When are data structures copied
in [C++]
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From: tohava on 9 Nov 2009 21:22 On Nov 9, 10:27 pm, SG <s.gesem...(a)gmail.com> wrote: > In C++0x you will be able to write > > m.emplace(move(s), move(v)); While I agree that this is the best solution, I think it would also make sense to have container::insert(const T &&value), which makes use of the move constructor, making m.insert(make_pair(move(s), move(v)) almost as efficient. (I do not remember if C++0X has this or not). -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: Bo Persson on 10 Nov 2009 04:26 Jeff Schwab wrote: > Jeff Flinn wrote: >> Do any implementations of STL implement copy-on-write >>> semantics for string or vector? >> >> MSVC(VC6)/Dinkumware used to do COW for std::string, but found that >> small string optimization provided better overall performance. > > How does that preclude CoW for non-small strings? It doesn't, it is a separate optimization, but with an overlap in effect for short strings. The original use of CoW for std::string was shown not to work in practice. Due to the semantics it becomes copy-on-potential-write, which is most often a net loss - especially for multi-threaded use. Herb Sutter did the measures that surprised most people: http://www.gotw.ca/gotw/045.htm Bo Persson -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: Seungbeom Kim on 10 Nov 2009 12:14
tohava wrote: > On Nov 9, 10:27 pm, SG <s.gesem...(a)gmail.com> wrote: >> In C++0x you will be able to write >> >> m.emplace(move(s), move(v)); > > While I agree that this is the best solution, I think it would also > make sense to have container::insert(const T &&value), which makes use > of the move constructor, making m.insert(make_pair(move(s), move(v)) > almost as efficient. (I do not remember if C++0X has this or not). The latest draft, N2960, has insert member functions that take rvalue references (P&&, not const P&&, though) for all containers that have insert member functions. -- Seungbeom Kim [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] |