Comments on: C++ Convert Int to String Speed

By: Roger

Roger — Thu, 07 Jul 2011 23:15:40 +0000

@6, Jeff:
Unfortunately Qt doesn’t have anything optimized. Qt is a library for easy and safe coding, it is far from optimized for speed. If anything Qt calls are MUCH slower than anything that’s mentioned above. Qt function calls are anywhere from 2x up to 10x slower than the examples above, depending on what function you use.

If you don’t believe me, do the tests yourself.
Also undocumented is the speed of some Qt functions. For examples from the QString class, the functions to convert to QByteArray.
::toLatin1() is actually the fastest, followed by ::toAscii() and the slowest being ::toLocal8Bit().

Qt has a lot, LOT of performance problems. If you’re only writing applications that execute 20function calls a second, Qt is great because it allows for agile development. However if you’re handling applications that execute millions of function calls every second, containing a few Qt calls in each of those functions, the performance problems become visible.

By: Tino Didriksen

Tino Didriksen — Tue, 23 Nov 2010 02:16:25 +0000

To get printf() formatting, I'd suggest using Boost Format. Whether that uses the fast Karma implementation, I dunno, but can read the documentation or ask Freenode's #boost.

By: QbProg

QbProg — Mon, 22 Nov 2010 09:25:20 +0000

Hello there,
I’m just converting my old int->string routines using Karma :)
A little question:
i need to implement double->string and

generate(Buf, double_ , d);

works well.

Is there a quick way to specify the number of decimals?
I was using
printf (“%0.3f”, myNumber)
to get 3 decimals numbers.

thank you!

By: Arash Partow

Arash Partow — Sun, 20 Jun 2010 22:39:19 +0000

Have you considered including StrTk in your listings?

The tests used in that library are a little more extensive.
http://www.codeproject.com/KB/recipes/Tokenizer.aspx

By: Jeff

Jeff — Thu, 25 Mar 2010 13:54:46 +0000

How about Qt? They generally have things optimized pretty well.

By: Jeff

Jeff — Fri, 12 Mar 2010 03:48:23 +0000

You should try against ‘fast format’ too, for completeness – Matt Wilson’s no slouch.

http://www.fastformat.org/performance.html

By: Tino Didriksen

Tino Didriksen — Thu, 11 Mar 2010 10:36:09 +0000

It has since been specialized for the basic integer and floating point types: http://www.boost.org/doc/libs/release/boost/lexical_cast.hpp (search for lcast_put_unsigned and below).

But for all other conversions, it is indeed dog slow. Just see http://tinodidriksen.com/2010/02/16/cpp-convert-string-to-int-speed/ where it trails by a huge margin.

By: Arzar

Arzar — Thu, 11 Mar 2010 10:04:34 +0000

I’m quite surprised to see lexical_cast performing this good too.

Most of all, if you define BOOST_LEXICAL_CAST_ASSUME_C_LOCALE the performance of lexical_cast increase even more. With MSCV10, I get ~30% instead of 70% !

(for reference, I get ~15% with itoa)

By: indranil banerjee

indranil banerjee — Wed, 10 Mar 2010 22:45:39 +0000

Interesting. Surprised to see that lexical_cast performed better than stringstream, I thought the former was implemented using the latter?

These results are quite contrary to the tests Sutter published back in 2001 http://www.gotw.ca/publications/mill19.htm. In those tests sprintf was the fastest, followed by strstream, stringstream & lexical_cast trailing. Have things changed so much?

By: Sumant

Sumant — Wed, 10 Mar 2010 15:36:32 +0000

Quite interesting! The performance results on g++ (4.4.3) hold as they are shown only when optimizations are turned on. With the increasing level of optimizations (-O1, -O2, -O3), performance improves.