From: "Eduardo Chao" <efchao@hotmail.com>
I am a new suscriber to the list. I want to report what I consider a bug in lexical_cast conversion template. First, I wanted to use it to convert things to std::string (Target). It throws a bad_lexical_cast_exception when I pass an empty std::string object (one that is default-constructed or constructed with "") as the Source argument. It throws it because the condition (interpreter >> result ) evaluates to false.
Yes, this is a bug in the current version, see (http://www.boost.org/libs/conversion/lexical_cast.htm#future). There's a proposition which fixes this and other problems here (http://groups.yahoo.com/group/boost/files/lexical_cast_proposition/lexical_ cast_proposition.zip). It's scheduled to be reviewed by the author, Kevlin Henney, or Björn Karlsson (Boost's maintenance wizard), before Boost release 1.30. If they find it ok, a notice will also be posted at the Boost list, so that others may give feedback before the release, as well.
My solution was to specialize the function template this way:
template<> lexical_cast<std::string>(const std::string &str) { return str; }
The proposition handles it in a similar way. However, it's not possible to partially specialise function templates (e.g. to handle all T -> std::basic_string conversions), so function template overloading is used, instead. This is because it needs to handle conversion from any type to std::basic_string, as there may be other types which output empty strings or whitespace, as well.
but it is not so general to support const std::string or const std::string& as Target type or perhaps std::string or const std::string etc as Source. Correct me if I am wrong, so I learn from you more experienced Boost (and then C++) users.
No, you're right. However, it's only necessary to handle T -> std::basic_string (not cv-qualification or reference) in this case, because: lexical_cast<std::string>(...); // Ok lexical_cast<const std::string>(...); // Error, can't write to a const string lexical_cast<const std::string &>(...); // Error, can't return a reference to the result (it would then return a dangling reference, as the result would be a temporary) lexical_cast<std::string>(const_cast<const std::string>(...)); // Ok, top-level cv-qualifiers are ignored when matching the call Regards, Terje