Experimental Physics and
Industrial Control System

"Jeff Hill" <[email protected]> · Wed, 27 Jul 2005 18:19:13 -0600

> Jeff: Would you accept something like this method in a string interface
> class?
> 
> protected:
>      virtual bool getSegment(size_t offset,
>                   const char * & segment, size_t & len) const = 0;

My reservation about this is # 29 in Effective C++ 2nd edition - avoid
returning handles to internal data. I think that these concerns are
magnified in systems that have a) dynamically created and deleted strings,
b) dynamically modifiable string values, c) dynamically modifiable string
storage sizes, and d) have multiple threads accessing shared strings.

> 
> If not, please describe how you would write an efficient comparison
> operator between two different segmented string implementations.
> 

In the String Segment interface we have the following:

    virtual StringDiff compare ( const StringSegment & ) const = 0;

My conclusion was that it would be easy, and relatively efficient enough (we
need to remind ourselves that nothing is particularly efficient when we use
strings), to inside of the implementation of striongSegment::compare()
attempt to downcast the incoming StringSegment typed parameter using
dynamic_cast to the local type deriving from interface StringSegment. If
that cast is successful we are comparing two implementations of the same
type that we have private access to and we can implement a very fast and
efficient version. If not then we drop back to a slower generic version that
requires copying into a scratchpad, but since that situation will occur only
when crossing between subsystems or when transitioning from an old string
implementation to a new one then perhaps the overhead would be acceptable.

PS: I would also like to see strings accessed through a pure virtual
interface class so that multiple implementations are possible. This is,
among other apple pie reasons, because DA is interfacing with the world, and
the world will use many different string implementations including
std::string.

Jeff

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System