The Complete Guide to C++ Strings, Part II - String Wrapper Classes

http://www.codeproject.com/string/cppstringguide1.asp
http://www.codeproject.com/string/cppstringguide2.asp

Introduction

Since C-style strings can be error-prone and difficult to manage, not to mention a target for hackers looking for buffer overrun bugs, there are lots of string wrapper classes. Unfortunately, it's not always clear which class should be used in some situations, nor how to convert from a C-style string to a wrapper class.

This article covers all the string types in the Win32 API, MFC, STL, WTL, and the Visual C++ runtime library. I will describe the usage of each class, how to construct objects, and how to convert to other classes. Nish has also contributed the section on managed strings and classes in Visual C++ 7.

In order to get the full benefit from this article, you must understand the different character types and encodings, as I covered in Part I.

Rule #1 of string classes

Casts are bad, unless they are explicitly documented.

What prompted me to write these two articles was the frequent questions about how to convert string type X to type Z, where the poster was using a cast and didn't understand why the code didn't work. The various string types, especially

BSTR

, are not concisely documented in any one place, so I imagine some people were throwing in casts and hoping it would work.

A cast does not do any conversion to a string, unless the source string is a wrapper class with an explicitly documented conversion operator. A cast of a string literal does nothing to the string, so writing something like:

void SomeFunc ( LPCWSTR widestr );
main()
{
SomeFunc ( (LPCWSTR) "C:\\foo.txt" );  // WRONG!
}

will fail 100% of the time. It will compile, because the cast overrides the compiler's type-checking. But just because it compiles, doesn't mean the code is correct.

In the examples that follow, I will point out when casts are legal.

C-style strings and typedefs

As I covered in Part I, Windows APIs are defined and documented in terms of

TCHAR

s, which can be MBCS or Unicode characters depending on whether you define the

_MBCS

or

_UNICODE

symbol when compiling. You should refer to Part I for a full description of

TCHAR

, but I will list the character typedefs here for convenience.

Type	Meaning
WCHAR	Unicode character ( wchar_t )
TCHAR	MBCS or Unicode character, depending on preprocessor settings
LPSTR	string of char ( char* )
LPCSTR	constant string of char ( const char* )
LPWSTR	string of WCHAR ( WCHAR* )
LPCWSTR	constant string of WCHAR ( const WCHAR* )
LPTSTR	string of TCHAR ( TCHAR* )
LPCTSTR	constant string of TCHAR ( const TCHAR* )

One additional character type is the

OLECHAR

. This represents the character type used in Automation interfaces (such as the interfaces exposed by Word so you can manipulate documents). This type is normally defined as

wchar_t

, however if you define the

OLE2ANSI

preprocessor symbol,

OLECHAR

will be defined as the

char

type. I know of no reason these days to define

OLE2ANSI

(it hasn't been used by Microsoft since the days of MFC 3), so from now on I will treat an

OLECHAR

as a Unicode character.

Here are the

OLECHAR

-related typedefs you will see:

Type	Meaning
OLECHAR	Unicode character ( wchar_t )
LPOLESTR	string of OLECHAR ( OLECHAR* )
LPCOLESTR	constant string of OLECHAR ( const OLECHAR* )

There are also two macros used around string and character literals so that the same code can be used for both MBCS and Unicode builds:

Type	Meaning
_T(x)	Prepends L to the literal in Unicode builds.
OLESTR(x)	Prepends L to the literal to make it an LPCOLESTR .

There are also variants on

_T

that you might encounter in documentation or sample code. There are four equivalent macros --

TEXT

,

_TEXT

,

__TEXT

, and

__T

-- that all do the same thing.

Strings in COM - BSTR and VARIANT

Many Automation and other COM interfaces use

BSTR

for strings, and

BSTR

s have a few pitfalls, so I will give

BSTR

its own section here.

BSTR

is a hybrid between Pascal-style strings (where the length is stored explicitly along with the data) and C-style strings (where the string length must be calculated by looking for a terminating zero character). A

BSTR

is a Unicode string that has its length prepended, and is also terminated by a zero character. Here is an example of "Bob" as a

BSTR

:

0600 00 00	42 00	6F 00	62 00	00 00
--length--	B	o	b	EOS

Notice how the length of the string is prepended to the string data. It is a

DWORD

, and holds the number of bytes in the string, not counting the terminating zero. In this case, "Bob" contains 3 Unicode characters (not counting the terminating zero), for a total of 6 bytes. The length field is present so that when a

BSTR

is marshaled between processes or computers, the COM library knows how much data to transfer. (As a side note, a

BSTR

can hold any arbitrary block of data, not just characters, and can contain embedded zero characters. However, for the purposes of this article, I will not consider such cases.)

A

BSTR

variable in C++ is actually a pointer to the first character of the string. In fact, the type

BSTR

is defined this way:

typedef OLECHAR* BSTR;

This is very unfortunate, because in reality a

BSTR

is not the same as a Unicode string. That typedef defeats type-checking and allows you to freely mix

LPOLESTR

s and

BSTR

s. Passing a

BSTR

to a function expecting a

LPCOLESTR

(or

LPCWSTR

) is safe, however the reverse is not. Therefore, it's important to be aware of the exact type of string that a function expects, and pass the correct type of string.

To see why it is not safe to pass a

LPCWSTR

to a function expecting a

BSTR

, remember that the four bytes immediately before the string must store its length. There is no such length with a

LPCWSTR

. If the

BSTR

needs to be marshaled to another process (for example, an instance of Word that you are controlling), the COM library will look for that length and find garbage, or some other variable on your stack, or other random data. This will either cause the method to fail, or even crash if the perceived length is too long.

There are several APIs that operate on

BSTR

s, however the two most important ones are the functions that create and destroy a

BSTR

. They are

SysAllocString()

and

SysFreeString()

.

SysAllocString()

copies a Unicode string into a

BSTR

, while

SysFreeString()

frees the memory used by a

BSTR

.

BSTR bstr = NULL;
bstr = SysAllocString ( L"Hi Bob!" );
if ( NULL == bstr )
// out of memory error
// Use bstr here...
SysFreeString ( bstr );

Naturally, the various

BSTR

wrapper classes take care of the memory management for you.

The other type used in Automation interfaces is

VARIANT

. This is used to send data between typeless languages like JScript and VBScript, as well as Visual Basic in some cases. A

VARIANT

can contain data of many different types, such as

long

and

IDispatch*

. When a

VARIANT

contains a string, it is stored as a

BSTR

. I will have more to say about

VARIANT

s when I cover the

VARIANT

wrapper classes later.

String wrapper classes

Now that I've covered the various types of strings, I'll demonstrate the wrapper classes. For each one, I'll show how to construct an object and how to convert it to a C-style string pointer. The C-style pointer is often necessary for an API call, or to construct an object of a different string class. I will not cover other operators the classes provide, such as sorting or comparison.

Once again, do not blindly cast objects unless you understand exactly what the resulting code will do.

Classes provided by the CRT

_bstr_t

_bstr_t

is a complete wrapper around a

BSTR

, and in fact it hides the underlying

BSTR

. It provides various constructors, as well as operators to access the underlying C-style string. However, there is no operator to access the

BSTR

itself, so a

_bstr_t

cannot be passed as an

[out]

parameter to COM methods. If you need a

BSTR*

to use as a parameter, it is easier to the ATL class

CComBSTR

.

A

_bstr_t

can be passed to a function that takes a

BSTR

, but only because of three coincidences. First,

_bstr_t

has a conversion function to

wchar_t*

; second,

wchar_t*

and

BSTR

appear the same to the compiler because of the definition of

BSTR

; and third, the

wchar_t*

that a

_bstr_t

keeps internally points to a block of memory that follows the

BSTR

format. So even though there is no documented conversion to

BSTR

, it happens to work.

// Constructing
_bstr_t bs1 = "char string";       // construct from a LPCSTR
_bstr_t bs2 = L"wide char string"; // construct from a LPCWSTR
_bstr_t bs3 = bs1;                 // copy from another _bstr_t
_variant_t v = "Bob";
_bstr_t bs4 = v;                   // construct from a _variant_t that has a string
// Extracting data
LPCSTR psz1 = bs1;              // automatically converts to MBCS string
LPCSTR psz2 = (LPCSTR) bs1;     // cast OK, same as previous line
LPCWSTR pwsz1 = bs1;            // returns the internal Unicode string
LPCWSTR pwsz2 = (LPCWSTR) bs1;  // cast OK, same as previous line
BSTR    bstr = bs1.copy();      // copies bs1, returns it as a BSTR
// ...
SysFreeString ( bstr );

Note that

_bstr_t

also has conversion operators for

char*

and

wchar_t*

. This is a questionable design, because even though those are non-constant string pointers, you must not use those pointers to modify the buffer, because that could break the internal

BSTR

structure.

_variant_t

_variant_t

is a complete wrapper around a

VARIANT

, and provides many constructors and conversion functions to operate on the multitude of types that a

VARIANT

can contain. I will only cover the string-related operations here.

// Constructing
_variant_t v1 = "char string";       // construct from a LPCSTR
_variant_t v2 = L"wide char string"; // construct from a LPCWSTR
_bstr_t bs1 = "Bob";
_variant_t v3 = bs1;                 // copy from a _bstr_t object
// Extracting data
_bstr_t bs2 = v1;           // extract BSTR from the VARIANT
_bstr_t bs3 = (_bstr_t) v1; // cast OK, same as previous line

Note that the

_variant_t

methods can throw exceptions if the type conversion cannot be made, so be prepared to catch

_com_error

exceptions.

Also note that there is no direct conversion from

_variant_t

to an MBCS string. You will need to make an interim

_bstr_t

variable, use another string class that provides the Unicode to MBCS conversion, or use an ATL conversion macro.

Unlike

_bstr_t

, a

_variant_t

can be passed directly as a parameter to a COM method.

_variant_t

derives from the

VARIANT

type, so passing a

_variant_t

in place of a

VARIANT

is allowed by C++ language rules.

STL classes

STL just has one string class,

basic_string

. A

basic_string

manages a zero-terminated array of characters. The character type is given in the

basic_string

template parameter. In general, a

basic_string

should be treated as an opaque object. You can get a read-only pointer to the internal buffer, but any write operations must use

basic_string

operators and methods.

There are two predefined specializations for

basic_string

:

string

, which contains

char

s, and

wstring

, which contains

wchar_t

s. There is no built-in

TCHAR

specialization, but you can use the one listed below.

// Specializations
typedef basic_string<TCHAR> tstring; // string of TCHARs
// Constructing
string str = "char string";         // construct from a LPCSTR
wstring wstr = L"wide char string"; // construct from a LPCWSTR
tstring tstr = _T("TCHAR string");  // construct from a LPCTSTR
// Extracting data
LPCSTR psz = str.c_str();    // read-only pointer to str's buffer
LPCWSTR pwsz = wstr.c_str(); // read-only pointer to wstr's buffer
LPCTSTR ptsz = tstr.c_str(); // read-only pointer to tstr's buffer

Unlike

_bstr_t

, a

basic_string

cannot directly convert between character sets. However, you can pass the pointer returned by

c_str()

to another class's constructor if the constructor accepts the character type, for example:

// Example, construct _bstr_t from basic_string
_bstr_t bs1 = str.c_str();  // construct a _bstr_t from a LPCSTR
_bstr_t bs2 = wstr.c_str(); // construct a _bstr_t from a LPCWSTR

ATL classes

CComBSTR

CComBSTR

is ATL's

BSTR

wrapper, and is more useful in some situations than

_bstr_t

. Most notably,

CComBSTR

allows access to the underlying

BSTR

, which means you can pass a

CComBSTR

object to COM methods, and the

CComBSTR

object will automatically manage the

BSTR

memory for you. For example, say you wanted to call methods of this interface:

// Sample interface:
struct IStuff : public IUnknown
{
// Boilerplate COM stuff omitted...
STDMETHOD(SetText)(BSTR bsText);
STDMETHOD(GetText)(BSTR* pbsText);
};

CComBSTR

has an

operator BSTR

method, so it can be passed directly to

SetText()

. There is also an

operator &

that returns a

BSTR*

, so you can use the

&

operator on a

CComBSTR

object to pass it to a function that takes a

BSTR*

.

CComBSTR bs1;
CComBSTR bs2 = "new text";
pStuff->GetText ( &bs1 );       // ok, takes address of internal BSTR
pStuff->SetText ( bs2 );        // ok, calls BSTR converter
pStuff->SetText ( (BSTR) bs2 ); // cast ok, same as previous line

CComBSTR

has similar constructors to

_bstr_t

, however there is no built-in converter to an MBCS string. For that, you can use an ATL conversion macro.

// Constructing
CComBSTR bs1 = "char string";       // construct from a LPCSTR
CComBSTR bs2 = L"wide char string"; // construct from a LPCWSTR
CComBSTR bs3 = bs1;                 // copy from another CComBSTR
CComBSTR bs4;
bs4.LoadString ( IDS_SOME_STR );  // load string from string table
// Extracting data
BSTR bstr1 = bs1;        // returns internal BSTR, but don't modify it!
BSTR bstr2 = (BSTR) bs1; // cast ok, same as previous line
BSTR bstr3 = bs1.Copy(); // copies bs1, returns it as a BSTR
BSTR bstr4;
bstr4 = bs1.Detach();  // bs1 no longer manages its BSTR
// ...
SysFreeString ( bstr3 );
SysFreeString ( bstr4 );

Note that in the last example, the

Detach()

method is used. After calling that method, the

CComBSTR

object no longer manages its

BSTR

or the associated memory. That's why the

SysFreeString()

call is necessary on

bstr4

.

As a footnote, the

operator &

override means you can't use

CComBSTR

directly in some STL collections, such as

list

. The collections require that the

&

operator return a pointer to the contained class, but applying

&

to a

CComBSTR

returns a

BSTR*

, not a

CComBSTR*

. However, there is an ATL class to overcome this,

CAdapt

. For example, to make a list of

CComBSTR

, declare it like this:

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

CAdapt

provides the operators required by the collection, but it is invisible to your code; you can use

bstr_list

just as if it were a list of

CComBSTR

.

CComVariant

CComVariant

is a wrapper around a

VARIANT

. However, unlike

_variant_t

, the

VARIANT

is not hidden, and in fact you need to access the members of the

VARIANT

directly.

CComVariant

provides many constructors to operate on the multitude of types that a

VARIANT

can contain. I will only cover the string-related operations here.

// Constructing
CComVariant v1 = "char string";       // construct from a LPCSTR
CComVariant v2 = L"wide char string"; // construct from a LPCWSTR
CComBSTR bs1 = "BSTR bob";
CComVariant v3 = (BSTR) bs1;          // copy from a BSTR
// Extracting data
CComBSTR bs2 = v1.bstrVal;            // extract BSTR from the VARIANT

Unlike

_variant_t

, there are no conversion operators to the various

VARIANT

types. As shown above, you must access the

VARIANT

members directly and ensure that the

VARIANT

holds data of the type you expect. You can call the

ChangeType()

method if you need to convert a

CComVariant

's data to a

BSTR

.

CComVariant v4 = ... // Init v4 from somewhere
CComBSTR bs3;
if ( SUCCEEDED( v4.ChangeType ( VT_BSTR ) ))
bs3 = v4.bstrVal;

As with

_variant_t

, there is no direct conversion to an MBCS string. You will need to make an interim

_bstr_t

variable, use another string class that provides the Unicode to MBCS conversion, or use an ATL conversion macro.

ATL conversion macros

ATL's string conversion macros are a very convenient way to convert between character encodings, and are especially useful in function calls. They are named according to the scheme

[source type]2[new type]

or

[source type]2C[new type]

. Macros named with the second form convert to a constant pointer (thus the "C" in the name). The type abbreviations are:

A: MBCS string,

char*

(A for ANSI)
W: Unicode string,

wchar_t*

(W for wide)
T:

TCHAR

string,

TCHAR*

OLE:

OLECHAR

string,

OLECHAR*

(in practice, equivalent to W)
BSTR:

BSTR

(used as the destination type only)

So, for example,

W2A()

converts a Unicode string to an MBCS string, and

T2CW()

converts a

TCHAR

string to a constant Unicode string.

To use the macros, first include the atlconv.h header file. You can do this even in non-ATL projects, since that header file has no dependencies on other parts of ATL, and doesn't require a

_Module

global variable. Then, when you use a conversion macro in a function, put the

USES_CONVERSION

macro at the beginning of the function. This defines some local variables used by the macros.

When the destination type is anything other than

BSTR

, the converted string is stored on the stack, so if you want to keep the string around for longer than the current function, you'll need to copy the string into another string class. When the destination type is

BSTR

, the memory is not automatically freed, so you must assign the return value to a

BSTR

variable or a

BSTR

wrapper class to avoid memory leaks.

Here are some examples showing various conversion macros:


Collapse

// Functions taking various strings:
void Foo ( LPCWSTR wstr );
void Bar ( BSTR bstr );
// Functions returning strings:
void Baz ( BSTR* pbstr );
#include <atlconv.h>
main()
{
using std::string;
USES_CONVERSION;    // declare locals used by the ATL macros
// Example 1: Send an MBCS string to Foo()
LPCSTR psz1 = "Bob";
string str1 = "Bob";
Foo ( A2CW(psz1) );
Foo ( A2CW(str1.c_str()) );
// Example 2: Send a MBCS and Unicode string to Bar()
LPCSTR psz2 = "Bob";
LPCWSTR wsz = L"Bob";
BSTR bs1;
CComBSTR bs2;
bs1 = A2BSTR(psz2);         // create a BSTR
bs2.Attach ( W2BSTR(wsz) ); // ditto, assign to a CComBSTR
Bar ( bs1 );
Bar ( bs2 );
SysFreeString ( bs1 );      // free bs1 memory
// No need to free bs2 since CComBSTR will do it for us.
// Example 3: Convert the BSTR returned by Baz()
BSTR bs3 = NULL;
string str2;
Baz ( &bs3 );          // Baz() fills in bs3
str2 = W2CA(bs3);      // convert to an MBCS string
SysFreeString ( bs3 ); // free bs3 memory
}

As you can see, the macros are very handy when passing parameters to a function if you have a string in one format and the function takes a different format.

MFC classes

CString

An MFC

CString

holds

TCHAR

s, so the exact character type depends on the preprocessor symbols you have defined. In general, a

CString

is like an STL

string

, in that you should treat it as an opaque object and modify it only with

CString

methods. One nice advantage

CString

has over the STL

string

is that it has constructors that accept both MBCS and Unicode strings, and it has a converter to

LPCTSTR

, so you can pass a

CString

object directly to a function that accepts an

LPCTSTR

; there is no

c_str()

method you have to call.

// Constructing
CString s1 = "char string";  // construct from a LPCSTR
CString s2 = L"wide char string";  // construct from a LPCWSTR
CString s3 ( ' ', 100 );  // pre-allocate a 100-byte buffer, fill with spaces
CString s4 = "New window text";
// You can pass a CString in place of an LPCTSTR:
SetWindowText ( hwndSomeWindow, s4 );
// Or, equivalently, explicitly cast the CString:
SetWindowText ( hwndSomeWindow, (LPCTSTR) s4 );

You can also load a string from your string table. There is a

CString

constructor that will do it, along with

LoadString()

. The

Format()

method can optionally read a format string from the string table as well.

// Constructing/loading from string table
CString s5 ( (LPCTSTR) IDS_SOME_STR );  // load from string table
CString s6, s7;
// Load from string table.
s6.LoadString ( IDS_SOME_STR );
// Load printf-style format string from the string table:
s7.Format ( IDS_SOME_FORMAT, "bob", nSomeStuff, ... );

That first constructor looks odd, but that is actually the documented that way to load a string.

Note that the only legal cast you can apply to a

CString

is a cast to

LPCTSTR

. Casting to an

LPTSTR

(that is, a non-

const

pointer) is wrong. Getting in the habit of casting a

CString

to an

LPTSTR

will only hurt yourself, as when the code does break later on, you might not see why, because you used the same code elsewhere and it happened to work. The correct way to get a non-const pointer to the buffer is the

GetBuffer()

method.

As an example of the correct usage, consider the case of setting the text of an item in a list control:

CString str = _T("new text");
LVITEM item = {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();  // return control of the buffer to str

The

pszText

member is an

LPTSTR

, a non-

const

pointer, therefore you call

GetBuffer()

on

str

. The parameter to

GetBuffer()

is the minimum length you want

CString

to allocate for the buffer. If for some reason you wanted a modifiable buffer large enough to hold 1K

TCHAR

s, you would call

GetBuffer(1024)

. Passing 0 as the length just returns a pointer to the current contents of the string.

The crossed-out line above will compile, and it will even work, in this case. But that doesn't mean the code is correct. By using the non-

const

cast, you're breaking object-oriented encapsulation and assuming something about the internal implementation of

CString

. If you make a habit of casting like that, you will eventually run into a case where the code breaks, and you'll wonder why it isn't working, because you use the same code everywhere else and it (apparently) works.

You know how people are always complaining about how buggy software is these days? Bugs are caused by the programmers writing incorrect code. Do you really want to write code you know is wrong, and thus contribute to the perception that all software is buggy? Take the time to learn the correct way of using a

CString

and have your code work 100% of the time.

CString

also has two functions that create a

BSTR

from the

CString

contents, converting to Unicode if necessary. They are

AllocSysString()

and

SetSysString()

. Aside from the

BSTR*

parameter that

SetSysString()

takes, they work identically.

// Converting to BSTR
CString s5 = "Bob!";
BSTR bs1 = NULL, bs2 = NULL;
bs1 = s5.AllocSysString();
s5.SetSysString ( &bs2 );
// ...
SysFreeString ( bs1 );
SysFreeString ( bs2 );

COleVariant

COleVariant

is pretty similar to

CComVariant

.

COleVariant

derives from

VARIANT

, so it can be passed to a function that takes a

VARIANT

. However, unlike

CComVariant

,

COleVariant

only has an

LPCTSTR

constructor. There are not separate constructors for

LPCSTR

and

LPCWSTR

. In most cases this is not a problem, since your strings will likely be

LPCTSTR

s anyway, but it is a point to be aware of.

COleVariant

also has a constructor that accepts a

CString

.

// Constructing
CString s1 = _T("tchar string");
COleVariant v1 = _T("Bob"); // construct from an LPCTSTR
COleVariant v2 = s1; // copy from a CString

As with

CComVariant

, you must access the

VARIANT

members directly, using the

ChangeType()

method if necessary to convert the

VARIANT

to a string. However,

COleVariant::ChangeType()

throws an exception if it fails, instead of returning a failure

HRESULT

code.

// Extracting data
COleVariant v3 = ...; // fill in v3 from somewhere
BSTR bs = NULL;
try
{
v3.ChangeType ( VT_BSTR );
bs = v3.bstrVal;
}
catch ( COleException* e )
{
// error, couldn't convert
}
SysFreeString ( bs );

WTL classes

CString

WTL's

CString

behaves exactly like MFC's

CString

, so refer to the description of the MFC

CString

above.

CLR and VC 7 classes

System::String

is the .NET class for handling strings. Internally, a

String

object holds an immutable sequence of characters. Any

String

method that supposedly manipulates the

String

object actually returns a new

String

object, because the original

String

is immutable. A peculiarity of

String

s is that if you have more than one

String

containing the same series, of characters all of them actually refer the same object. The Managed Extensions to C++ have a new string literal prefix

S

, which is used to represent a managed string literal.

// Constructing
String* ms = S"This is a nice managed string";

You can construct a

String

object by passing an unmanaged string, but this is slightly less efficient than when you construct a

String

object by passing a managed string. This is because all instances of identical

S

prefixed strings represent the same object, but this is not true for unmanaged strings. The following code will make this clear:

String* ms1 = S"this is nice";
String* ms2 = S"this is nice";
String* ms3 = L"this is nice";
Console::WriteLine ( ms1 == ms2 ); // prints true
Console::WriteLine ( ms1 == ms3);  // prints false

The right way to compare strings that may not have been created using

S

prefixed strings is to use the

String::CompareTo()

method as shown below:

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

Both the above lines will print 0, which means the strings are equal.

Converting between a

String

and the MFC 7

CString

is easy.

CString

has a converter to

LPCTSTR

and

String

has two constructors that take a

char*

and

wchar_t*

, therefore you can pass a

CString

straight to a

String

constructor.

CString s1 ( "hello world" );
String* s2 ( s1 );  // copy from a CString

Converting the other way works similarly:

String* s1 = S"Three cats";
CString s2 ( s1 );

This might puzzle you a bit, but it works because starting with VS.NET,

CString

has a constructor that accepts a

String

object:

CStringT ( System::String* pString );

For some speedy manipulations, you might sometimes want to access the underlying string:

String* s1 = S"Three cats";
Console::WriteLine ( s1 );
const __wchar_t __pin* pstr = PtrToStringChars(s1);
for ( int i = 0; i < wcslen(pstr); i++ )
(*const_cast<__wchar_t*>(pstr+i))++;
Console::WriteLine ( s1 );

PtrToStringChars()

returns a

const __wchar_t*

to the underlying string which we need to pin down as otherwise the garbage collector might move the string in memory while we are manipulating its contents.

Using string classes with printf-style formatting functions

You must pay careful attention when using string wrapper classes with

printf()

or any function that works the way

printf()

does. This includes

sprintf()

and its variants, as well as the

TRACE

and

ATLTRACE

macros. Because there is no type-checking done on the additional parameters to the functions, you must be careful to only pass a C-style string pointer, not a complete string object.

So for example, to pass a string in a

_bstr_t

to

ATLTRACE()

, you must explicitly write the

(LPCSTR)

or

(LPCWSTR)

cast:

_bstr_t bs = L"Bob!";
ATLTRACE("The string is: %s in line %d\n", (LPCSTR) bs, nLine);

If you forget the cast and pass the entire

_bstr_t

object, the trace message will display meaningless output, since what will be pushed on the stack is whatever internal data the

_bstr_t

variable keeps.

Summary of all the classes

The usual way of converting between two string classes is to take the source string, convert it to a C-style string pointer, and then pass the pointer to a constructor in the destination type. So here is a chart showing how to convert a string to a C-style pointer, and which classes can be constructed from C-style pointers.

Class	string type	convert to char* ?	convert to const char* ?	convert to wchar_t* ?	convert to const wchar_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	cast to _bstr_t 3	cast to _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	in MBCS builds, cast	no6	in Unicode builds, cast	no5	yes	yes
COleVariant	BSTR	no	no	no	yes4	yes4	in MBCS builds	in Unicode builds
1 Even though _bstr_t provides conversion operators to non- const pointers, modifying the underlying buffer may cause a GPF if you overrun the buffer, or a leak when the BSTR memory is freed. 2 A _bstr_t holds a BSTR internally in a wchar_t* variable, so you can use the const wchar_t* converter to retrieve the BSTR . This is an implementation detail, so use this with caution, as it may change in the future. 3 This will throw an exception if the data cannot be converted to a BSTR . 4 Use ChangeType() then access the bstrVal member of the VARIANT . In MFC, this will throw an exception if the data cannot be converted. 5 There is no BSTR conversion function, however the AllocSysString() method returns a new BSTR . 6 You can temporarily get a non-const TCHAR pointer using the GetBuffer() method.