,aswellasoperatorstoaccessthe
underlyingC-stylestring.However,thereisnooperatortoaccessthe

BSTR

itself,
soa

_bstr_t

cannotbepassedasan

[out]

parametertoCOMmethods.Ifyouneeda

BSTR*

touseasaparameter,itiseasiertotheATLclass

CComBSTR

.
A

_bstr_t

canbepassedtoafunctionthattakesa

BSTR

,butonlybecauseof
threecoincidences.First,

_bstr_t

hasaconversionfunctionto

wchar_t*

;second,

wchar_t*

and

BSTR

appearthesametothecompilerbecauseofthedefinitionof

BSTR

;
andthird,the

wchar_t*

thata

_bstr_t

keepsinternallypointstoablockofmemory
thatfollowsthe

BSTR

format.Soeventhoughthereisnodocumentedconversionto

BSTR

,ithappenstowork.

//Constructing
_bstr_tbs1="charstring";//constructfromaLPCSTR
_bstr_tbs2=L"widecharstring";//constructfromaLPCWSTR
_bstr_tbs3=bs1;//copyfromanother_bstr_t
_variant_tv="Bob";
_bstr_tbs4=v;//constructfroma_variant_tthathasastring

//Extractingdata
LPCSTRpsz1=bs1;//automaticallyconvertstoMBCSstring
LPCSTRpsz2=(LPCSTR)bs1;//castOK,sameaspreviousline
LPCWSTRpwsz1=bs1;//returnstheinternalUnicodestring
LPCWSTRpwsz2=(LPCWSTR)bs1;//castOK,sameaspreviousline
BSTRbstr=bs1.copy();//copiesbs1,returnsitasaBSTR

//...
SysFreeString(bstr);

Notethat

_bstr_t

alsohasconversionoperatorsfor

char*

and

wchar_t*

.Thisis

aquestionabledesign,becauseeventhoughthosearenon-constantstringpointers,

youmustnotusethosepointerstomodifythebuffer,becausethatcouldbreakthe

internal

BSTR

structure.

_variant_t

_variant_t

isacompletewrapperarounda

VARIANT

,andprovidesmanyconstructors

andconversionfunctionstooperateonthemultitudeoftypesthata

VARIANT

can

contain.Iwillonlycoverthestring-relatedoperationshere.

//Constructing
_variant_tv1="charstring";//constructfromaLPCSTR
_variant_tv2=L"widecharstring";//constructfromaLPCWSTR
_bstr_tbs1="Bob";
_variant_tv3=bs1;//copyfroma_bstr_tobject

//Extractingdata
_bstr_tbs2=v1;//extractBSTRfromtheVARIANT
_bstr_tbs3=(_bstr_t)v1;//castOK,sameaspreviousline

Notethatthe

_variant_t

methodscanthrowexceptionsifthetypeconversioncannot

bemade,sobepreparedtocatch

_com_error

exceptions.

Alsonotethatthereisnodirectconversionfrom

_variant_t

toanMBCSstring.

Youwillneedtomakeaninterim

_bstr_t

variable,useanotherstringclassthat

providestheUnicodetoMBCSconversion,oruseanATLconversionmacro.

Unlike

_bstr_t

,a

_variant_t

canbepasseddirectlyasaparametertoaCOM

method.

_variant_t

derivesfromthe

VARIANT

type,sopassinga

_variant_t

inplace

ofa

VARIANT

isallowedbyC++languagerules.

STLclasses

STLjusthasonestringclass,

basic_string

.A

basic_string

managesazero-terminated

arrayofcharacters.Thecharactertypeisgiveninthe

basic_string

templateparameter.

Ingeneral,a

basic_string

shouldbetreatedasanopaqueobject.Youcangeta

read-onlypointertotheinternalbuffer,butanywriteoperationsmustuse

basic_string

operatorsandmethods.

Therearetwopredefinedspecializationsfor

basic_string

:

string

,which

contains

char

s,and

wstring

,whichcontains

wchar_t

s.Thereisnobuilt-in

TCHAR

specialization,butyoucanusetheonelistedbelow.

//Specializations
typedefbasic_string<TCHAR>tstring;//stringofTCHARs

//Constructing
stringstr="charstring";//constructfromaLPCSTR
wstringwstr=L"widecharstring";//constructfromaLPCWSTR
tstringtstr=_T("TCHARstring");//constructfromaLPCTSTR

//Extractingdata
LPCSTRpsz=str.c_str();//read-onlypointertostr'sbuffer
LPCWSTRpwsz=wstr.c_str();//read-onlypointertowstr'sbuffer
LPCTSTRptsz=tstr.c_str();//read-onlypointertotstr'sbuffer

Unlike

_bstr_t

,a

basic_string

cannotdirectlyconvertbetweencharactersets.

However,youcanpassthepointerreturnedby

c_str()

toanotherclass'sconstructor

iftheconstructoracceptsthecharactertype,forexample:

//Example,construct_bstr_tfrombasic_string
_bstr_tbs1=str.c_str();//constructa_bstr_tfromaLPCSTR
_bstr_tbs2=wstr.c_str();//constructa_bstr_tfromaLPCWSTR

ATLclasses

CComBSTR

CComBSTR

isATL's

BSTR

wrapper,andismoreusefulinsomesituationsthan

_bstr_t

.

Mostnotably,

CComBSTR

allowsaccesstotheunderlying

BSTR

,whichmeansyoucan

passa

CComBSTR

objecttoCOMmethods,andthe

CComBSTR

objectwillautomatically

managethe

BSTR

memoryforyou.Forexample,sayyouwantedtocallmethodsofthis

interface:

//Sampleinterface:

structIStuff:publicIUnknown
{
//BoilerplateCOMstuffomitted...
STDMETHOD(SetText)(BSTRbsText);
STDMETHOD(GetText)(BSTR*pbsText);
};

CComBSTR

hasan

operatorBSTR

method,soitcanbepasseddirectlyto

SetText()

.

Thereisalsoan

operator&

thatreturnsa

BSTR*

,soyoucanusethe

&

operatoron

a

CComBSTR

objecttopassittoafunctionthattakesa

BSTR*

.

CComBSTRbs1;
CComBSTRbs2="newtext";

pStuff->GetText(&bs1);//ok,takesaddressofinternalBSTR
pStuff->SetText(bs2);//ok,callsBSTRconverter
pStuff->SetText((BSTR)bs2);//castok,sameaspreviousline

CComBSTR

hassimilarconstructorsto

_bstr_t

,howeverthereisnobuilt-inconverter

toanMBCSstring.Forthat,youcanuseanATLconversionmacro.

//Constructing
CComBSTRbs1="charstring";//constructfromaLPCSTR
CComBSTRbs2=L"widecharstring";//constructfromaLPCWSTR
CComBSTRbs3=bs1;//copyfromanotherCComBSTR
CComBSTRbs4;

bs4.LoadString(IDS_SOME_STR);//loadstringfromstringtable

//Extractingdata
BSTRbstr1=bs1;//returnsinternalBSTR,butdon'tmodifyit!
BSTRbstr2=(BSTR)bs1;//castok,sameaspreviousline
BSTRbstr3=bs1.Copy();//copiesbs1,returnsitasaBSTR
BSTRbstr4;

bstr4=bs1.Detach();//bs1nolongermanagesitsBSTR

//...
SysFreeString(bstr3);
SysFreeString(bstr4);

Notethatinthelastexample,the

Detach()

methodisused.Aftercallingthat

method,the

CComBSTR

objectnolongermanagesits

BSTR

ortheassociatedmemory.

That'swhythe

SysFreeString()

callisnecessaryon

bstr4

.

Asafootnote,the

operator&

overridemeansyoucan'tuse

CComBSTR

directlyinsome

STLcollections,suchas

list

.Thecollectionsrequirethatthe

&

operatorreturn

apointertothecontainedclass,butapplying

&

toa

CComBSTR

returnsa

BSTR*

,not

a

CComBSTR*

.However,thereisanATLclasstoovercomethis,

CAdapt

.Forexample,

tomakealistof

CComBSTR

,declareitlikethis:

std::list<CAdapt<CComBSTR>>bstr_list;

CAdapt

providestheoperatorsrequiredbythecollection,butitisinvisibleto

yourcode;youcanuse

bstr_list

justasifitwerealistof

CComBSTR

.

CComVariant

CComVariant

isawrapperarounda

VARIANT

.However,unlike

_variant_t

,the

VARIANT

isnothidden,andinfactyouneedtoaccessthemembersofthe

VARIANT

directly.

CComVariant

providesmanyconstructorstooperateonthemultitudeoftypesthata

VARIANT

cancontain.Iwillonlycoverthestring-relatedoperationshere.

//Constructing
CComVariantv1="charstring";//constructfromaLPCSTR
CComVariantv2=L"widecharstring";//constructfromaLPCWSTR
CComBSTRbs1="BSTRbob";
CComVariantv3=(BSTR)bs1;//copyfromaBSTR

//Extractingdata
CComBSTRbs2=v1.bstrVal;//extractBSTRfromtheVARIANT

Unlike

_variant_t

,therearenoconversionoperatorstothevarious

VARIANT

types.

Asshownabove,youmustaccessthe

VARIANT

membersdirectlyandensurethatthe

VARIANT

holdsdataofthetypeyouexpect.Youcancallthe

ChangeType()

methodif

youneedtoconverta

CComVariant

'sdatatoa

BSTR

.

CComVariantv4=...//Initv4fromsomewhere
CComBSTRbs3;

if(SUCCEEDED(v4.ChangeType(VT_BSTR)))
bs3=v4.bstrVal;

Aswith

_variant_t

,thereisnodirectconversiontoanMBCSstring.Youwillneed

tomakeaninterim

_bstr_t

variable,useanotherstringclassthatprovidesthe

UnicodetoMBCSconversion,oruseanATLconversionmacro.

ATLconversionmacros

ATL'sstringconversionmacrosareaveryconvenientwaytoconvertbetweencharacter

encodings,andareespeciallyusefulinfunctioncalls.Theyarenamedaccordingto

thescheme

[sourcetype]2[newtype]

or

[sourcetype]2C[newtype]

.Macrosnamedwith

thesecondformconverttoaconstantpointer(thusthe"C"inthename).Thetype

abbreviationsare:

A:MBCSstring,

char*

(AforANSI)
W:Unicodestring,

wchar_t*

(Wforwide)
T:

TCHAR

string,

TCHAR*

OLE:

OLECHAR

string,

OLECHAR*

(inpractice,equivalenttoW)
BSTR:

BSTR

(usedasthedestinationtypeonly)

So,forexample,

W2A()

convertsaUnicodestringtoanMBCSstring,and

T2CW()

convertsa

TCHAR

stringtoaconstantUnicodestring.

Tousethemacros,firstincludetheatlconv.hheaderfile.Youcandothisevenin

non-ATLprojects,sincethatheaderfilehasnodependenciesonotherpartsofATL,

anddoesn'trequirea

_Module

globalvariable.Then,whenyouuseaconversion

macroinafunction,putthe

USES_CONVERSION

macroatthebeginningofthefunction.

Thisdefinessomelocalvariablesusedbythemacros.

Whenthedestinationtypeisanythingotherthan

BSTR

,theconvertedstringis

storedonthestack,soifyouwanttokeepthestringaroundforlongerthanthe

currentfunction,you'llneedtocopythestringintoanotherstringclass.When

thedestinationtypeis

BSTR

,thememoryisnotautomaticallyfreed,soyoumust

assignthereturnvaluetoa

BSTR

variableora

BSTR

wrapperclasstoavoidmemory

leaks.

Herearesomeexamplesshowingvariousconversionmacros:


Collapse
//Functionstakingvariousstrings:
voidFoo(LPCWSTRwstr);
voidBar(BSTRbstr);
//Functionsreturningstrings:
voidBaz(BSTR*pbstr);

#include<atlconv.h>

main()
{
usingstd::string;
USES_CONVERSION;//declarelocalsusedbytheATLmacros

//Example1:SendanMBCSstringtoFoo()
LPCSTRpsz1="Bob";
stringstr1="Bob";

Foo(A2CW(psz1));
Foo(A2CW(str1.c_str()));

//Example2:SendaMBCSandUnicodestringtoBar()
LPCSTRpsz2="Bob";
LPCWSTRwsz=L"Bob";
BSTRbs1;
CComBSTRbs2;

bs1=A2BSTR(psz2);//createaBSTR
bs2.Attach(W2BSTR(wsz));//ditto,assigntoaCComBSTR

Bar(bs1);
Bar(bs2);

SysFreeString(bs1);//freebs1memory
//Noneedtofreebs2sinceCComBSTRwilldoitforus.

//Example3:ConverttheBSTRreturnedbyBaz()
BSTRbs3=NULL;
stringstr2;

Baz(&bs3);//Baz()fillsinbs3

str2=W2CA(bs3);//converttoanMBCSstring
SysFreeString(bs3);//freebs3memory
}[/code]
Asyoucansee,themacrosareveryhandywhenpassingparameterstoafunctionif

youhaveastringinoneformatandthefunctiontakesadifferentformat.

MFCclasses

CString

AnMFC

CString

holds

TCHAR

s,sotheexactcharactertypedependsonthepreprocessor

symbolsyouhavedefined.Ingeneral,a

CString

islikeanSTL

string

,inthatyou

shouldtreatitasanopaqueobjectandmodifyitonlywith

CString

methods.One

niceadvantage

CString

hasovertheSTL

string

isthatithasconstructorsthat

acceptbothMBCSandUnicodestrings,andithasaconverterto

LPCTSTR

,soyou

canpassa

CString

objectdirectlytoafunctionthatacceptsan

LPCTSTR

;thereis

no

c_str()

methodyouhavetocall.

//Constructing
CStrings1="charstring";//constructfromaLPCSTR
CStrings2=L"widecharstring";//constructfromaLPCWSTR
CStrings3('',100);//pre-allocatea100-bytebuffer,fillwithspaces
CStrings4="Newwindowtext";

//YoucanpassaCStringinplaceofanLPCTSTR:
SetWindowText(hwndSomeWindow,s4);

//Or,equivalently,explicitlycasttheCString:
SetWindowText(hwndSomeWindow,(LPCTSTR)s4);

Youcanalsoloadastringfromyourstringtable.Thereisa

CString

constructor

thatwilldoit,alongwith

LoadString()

.The

Format()

methodcanoptionallyreada

formatstringfromthestringtableaswell.

//Constructing/loadingfromstringtable
CStrings5((LPCTSTR)IDS_SOME_STR);//loadfromstringtable
CStrings6,s7;

//Loadfromstringtable.
s6.LoadString(IDS_SOME_STR);

//Loadprintf-styleformatstringfromthestringtable:
s7.Format(IDS_SOME_FORMAT,"bob",nSomeStuff,...);

Thatfirstconstructorlooksodd,butthatisactuallythedocumentedthatwayto

loadastring.

Notethattheonlylegalcastyoucanapplytoa

CString

isacastto

LPCTSTR

.

Castingtoan

LPTSTR

(thatis,anon-

const

pointer)iswrong.Gettinginthehabit

ofcastinga

CString

toan

LPTSTR

willonlyhurtyourself,aswhenthecodedoes

breaklateron,youmightnotseewhy,becauseyouusedthesamecodeelsewhereand

ithappenedtowork.Thecorrectwaytogetanon-constpointertothebufferis

the

GetBuffer()

method.

Asanexampleofthecorrectusage,considerthecaseofsettingthetextofanitem

inalistcontrol:

CStringstr=_T("newtext");
LVITEMitem={0};

item.mask=LVIF_TEXT;
item.iItem=1;
item.pszText=(LPTSTR)(LPCTSTR)str;//WRONG!
item.pszText=str.GetBuffer(0);//correct

ListView_SetItem(&item);
str.ReleaseBuffer();//returncontrolofthebuffertostr

The

pszText

memberisan

LPTSTR

,anon-

const

pointer,thereforeyoucall

GetBuffer()

on

str

.Theparameterto

GetBuffer()

istheminimumlengthyouwant

CString

to

allocateforthebuffer.Ifforsomereasonyouwantedamodifiablebufferlarge

enoughtohold1K

TCHAR

s,youwouldcall

GetBuffer(1024)

.Passing0asthelength

justreturnsapointertothecurrentcontentsofthestring.

Thecrossed-outlineabovewillcompile,anditwillevenwork,inthiscase.But

thatdoesn'tmeanthecodeiscorrect.Byusingthenon-

const

cast,you'rebreaking

object-orientedencapsulationandassumingsomethingabouttheinternalimplementation

of

CString

.Ifyoumakeahabitofcastinglikethat,youwilleventuallyruninto

acasewherethecodebreaks,andyou'llwonderwhyitisn'tworking,becauseyou

usethesamecodeeverywhereelseandit(apparently)works.

Youknowhowpeoplearealwayscomplainingabouthowbuggysoftwareisthesedays?

Bugsarecausedbytheprogrammerswritingincorrectcode.Doyoureallywantto

writecodeyouknowiswrong,andthuscontributetotheperceptionthatallsoftware

isbuggy?Takethetimetolearnthecorrectwayofusinga

CString

andhaveyour

codework100%ofthetime.

CString

alsohastwofunctionsthatcreatea

BSTR

fromthe

CString

contents,

convertingtoUnicodeifnecessary.Theyare

AllocSysString()

and

SetSysString()

.

Asidefromthe

BSTR*

parameterthat

SetSysString()

takes,theyworkidentically.

//ConvertingtoBSTR
CStrings5="Bob!";
BSTRbs1=NULL,bs2=NULL;

bs1=s5.AllocSysString();
s5.SetSysString(&bs2);

//...
SysFreeString(bs1);
SysFreeString(bs2);

COleVariant

COleVariant

isprettysimilarto

CComVariant

.

COleVariant

derivesfrom

VARIANT

,so

itcanbepassedtoafunctionthattakesa

VARIANT

.However,unlike

CComVariant

,

COleVariant

onlyhasan

LPCTSTR

constructor.Therearenotseparateconstructors

for

LPCSTR

and

LPCWSTR

.Inmostcasesthisisnotaproblem,sinceyourstrings

willlikelybe

LPCTSTR

sanyway,butitisapointtobeawareof.

COleVariant

also

hasaconstructorthatacceptsa

CString

.

//Constructing
CStrings1=_T("tcharstring");
COleVariantv1=_T("Bob");//constructfromanLPCTSTR
COleVariantv2=s1;//copyfromaCString

Aswith

CComVariant

,youmustaccessthe

VARIANT

membersdirectly,usingthe

ChangeType()

methodifnecessarytoconvertthe

VARIANT

toastring.However,

COleVariant::ChangeType()

throwsanexceptionifitfails,insteadofreturninga

failure

HRESULT

code.

//Extractingdata
COleVariantv3=...;//fillinv3fromsomewhere
BSTRbs=NULL;

try
{
v3.ChangeType(VT_BSTR);
bs=v3.bstrVal;
}
catch(COleException*e)
{
//error,couldn'tconvert
}

SysFreeString(bs);

WTLclasses

CString

WTL's

CString

behavesexactlylikeMFC's

CString

,sorefertothedescriptionofthe

MFC

CString

above.

CLRandVC7classes

System::String

isthe.NETclassforhandlingstrings.Internally,a

String

object

holdsanimmutablesequenceofcharacters.Any

String

methodthatsupposedly

manipulatesthe

String

objectactuallyreturnsanew

String

object,becausethe

original

String

isimmutable.Apeculiarityof

String

sisthatifyouhavemore

thanone

String

containingthesameseries,ofcharactersallofthemactually

referthesameobject.TheManagedExtensionstoC++haveanewstringliteral

prefix

S

,whichisusedtorepresentamanagedstringliteral.

//Constructing
String*ms=S"Thisisanicemanagedstring";

Youcanconstructa

String

objectbypassinganunmanagedstring,butthisis

slightlylessefficientthanwhenyouconstructa

String

objectbypassinga

managedstring.Thisisbecauseallinstancesofidentical

S

prefixedstrings

representthesameobject,butthisisnottrueforunmanagedstrings.Thefollowing

codewillmakethisclear:

String*ms1=S"thisisnice";
String*ms2=S"thisisnice";
String*ms3=L"thisisnice";

Console::WriteLine(ms1==ms2);//printstrue
Console::WriteLine(ms1==ms3);//printsfalse

Therightwaytocomparestringsthatmaynothavebeencreatedusing

S

prefixed

stringsistousethe

String::CompareTo()

methodasshownbelow:

Console::WriteLine(ms1->CompareTo(ms2));
Console::WriteLine(ms1->CompareTo(ms3));

Boththeabovelineswillprint0,whichmeansthestringsareequal.

Convertingbetweena

String

andtheMFC7

CString

iseasy.

CString

hasaconverter

to

LPCTSTR

and

String

hastwoconstructorsthattakea

char*

and

wchar_t*

,

thereforeyoucanpassa

CString

straighttoa

String

constructor.

CStrings1("helloworld");
String*s2(s1);//copyfromaCString

Convertingtheotherwayworkssimilarly:

String*s1=S"Threecats";
CStrings2(s1);

Thismightpuzzleyouabit,butitworksbecausestartingwithVS.NET,

CString

has

aconstructorthatacceptsa

String

object:

CStringT(System::String*pString);

Forsomespeedymanipulations,youmightsometimeswanttoaccesstheunderlying

string:

String*s1=S"Threecats";

Console::WriteLine(s1);

const__wchar_t__pin*pstr=PtrToStringChars(s1);

for(inti=0;i<wcslen(pstr);i++)
(*const_cast<__wchar_t*>(pstr+i))++;

Console::WriteLine(s1);

PtrToStringChars()

returnsa

const__wchar_t*

totheunderlyingstringwhichweneed

topindownasotherwisethegarbagecollectormightmovethestringinmemorywhile

wearemanipulatingitscontents.

Usingstringclasseswithprintf-styleformattingfunctions

Youmustpaycarefulattentionwhenusingstringwrapperclasseswith

printf()

or

anyfunctionthatworkstheway

printf()

does.Thisincludes

sprintf()

andits

variants,aswellasthe

TRACE

and

ATLTRACE

macros.Becausethereisnotype-checking

doneontheadditionalparameterstothefunctions,youmustbecarefultoonlypassaC-stylestringpointer,notacompletestringobject.

Soforexample,topassastringina

_bstr_t

to

ATLTRACE()

,youmustexplicitly

writethe

(LPCSTR)

or

(LPCWSTR)

cast:

_bstr_tbs=L"Bob!";

ATLTRACE("Thestringis:%sinline%d/n",(LPCSTR)bs,nLine);

Ifyouforgetthecastandpasstheentire

_bstr_t

object,thetracemessagewill

displaymeaninglessoutput,sincewhatwillbepushedonthestackiswhatever

internaldatathe

_bstr_t

variablekeeps.

Summaryofalltheclasses

Theusualwayofconvertingbetweentwostringclassesistotakethesourcestring,convertittoaC-stylestringpointer,andthenpassthepointertoaconstructorinthedestinationtype.SohereisachartshowinghowtoconvertastringtoaC-stylepointer,andwhichclassescanbeconstructedfromC-stylepointers.

Class	string type	convert to char* ?	convertto constchar* ?	convertto wchar_t* ?	convertto constwchar_t* ?	convert to BSTR ?	construct from char* ?	construct from wchar_t* ?
_bstr_t	BSTR	yes,cast1	yes,cast	yes,cast1	yes,cast	yes2	yes	yes
_variant_t	BSTR	no	no	no	castto _bstr_t 3	castto _bstr_t 3	yes	yes
string	MBCS	no	yes, c_str() method	no	no	no	yes	no
wstring	Unicode	no	no	no	yes, c_str() method	no	no	yes
CComBSTR	BSTR	no	no	no	yes,cast to BSTR	yes,cast	yes	yes
CComVariant	BSTR	no	no	no	yes4	yes4	yes	yes
CString	TCHAR	no6	inMBCS builds,cast	no6	inUnicode builds,cast	no5	yes	yes
COleVariant	BSTR	no	no	no	yes4	yes4	inMBCSbuilds	inUnicodebuilds
1Eventhough _bstr_t providesconversionoperatorstonon- const pointers,modifyingtheunderlyingbuffermaycauseaGPFifyouoverrunthebuffer,oraleakwhenthe BSTR memoryisfreed. 2A _bstr_t holdsa BSTR internallyina wchar_t* variable,soyoucanusethe constwchar_t* convertertoretrievethe BSTR .Thisisanimplementationdetail,sousethiswithcaution,asitmaychangeinthefuture. 3Thiswillthrowanexceptionifthedatacannotbeconvertedtoa BSTR . 4Use ChangeType() thenaccessthe bstrVal memberofthe VARIANT .InMFC,thiswillthrowanexceptionifthedatacannotbeconverted. 5Thereisno BSTR conversionfunction,howeverthe AllocSysString() methodreturnsanew BSTR . 6Youcantemporarilygetanon-const TCHAR pointerusingthe GetBuffer() method.

http://www.codeproject.com/KB/string/cppstringguide2.aspx