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:
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#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:
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| #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.
Source
Utiliser Qt peut-il améliorer les performances ?
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