I've noticed that calling std::string::reserve() in a shrink- to-fit request can sometimes end up allocating exactly the same amount of memory as the string already had.
This is because _S_create() rounds up allocations of over 128 bytes to 128 byte subpages, and of over 4k to 4k pages, for more efficient use of malloc(). When the reallocated block is the same size as the original one, the entire contents of the string are effectively being copied unnecessarily. Further shrink-to-fit calls keep doing it, too, since the requested capacity remains less than the actual capacity. NOTES: 1. I first noticed this in February 2004, and submitted a possible patch for it at the time - the original mailing list thread started here: http://gcc.gnu.org/ml/libstdc++/2004-02/msg00179.html Unfortunately, problems at my end caused the patch to get hung up on FSF assignment issues, so it couldn't go in. I've now resolved these issues, and Benjamin Kosnik has asked me to put the original report here in Bugzilla for further discussion. 2. Recent CVS (as of November 2004) no longer uses the 128 byte subpages, courtesy of Paolo Carlini's work to improve std::string. -- Summary: Shrink-to-fit std::string::reserve() calls can reallocate & copy string contents unnecessarily Product: gcc Version: 3.4.1 Status: UNCONFIRMED Severity: normal Priority: P2 Component: libstdc++ AssignedTo: unassigned at gcc dot gnu dot org ReportedBy: nferguso at eso dot org CC: gcc-bugs at gcc dot gnu dot org GCC build triplet: i686-pc-linux-gnu GCC host triplet: i686-pc-linux-gnu GCC target triplet: i686-pc-linux-gnu http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18654
