El 31/01/2012 14:47, Stephan T. Lavavej escribió:
[Thorsten Ottosen]
However, consider const std::string& ext = iter->path().extension().string(); This is fine on windows, as a temporary is returned, and its lifetime is extended; on linux, extension() returns a temp path object, from where we get a reference. I'm not a 100% sure, but I think the C++ standard does not guarantee that this path object be kept alive.
* The Standard requires the temporary path to be destroyed at the semicolon.
* Replace string with wstring, and this becomes problematic on Windows.
* Additionally, returning const values like const string/const wstring inhibits move semantics.
I know I repeat myself too many times, but I can't resist! One of the issues I find with move semantics is that sometimes we assume returning by value is free. Returning by value objects with dynamic allocation avoids reusing those dynamic resources (specially in loops). I think returning by value has a nice syntax and properties (you don't need a default-constructed object when you need to create a new object instead of assigning it), but IMHO is premature pessimization. Although returning vectors by value might be a good idea for factories, and such things, it's always more efficient to pass a reference to a vector, and clear() + fill or assign, or even better, reusing already constructed values (which can be vectors of vectors). Using filesystem::path in a loop might trigger a lot of allocations that could be avoided if the string could be passed as an argument, reusing already allocated memory for each path. In this question I totally agree with Alexandrescu (skip until 41:00 if downloaded, or 1:41:00 in the online video according to my web browser): http://channel9.msdn.com/Shows/Going+Deep/C-and-Beyond-2011-C11-Panel-Scott-... Apart from the arguments explained by Alexandrescu and Meyers, I'd say that you must also avoid returning by value objects without dynamic allocation but big sizeof(T) (say std::array<T, 1000>, instead of returning std::vector<T>(1000)), you are going to waste a lot of stack space with that extra temporary. Ion