WinAPI: A and W functions

Most (all?) string related WinAPI functions come in two versions: FuncNameA and FuncNameW which are the ansi and wide character variant of the same function.

The FuncNameA variants are basically wrapper functions that convert strings from ansi to wide char representation and then call the corresponding FuncNamew.

I assume that the conversion from ansi to wide char is dependent on the current Language for Non-Unicode Programs.

Defining UNICODE

In order to use the wide character (FuncNameW) versions of WinAPI functions, the macro UNICODE must be defined before including <windows.h>.

For example, the include file WinUser.h contains the following lines (edited for better readability).

WINUSERAPI int WINAPI MessageBoxA(
    _In_opt_ HWND   hWnd,
    _In_opt_ LPCSTR lpText,
    _In_opt_ LPCSTR lpCaption,
    _In_     UINT   uType
);

WINUSERAPI int WINAPI MessageBoxW(
    _In_opt_ HWND    hWnd,
    _In_opt_ LPCWSTR lpText,
    _In_opt_ LPCWSTR lpCaption,
    _In_     UINT    uType
);

#ifdef UNICODE
  #define MessageBox  MessageBoxW
#else
  #define MessageBox  MessageBoxA
#endif // !UNICODE

Here, the two WinAPI functions MessageBoxA and MessageBoxB are defined. The ansi version has LPCSTR parameters while the wide character version has LPCWSTR parameters.

Then, depending whether UNICODE is defined, the macro MessageBox either expands to MessageBoxA or MessageBoxB.

Now, when MessageBox (or any other WinAPI function that takes a string) is called, the strings should be initialized with the TEXT macro (which depending on the defined value of UNICODE) creates an ansi char* or wide character wchar_t* string.

WinAPI: A and W functions

Defining UNICODE

See also