Re: Casting from a string

James Kanze <>
Fri, 5 Sep 2008 02:28:09 -0700 (PDT)
On Sep 5, 3:33 am, Alexander Dong Back Kim <>

On Sep 5, 1:15 am, James Kanze <> wrote:

The main reason I want to make the "complex" class cast-able
with std::string is I want to convert into a XML file format.
For example,

I'm not sure I understand the abstraction of your class.
Because of the name, I supposed that it was a complex number, in
which case, just about any support for external formats would be
out of place. If it is in fact an XMLComplex, which is used to
map between XML and std::complex, it's a different issue.
Still, conversion is *not* the way C++ handles this problem; you
should define such a class, and then << and >> operators on it.

If I have three instance of the class in a vector or some container

vector<Complex> vComplex;
Complex a, b, c;
a = 10;
b = "100";
c = 0;


SaveToXML(vComplex); // Saving function that generates an XML file


I expect the XML look something like...

    <Complex> "10" </Complex>
    <Complex> "100" </Complex>
    <Compelx> "0" </Complex>

And when you need to support BER, you'll add conversion to a BER
string, and when you need to support XDR, you'll add conversion
to an XDR string, and...

External formats should be separate behavior. You don't want to
mimic the behavior of an existing class (e.g. std::complex) just
to support some particular external format. Conceptually, they
really require some form of double dispatch: to format a value,
you dispatch on the type of the value and a type implementing
the format. In C++, this is usually done by using function or
operator overloading (since the types are normally known
statically anyway, and C++ doesn't support dynamic double
dispatch). Thus, the idiomatic way of supporting this would be
to define an XMLstream, and overload >> and << to input from and
output to this stream. (For input, this might not be the ideal
solution, since XML contains type information that you might
want to exploit dynamically.)

In the case of XML, because the actual stream is text, I'd
probably have the oXMLstream contain a pointer to an
std::ostream, which handled the lower level textual formatting.
So you might end up with functions like:

    template< typename T >
    operator<<( oXMLstream& dest, std::vector< T > const& vect )
        dest.raw() << "<Container>\n" ;
        dest.incrementIndentation() ;
        for ( std::vector< T >::const_iterator iter = vect.begin() ;
                iter != vect.end() ;
                ++ iter ) {
            dest << *iter ;
        dest.decrementIndentation() ;
        dest.raw() << "</Container>\n" ;
        return dest ;

    operator<<( oXMLstream& dest, Complex const& obj )
        dest.raw() << "<Complex> " << obj.value << " </Complex>\n" ;
        return dest ;

The constructor of oXMLstream would insert a filtering streambuf
between the ostream and its original streambuf, which takes care
of the indentation, with oXMLstream::incrementIndentation and
oXMLstream::decrementIndentation forwarding to this streambuf.
The destructor, of course, would remove the filtering streambuf.
(For more on filtering streambuf's, see and; the first
should be largely sufficient for what you are doing. And
there's significant code available for them at my site, or in
boost::iostream; enough so that you shouldn't have to write more
than about 10 lines of code to implement the filtering

Note that this technique allows you to output pre-existing
types (although I suspect that most of the time, you'll want to
wrap them anyway, to provide semantic specific entity labels).

In order to do this, in my opinion, std::string compatibility
would be the thing really nice to have for further
string-related operations. I'm sorry if my explanation is not
clear enough. If you think my approach is wrong or you think
there are other approaches for doing this kind of thing,
please feel absolutely free to tell and let me know about
them. =)

Well, first, implicit conversions will cause problems in the
long run. And the C++ idiom for formatting doesn't use
std::string, but std::ostream (which, of course, can be an
std::ostringstream). IMHO, this provides all the chaining you
need, and is a lot more flexible than implicit conversions.

James Kanze (GABI Software)
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