Re: Efficient use of C++ Strings: Request for Comments
Scott McKellar wrote:
Having put up some web pages about how to use C++ strings
efficiently, I hereby invite my betters to rip them to shreds:
http://home.swbell.net/mck9/effstr/
In my job I see code written by other people. Much of it
is needlessly inefficient, where more efficient code would be
just as readable and maintainable. The string handling in
particular is bloated by the the unnecessary construction and
destruction of strings.
Which may or may not be that expensive, depending on the
implementation. I've never found it to be an important issue
when measured with a profiler.
In hopes of encouraging better practices, I Googled around for
recommendations on how to use strings efficiently, but I
didn't find much. So I wrote my own.
I don't claim to be an expert, but there are certainly
experts in this group. I'm hoping that someone will point
out whatever blunders I have committed so that I can correct
them.
Some of them are pratically anti-patterns; allocating strings
statically is a good way to get you into trouble, quickly, for
example. And if the function constructs a string, how do you
return it, if not by value. The operator+ and
std::string::substr are also very useful functions, and
certainly not to be avoided. In the implementations I'm
familiar with, std::string::empty() just calls
std::string::size(), so there's no advantage there. (It
typically isn't measurably faster than comparing with "", but it
does seem to say more clearly what you are testing.) And about
the only times I'll use C style character arrays is when order
of initialization or object lifetime is an issue.
As for the example you give immediately after the points, all of
the compilers I have access to generate exactly the same code
for the two cases, so I don't see how one could be faster than
the other.
Yes, I do realize that raw machine efficiency is not always
very important. But when it is important, or at least useful,
it's good to know how to achieve it.
But it's widely known how to achieve it: profile. You can't
improve efficiency until you know where the time is being spent.
--
James Kanze (Gabi Software) email: james.kanze@gmail.com
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