What is the difference in using CStringW/CStringA and CT2W/CT2A to convert strings?

Question

In the past, I used CT2W and CT2A to convert string between Unicode & Ansi. Now It seems that CStringW and CStringA can also do the same task.

I write the following code snippet:

CString str = _T("Abc");
CStringW str1;
CStringA str2;
CT2W str3(str);
CT2A str4(str);

str1 = str;
str2 = str;

It seems CStringW and CStringA also perform conversions by using WideCharToMultibyte when I assign str to them.

So, what is the advantages of using CT2W/CT2A instead of CStringW/CStringA, since I have never heard of the latter pair. Neither MS recommend the latter pair to do the conversion.

Answer 1

CString offers a number of conversion constructors to convert between ANSI and Unicode encoding. They are as convenient as they are dangerous, often masking bugs. MFC allows you to disable implicit conversion by defining the _CSTRING_DISABLE_NARROW_WIDE_CONVERSION preprocessor symbol (which you probably should). Conversions always involve creating of a new CString object with heap-allocated storage (ignoring the short string optimization).

By contrast, the Cs2d macros (where s = source, d = destination) work on raw C-style strings; no CString instances are created in the process of converting between character encodings. A temporary buffer of 128 code units is always allocated on the stack, in addition to a heap-allocated buffer in case the conversion requires more space.

Both of the above perform a conversion with an implied ANSI codepage (either CP_THREAD_ACP or CP_ACP in case the _CONVERSION_DONT_USE_THREAD_LOCALE preprocessor symbol is defined). CP_ACP is particularly troublesome, as it's a process-global setting, that any thread can change at any time.

Which one should you choose for your conversions? Neither of the above. Use the EX versions instead (see string and text classes for a full list). Those are implemented as class templates that give you a lot more control you need to reliably perform your conversions. The template non-type parameter lets you control the static buffer. More importantly, those class templates have constructors with an explicit codepage parameter, so you can perform the conversion you want (including from and to UTF-8), and document your intent in code.