Experimental Physics and
Industrial Control System

Andrew Johnson <[email protected]> · Thu, 28 Jul 2005 18:00:30 -0500

Jeff Hill wrote:

I'm not sure that there is a compelling reason - that there needs to be as
much performance hit as one might initially think. Consider if the string
interface has a call in it "virtual StringInterface::copy ( const
StringInterface & stringIn )". If we had that, then the native string
storage interface could, in its copy virtual method, use the block copy
method in stringIn to copy into its private storage w/o need for a scratch
pad. What do you think?

I concede the point. I can also implement comparisons efficiently in a

similar fashion, *but* I need another member function that does a

comparison of part of the string with the contents of an external buffer

in order to do this.

BTW your StringInterface::copy() function is named assign() in all the

C++ standard library containers including std::string. I want to use

standard names where the operations match, as this makes our code easier

for others to understand.

Thanks for the DA string requirements. I don't accept that all of them

have to be implemented as string member functions, but I will make sure

I cover all your needs.

Ok, my next point:  char operator[] (size_t pos) const

I would like to provide indexed access to individual characters, so a

user can write myString[5] to get the 6th character of myString. If the

index is too big, operator[] will throw an exception, probably

std::out_of_range since that's what std::at() throws.

I realize that this is not going to be a very efficient way to scan a

long segmented string, especially if operator[] has to go through a list

of segments each time you call it to locate the one that actually

contains the 723rd character.

On implementations that use a single buffer however, the performance of

operator[] will be fast, as it requires just one comparison and one

addition (note that std::string doesn't even bother with the comparison;

behaviour is undefined if you try to access beyond the end of the string).

I think we should provide this method since many programmers will expect

to see something like it in a string API, but we should recommend that

it not be used on segmented strings where performance could be an issue.

I'm going to have to stop here; for the next 8 working days I'll be on

vacation, but will pick up again with this discussion when I get back.

- Andrew
--
Podiabombastic: The tendency to shoot oneself in the foot.

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System