Re: [boost] [filesystem] Some questions about string use

From: Ion Gaztañaga (igaztanaga_at_[hidden])
Date: 2006-03-06 14:41:31

> In the interests of science, I wrote a small "file finder" using
> Boost.Filesystem and a comparable version using POSIX functions (e.g. stat,
> readdir, etc).  

Wow, that's a fast test implementation!

> Looking at the profiling output from the "finder-fs" program, it appears a
> bulk of the time is spent in fs::basic_path::operator/=() which may bear out
> Ion's fears.

Even if the string manipulation was not the most heavy part of the test, 
I would like to point out two ideas:

-> Even if the filesystem is slower than returning a temporary object, 
that doesn't mean that you shouldn't optimize it. Your program is not 
alone in the computer, so when your program is waiting for I/O (surely 
via DMA) other processes are taking CPU, but when you create a lot of 
temporary strings you are stressing the memory allocator (so you can 
have more fragmentation in a long live program) and degrading overall 
system performance. Maybe your program is not much slower, but the CPU 
consumption is much higher.

-> The good old C++ principle: "Pass heavy objects always by reference" 
is a very good advice to avoid performance problems. You can pass 
strings by value, but you never do that. But in my opinion this advice 
is also valid for the inverse sense: "Obtain heavy values always by 
reference". You never know how your code will be used, how many times 
your functions will be called. If you want your library to be useful 
even in situations that you don't predict, it's always helpful to use 
references. The difference is that passing parameters by reference does 
not change syntax whereas obtaining by values by reference is uglier.

Caleb, I don't know if this is too much work but, can we know how many 
calls to "new" this application makes or even better, how many 
std::strings it constructs?

Thanks and great job!

Ion