Qt - Write Speed - What's the fastest method to write a file in Qt?

Question

I already found some posts relating to Qt and writing a file with C++, but I didn't find a comparison of existing methods and a satisfying answer to the question "What's the fastest method writing a file in Qt?"...

The task: I need to write a whole bunch of double values (around 500.000 up to 1.000.000) to file to a file. Due to my program structure, this values are saved in a QList, which contains some QVectors (every QVector has same size). The QVectors contains the double values. Additionally, each column (each QList element is a column) has to be written with different precision. Each column is seperated with a \\t.

I tried several methods:

• QTextStream to a QFile with operator <<
• Directly writing to a QFile with QFile::write
• FILE and fwrite

In the end I got the best results with QTextStream and FILE using fwrite (nearly same speed), QFile with the QFile::write was a little bit slower (maybe 30%).

But I still think that there must be a faster method. For 500.000 double values my PC (Core2Duo) needs about 2s, and that's quite very long.

I also thought about using the Boost Karma C++ Library - can you recommend it? http://www.boost.org/doc/libs/1_51_0/libs/spirit/doc/html/spirit/karma.html

Answer 1

Assuming you are doing it like below, it is unlikely you will find anything faster than fwrite.

double values[1000] = {...};

fwrite(values, sizeof(double), 1000, f);