From: Rob Stewart (stewart_at_[hidden])
Date: 2004-06-03 11:50:09
From: "Reece Dunn" <msclrhd_at_[hidden]>
> Martin wrote:
> >1. The abstract base class.
> >Having an ABC to handle fixed strings of various lengths is a good idea but
> >is not very practical. I understand that the main purpose is to allow fixed
> >strings to be passed to functions without using templates.
> >int countwords(const boost::char_string & str);
> >unfortunatly the ABC makes it difficult to pass anything but fixed strings.
> >you want to pass a string literal, a character array or a std::string you
> >create a temporary fixed_string<>.
> >countwords(boost::fixed_string<20>("The quick brown fox"));
> This is an unfortunate side-affect of using the ABC technique that affects
> John's implementation as well. Your proposed solution looks interesting, but
> I don't know how practical it will be for the string manipulation operations
> (w.r.t. buffer safety).
> Rob proposed a "make_string" function, which would simplify this a little.
There's nothing to be done here, except to forcibly create an
appropriate derived instance of char_string when calling such
functions. It doesn't have to be computed manually, though, as
Reece has alluded to. The make_string() I suggested would cause
the compiler to deduce N for fixed_string<N>. Unfortunately,
that object would have to be returned by value or via a smart
pointer. Either way, it would contain a copy of the original
To do more would require that the strings be kept in the free
store. Then, the base class would contain a pointer to its
implementation object which would be a fixed_string<N>. Thus,
you could add a parameterized ctor to char_string that could
instantiate the appropriate fixed_string<N> against that pointer.
That's not so terrible, but it doesn't leave fixed_string with
much benefit over std::string!
Martin's suggestion of having char_string maintain a possible
null pointer would entail every function having to determine
whether to forward to the derived class or interact with the
buffer referenced by the pointer. That would add quite a bit of
overhead to each operation and certainly complicate matters.
> >2. Substrings
> >One of the applications I can see for fixed_string is when you want to
> >dynamic allocation but still want to use the std::string interface and
> >algorithms. In that perspective it seems strange that substr returns a
> >std::string instead of a fixed_string.
> This is related to the above problem. You cannot create an instance of
> boost::char_string, etc. and as such cannot implement substr in the usual
> way (this is the proplem I was having with operator+). The solution would be
> to defione an alternate substr function in fixed_string, but this would not
> solve the problem for boost::char_string, etc.
> The alternative would be to use a ranged string (pointer to the fixed_string
> buffer and length), but the problem with this is null-termination.
> I do not see an easy way around this without removing the ABC design, since
> it is not possible to do fixed_string< length() > or something similar.
Couldn't substr() be a pure virtual function in char_string that
deferred to the derived class to create a copy of itself? The
derived class can just create a duplicate of itself, including
the current capacity. That capacity is guarranteed to be
sufficient to hold any substring requested.
-- Rob Stewart stewart_at_[hidden] Software Engineer http://www.sig.com Susquehanna International Group, LLP using std::disclaimer;
Boost list run by bdawes at acm.org, gregod at cs.rpi.edu, cpdaniel at pacbell.net, john at johnmaddock.co.uk