With or Without Qt

I wanted to do a certain bit of benchmarking for quite a while - years actually - but triggered by one of those friendly discussions on the ##c++ channel on FreeNode I finally figured I should sit down and actually do it. I was expecting some interesting results, but the not at the scale that we will see below.

If you ask the resident channel bot on ##c++ how to tokenize a std::string you'll get offered the following (slightly compacted) code snippet:

  #include <sstream>

  #include <vector>

  #include <string>
  std::string str("abc:def");

  std::istringstream split(str);

  std::vector<std ::string> tokens;

  for (std::string each; std::getline(split, each, ':'); tokens.push_back(each));

Obviously, that's the "Without Qt" version: Clean Standard C++, as straight-forward as it can get. The contender "With Qt" is:

  #include <QString>

  #include <QStringList>
  QString str("abc:def");

  QStringList tokens = str.split(':');

From the source alone we can collect a few numbers:

To compare performance I use a benchmark that I just checked into the Qt source base, under tests/benchmark/qstringlist. It basically consists of running the above mentioned snippets on a string "unit:unit:unit:...." with 10, 100, 1000, and 10000 "unit" chunks and record callgrind's "instruction loads per iteration" as follows:

In this case, bigger numbers mean more time needed to execute. Interesting, isn't it?

After reading Thiago's latest posts I got the impression that people like conclusions. The verbose version of a conclusion might be something along the lines of

Using Qt's functions to split a string you need about a third of the effort to write code, and get a 47% performance boost at runtime.

Or shorter (Qt way): "Code less, create more".

André

[1] Counted with "g++ -E -DQT_NO_STL -I$QTDIR/include/QtCore -I$QTDIR/include qt.cpp | wc" using g++ Ubuntu 4.3.3-5ubuntu4.

[2] Fastest result of "real" time, out of 20 runs each. "user" times in both scenarios smaller, with similar ratio.