L prefix for strings in C++

Question

I have a static library. This library have the following function defined

int WriteData(LPTSTR s)

The sample to call the function is

Answer 1

Try:

[DllImport("Test.dll")]
private static extern int TestFun([MarshalAs(UnmanagedType.LPTStr)] string s);

More information on marshaling with the MarshalAsAttribute on MSDN.

Answer 2

The L prefix makes the string a wchar_t string. You can use the Windows API function MultiByteToWideChar to convert an ANSI string to a wchar_t string.

Answer 3

The specific "function" to perform the L prefix is a macro TEXT() or _T(). (TEXT is defined by the Windows SDK, _T is an extension of the Microsoft C Runtime).

These functions automatically add the L prefix when your project is built with unicode support on (which is the default for new projects now in most MS Dev environments) - or leave it off for non unicode (or ansi) configured projects.

Don't do this cast:

LPTSTR s = (LPTSTR) L"ABC";   // Working fine
     WriteData(s);

If the project was ever configured for non Unicode, then L"ABC" would still be an array of wide-characters, but LPTSTR would become a pointer to an array of 8bit characters.

This is how to correctly assign a string to an Ansi, Unicode, or "Text" string. (Text can be Ansi or Unicode depending on project settings) (I left off the L because its redundant, and added a C, because string literals should be constant).

PCSTR p1 = "ABC";
PCWSTR p2 = L"ABC";
PCTSTR p3 = TEXT("ABC");

Answer 4

I think you're confused, as your function should work just fine:

int TestFun(LPTSTR lpData)
{
   return  WriteData(lpData); // Should be happy
}

But when you call your function, you'll have to be careful:

TestFun((LPTSTR) L"ABC"); // Will work fine
TestFun((LPTSTR) "ABC");  // Will not work

This is because "ABC" and L"ABC" are two different things. If you look at them in memory:

"ABC"  | 65 66 67 00
L"ABC" | 65 00 66 00 67 00 00 00

Edited to add:

There is nothing like L prefix in .Net

This is just wrong. I just opened "New Project->C++->CLR Console" in VisualStudio, and the first line is:

Console::WriteLine(L"Hello World");

Answer 5

I would wrap your strings in the _T(...) macro. That way its portable between ANSI and UNICODE builds.

Note that you are using the portable string type - LPTSTR - note the T. It will change between ANSI and UNICODE based on build settings.