Re: non-const reference to temporary

From:

Seungbeom Kim <musiphil@bawi.org>

Newsgroups:

comp.lang.c++.moderated

Date:

Thu, 4 Sep 2008 03:29:09 CST

Message-ID:

<g9mutr$8v3$1@news.stanford.edu>

Dave Harris wrote:

chris.morley@lineone.net (Chris Morley) wrote (abridged):

You can cast like blargg says but (if I had to and didn't use MS
compilers) I'd do it like this rather than embedded in the call...

void WriteEverythingToFile( const File& file, const std::string&
text)
{
File& myfile = const_cast<File&>(file);
myfile.write(text);
}

An alternative is to just overload the function to take a filename:

void WriteEverythingToFile( File &file, const std::string &text );

void WriteEverythingToFile( const char *name, const std::string &text ) {
    File myfile( name );
    WriteEverythingToFile( myFile, text );
}

Now you can call with:
    WriteEverythingToFile( "filename.txt",
             std::string("stuff to write") );

which makes the short case even shorter, and doesn't involve unsafe
const_casts, mutable etc.

That's not always possible or desirable; in fact, I often find it
much more flexible to have the subroutine accept a stream reference
parameter (std::istream& or std::ostream&) instead of a filename,
because that allows the subroutine to work on any kind of streams.
A typical pattern occurring very often looks like this:

void subroutine(std::istream&);

int main(int argc, char** argv)
{
     if (argc < 2)
         subroutine(std::cin);
     else while (++argv, --argc) {
         if (std::strcmp(*argv, "-") == 0)
             subroutine(std::cin);
         else {
             std::ifstream f(*argv);
             if (f)
                 subroutine(f);
             else
                 std::cerr << *argv << ": " << strerror(errno) << '\n';
         }
     }
}

where you can call subroutine() with either a std::ifstream object
or std::cin.

--
Seungbeom Kim

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