Is Win32 error code 122 somewhat benign in this case? - c++

Question

The following line results in GetLastError() return error code 122 (=ERROR_INSUFFICIENT_BUFFER)
CString str = CString("'") + _T("%s") + CString("'");
But this happens only under VS2005 and doesn't happen in VS2015. Still I see no memory corruption or anything in VS2005 and the str variable does contain the correct value. Is that still an issue to be concerned about give the error code?
The reason this seems to happen is because of concatenation of wide character and simple character strings and the fix is to simply sorround both remaining strings with _T("") so the code line would look like:
CString str = CString(_T("'")) + _T("%s") + CString(_T("'"));
But what does the error code 122 really means in original line when only one string was Unicode? What wrong has really happened or is it more like a warning in this case?

Answer 1

GetLastError() is only meaningful after some system call has returned an error. Since your code does not have any system call, GetLastError() can return anything.
Maybe the value you see is the last error from the last system call that failed. Or maybe it is some error that happened from inside the CString class, but it is handled there.
TL;DR; No error here.

Answer 2

In VS 2015 you can reproduce the error with CString("a") (if Unicode is set) or just CStringW("a")
#include <iostream>
#include <atlstr.h>
int main()
{
CStringW("a");
DWORD err = GetLastError();
std::cout << err << "\n"; //<= error 122, ERROR_INSUFFICIENT_BUFFER
return 0;
}
This happens because CString uses WinAPI MultiByteToWideChar to convert ANSI "a" to Unicode L"a". Debugging through the source code in "atlmfc\include\cstringt.h", we see that at some point it calls the following function:
static int __cdecl GetBaseTypeLength(_In_z_ LPCSTR pszSrc) throw()
{
// Returns required buffer size in wchar_ts
return ::MultiByteToWideChar( _AtlGetConversionACP(), 0, pszSrc, -1, NULL, 0 )-1;
}
For some reason there is a -1 at the end. I don't know why that's there, it might be necessary for other CString functions but in this case it ends up causing ERROR_INSUFFICIENT_BUFFER error in the next call to MultiByteToWideChar. The conversion can be roughly simplified to following:
int main()
{
int nDestLength = MultiByteToWideChar(CP_ACP, 0, "a", -1, NULL, 0) - 1;
wchar_t *pszDest = new wchar_t[32];
//ERROR_INSUFFICIENT_BUFFER occurs here because nDestLength is short by 1:
MultiByteToWideChar(CP_ACP, 0, "a", -1, pszDest, nDestLength);
DWORD err = GetLastError();
std::cout << err << "\n";
return 0;
}
nDestLength is too small because it doesn't account for null terminator. CString sorts this out later but the error remains. That's a good reason not to pay attention to GetLastError unless the function fails.
As you noted, this error can be avoided by using the _T macro, because CString would no longer need MultiByteToWideChar. Or better yet, use the L prefix, or CString::Format
CString str = CString(L"'") + L"%s" + CString(L"'");