String class allocating on stack for small strings?

Question

Do anyone know if there is a STL interface compatible string class that allocates memory for small strings on the stack (up to a certain threshold) and the heap for larger strings ?

I'm looking to optimize a program and I'm using allot of small local strings that easily could fit on the stack, instead of being allocated on the heap.

Visual Studio has this optimization (for very small string, just a couple characters) while gcc preferred the Copy-On-Write. — Matthieu M.
– Matthieu M., Commented Mar 24, 2011 at 12:50
We use Visual Studio 2010 , is there anywhere you can read about this optimization (MSDN or elsewhere) ? — ROAR
– ROAR, Commented Mar 24, 2011 at 12:59
Most compilers will do that, if you have a recent enough version. — Bo Persson
– Bo Persson, Commented Mar 24, 2011 at 13:02
Looked it up and it appears to be 16 bytes in VS 2010 , no option to set it to anything else. — ROAR
– ROAR, Commented Mar 24, 2011 at 13:21

Community · Accepted Answer · 2017-05-23 12:04:13Z

2

You can provide a custom allocator for std::basic_string (it is the third template argument). This answer explains how use that and links to an implementation of a stack-allocator that can be used.

edited May 23, 2017 at 12:04

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answered Mar 24, 2011 at 12:36

Björn Pollex

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Pete Over a year ago

Some implementation of std::string do this, it's called the small string optimisation. A quick google should tell you if yours does or if there is an alternative you can use.

NoSenseEtAl · Accepted Answer · 2012-10-04 15:14:32Z

This is antique question, but I feel that this is better that any of the current answers.

http://llvm.org/docs/ProgrammersManual.html#dss_smallstring

Basically it is what you want. BTW tcmalloc increased perf in my (badly designed :D) string alloc intensive program 10%. Also you should profile to prove allocs are your perf problem.

Logan Capaldo · Accepted Answer · 2011-03-24 12:50:33Z

The vstring (__versa_string) implementation from gcc can do the small string optimization and has a std string interface. If you happen to be using gcc it's easy enough to include ext/vstring. Otherwise you may be able to adapt it to your compiler/enviroent.

ArtemGr · Accepted Answer · 2012-05-24 06:19:27Z

__versa_string SSO version can store no more than 15 bytes on the stack and if it does, it reserves 16 bytes regardless of the string size ( http://codepad.org/2M7N9cTu ).

http://www.and.org/ustr/ can reuse a stack buffer, but I had problems linking with it under Debian Wheezy 64bit.

http://freecode.com/projects/str-class can reuse a stack buffer.

I wrote a header-only string class which uses just four bytes and can reuse a stack buffer: http://code.google.com/p/libglim/source/browse/trunk/gstring.hpp
It has a limited STL compatibility: basic_streambuf implemented for use with std::ostream.

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