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 |
|---|---|
| Unicode character ( |
| MBCS or Unicode character, depending on preprocessor settings |
| string of |
| constant string of |
| string of |
| constant string of |
| string of |
| constant string of |
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 |
|---|---|
| Unicode character ( |
| string of |
| constant string of |
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 |
|---|---|
| Prepends |
| Prepends |
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
:
| | | | |
| | | | |
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 | convert to | convert to | convert to | convert to | construct from | construct from |
|---|---|---|---|---|---|---|---|---|
| | yes, cast1 | yes, cast | yes, cast1 | yes, cast | yes2 | yes | yes |
| | no | no | no | cast to | cast to | yes | yes |
| MBCS | no | yes, | no | no | no | yes | no |
| Unicode | no | no | no | yes, | no | no | yes |
| | no | no | no | yes, cast to | yes, cast | yes | yes |
| | no | no | no | yes4 | yes4 | yes | yes |
| | no6 | in MBCS builds, cast | no6 | in Unicode builds, cast | no5 | yes | yes |
| | no | no | no | yes4 | yes4 | in MBCS builds | in Unicode builds |
1 Even though memory is freed. 2 A . 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 . 4 Use . In MFC, this will throw an exception if the data cannot be converted. 5 There is no . 6 You can temporarily get a non-const |
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