pros and cons of returning const ref to string instead of string by value
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From: Martin B. on 3 Dec 2009 22:57 Zachary Turner wrote: > On Dec 3, 2:36 am, Andrew <marlow.and...(a)googlemail.com> wrote: >> On 2 Dec, 22:00, Zachary Turner <divisorthe...(a)gmail.com> wrote: >> >>> On Dec 2, 7:06 am, Andrew <marlow.and...(a)googlemail.com> wrote: >>> Maybe I'm overlooking something, but I see little reason to return by >>> value. What matters is whether you assign it to a const reference or >>> to a value. >> The view I am running into is that a function should return a const >> ref to a string so that it can be assigned to a const ref to a string. >> The argument is that doing this avoids string copying. I should have >> made that clearer. >> > > Right. If you return by value, it always makes a copy. If you return > by const reference, it makes a copy when you want it to make a copy, > and doesn't make a copy when you don't want it to make a copy. > It is not generally true that "If you return by value, it always makes a copy." - the reason is RVO. In the presence of RVO, Initializing a string object with return-by-value is exactly the same as return-by-reference (Assigning is not however), i.e.: // e.g. with VC8 or later *without* optimizations Foo f; string s1("s1"); string s2("s2"); s1 = f.get_byval(); // Needs temporary! s2 = f.get_byref(); // No need for temporary string s3 = f.get_byval(); // Does NOT need temporary string s4 = f.get_byval(); // Does NOT need temporary br, Martin -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] |