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Convert C++Builder AnsiString to std::string via boost::lexical_cast

Answer 1

A:

Your conversion converts one line of the input. That's a serious problem if there are two lines, and a rather fatal bug if there are none. The best solution in this case is not a new operator>>, but a template specialization. Top of my head:

template< > AnsiString lexical_cast<AnsiString, std::string>(std::string const& s)
{
    return AnsiString(s.c_str());
}

(You're not supposed to overload other peoples templates. In std::, it's illegal, elsewhere just bad practice)

MSalters 2009-05-22 14:48:53

@James: Of course you can specialize template functions - only partial specialization is not possible for them.

Georg Fritzsche 2010-05-30 18:26:12

Answer 2

+1 A:

Still not using boost, but may be a first step to solve your current problem. You may try this:

std::istream& operator>>(std::istream& istr, AnsiString& str) {
    istr.exceptions(std::ios::badbit | std::ios::failbit | std::ios::eofbit);
    std::string s;
    istr >> s;
    str = AnsiString(s.c_str());
    return istr;
}

EDIT: more complete solution, taking op's comments into account:

std::istream& operator>> (std::istream& istr, AnsiString& str) 
{ 
    std::string tmp;

    std::istreambuf_iterator<char> it(istr), end; 
    std::copy(it, end, std::inserter(tmp, tmp.begin())); 

    str = AnsiString(tmp.c_str());

    return istr; 
}

However, having different operator >> behavior for std::string and AnsiString is probably ok for your needs, but not very nice in general. You can still give it an explicit name.

Jem 2009-05-22 14:51:48

I think that will only copy a single word

James Hopkin 2009-05-22 15:04:39

Yes, but it may be the expected behaviour (or not). Reading the entire buffer could be done at an upper level.I do not know streams by heart, but it is probably possible here to even avoid the temporary string, by using stream.get()

Jem 2009-05-22 15:15:12

I switched from >> to getline() some time ago because I want to read the entire string. So reading the whole string is what I want :)

David Klein 2009-05-22 15:23:38

If I put the istr.exceptions(std::ios::badbit | std::ios::failbit | std::ios::eofbit); call into your edited suggestion it crashes with "ios_base::failbit set", strange :)

David Klein 2009-05-22 19:32:14

I think that it is normal. As you read everything in the stream, by definition you reach the end of it, and an exception is thrown. If you do not catch it (using try / catch), it causes the crash you're experiencing (a unhandled exception error ?).With my example, you should either not set the exceptions bits, or catch exceptions but only report those which are actually errors.

Jem 2009-05-22 21:32:10

Answer 3

A:

In addition to MSalters comments, It would be more efficient to pass the std::string's actual length to AnsiString as well, so that it does not need waste CPU cycles to calculating the length manually:

template<> System::AnsiString lexical_cast<System::AnsiString, std::string>(std::string const& s)
{
    return System::AnsiString(s.c_str(), s.length());
}

Remy Lebeau - TeamB 2009-06-01 20:16:21

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Convert C++Builder AnsiString to std::string via boost::lexical_cast

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