An Introduction to Unicode

This post extracts some knowledge from Programming Windows Chapter 2 – An Introduction to Unicode.

Maintaining a Single Source

One answer is to use the TCHAR.H header file included with Microsoft Visual C++. This header file is not part of the ANSI C standard, so every function and macro definition defined therein is preceded by an underscore. TCHAR.H provides a set of alternative names for the normal run-time library functions requiring string parameters (for example, _tprintf and _tcslen ). These are sometimes referred to as “generic” function names because they can refer to either the Unicode or non-Unicode versions of the functions.

If an identifier named _UNICODE is defined and the TCHAR.H header file is included in your program, _tcslen is defined to be wcslen:

#define _tcslen wcslen

If UNICODE isn’t defined, _tcslen is defined to be strlen:

#define _tcslen strlen

If the _UNICODE identifier is defined, a macro called __T is defined like this:

#define __T(x) L##x

This is fairly obscure syntax, but it’s in the ANSI C standard for the C preprocessor. That pair of number signs is called a “token paste,” and it causes the letter L to be appended to the macro parameter. Thus, if the macro parameter is “Hello!”, then L##x is L"Hello!".

If the _UNICODE identifier is not defined, the __T macro is simply defined in the following way:

#define __T(x) x

Regardless, two other macros are defined to be the same as __T:

#define _T(x) __T(x)
#define _TEXT(x) __T(x)

UNICODE vs _UNICODE

In Microsoft Visual Studio, _UNICODE is a preprocessor macro that is typically used with UNICODE to control whether to use the wide character (UTF-16) version of the Windows API.

In Visual Studio: Enabling the Unicode character set will automatically add the UNICODE;_UNICODE macro.