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With or Without Qt

Published on Monday August 24, 2009 by André in Qt | Comments

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:

Property   Without Qt   With Qt   Ratio
Code to type   3 lines   1 line   3.0
  147 char   35 chars   4.2
Code usable as sub-expression   no   yes
Size of compilation unit [1]   22215 lines   7590 lines   2.9
Compile time [2]   1.64s   1.02s   1.6

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:

Chunk count   Without Qt   With Qt   Ratio
10 chunks     18,455   9,827   1.9
100 chunks     134,578   71,008   1.9
1000 chunks     1,244,425   641,174   1.9
10000 chunks     13,161,115   7,053,633   1.9

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.

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