I have written some C style routines that convert strings to doubles,
ints, etc. using the C++ string stream objects. Each of the methods
use a locally constructed istringstream object which uses the >>
operator to make the conversions. A typical method looks like:
double ToDouble( const string & str)
{
std::istringstream iss( str);
double num;
iss >> num;
return num;
}
I have heard that the C routines (sprintf and sscanf) are faster that
the corresponding C++ objects. I did a couple of test and found that:
* sprintf and ostreamstring are about the same in performance
(within a couple of percents).
* istringstream (ISS) takes about 82% longer than the sscanf
times. I tried a couple of variations where I used the ISS clear and
str methods instead of the constructor, but that didn't change the
performance. When I made the ISS object static, it took only 12%
longer than as sscanf. The times for each test for 5 million
conversions were:
ISS local 2.79 sec
ISS static 1.78 sec
sscanf 1.52 sec
The following static ISS mods made a big difference in performance and
shortened the run by 35%.
double ToDouble( const string & str)
{
static std::istringstream iss;
iss.clear(); iss.str( str);
double num;
iss >> num;
return num;
}
For some reason, running the default ISS constructor takes up a lot of
time. Why is this? I have looked at some of the gnu g++ header
source files, but nothing obvious shows up. Could this an issue with
the basic_stringbuf or basic_streambuf classes?
Any ideas on how to fix this? I realize an converter object could be
created with an ISS object, but at this point I prefer the ease of use
of the C style methods. I'm looking for ways to make the construction
of the ISS object much faster.
Is the solution with a different basic_streambuf class?
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