Converting wide char string to lowercase in C++

c++ lowercase widestring
Nitramk picture Nitramk · Oct 23, 2009 · Viewed 16.2k times · Source

How do I convert a wchar_t string from upper case to lower case in C++?

The string contains a mixture of Japanese, Chinese, German and Greek characters.

I thought about using towlower...

http://msdn.microsoft.com/en-us/library/8h19t214%28VS.80%29.aspx

.. but the documentation says that:

The case conversion of towlower is locale-specific. Only the characters relevant to the current locale are changed in case.

Edit: Maybe I should describe what I'm doing. I receive a Unicode search query from a user. It's originally in UTF-8 encoding, but I'm converting it to a widechar (I may be wrong on the wording). My debugger (VS2008) correctly shows the Japanese, German, etc characters in in the "variable quick watch". I need to go through another set of data in Unicode and find matches of the search string. While this is no problem for me to do when the search is case sensitive, it's more problematic to do it case insensitive. My (maybe naive) approach to solve the problem would be to convert all input data and output data to lower case and then compare it.

Answer

Jonathan Leffler picture Jonathan Leffler · Oct 23, 2009

If your string contains all those characters, the codeset must be Unicode-based. If implemented properly, Unicode (Chapter 4 'Character Properties') defines character properties including whether the character is upper case and the lower case mapping, and so on.

Given that preamble, the towlower() function from <wctype.h> is the correct tool to use. If it doesn't do the job, you have a QoI (Quality of Implementation) problem to discuss with your vendor. If you find the vendor unresponsive, then look at alternative libraries. In this case, you might consider ICU (International Components for Unicode).

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