Re: What's your preferred way of returning a list of items?
On May 13, 8:07 pm, "Leigh Johnston" <le...@i42.co.uk> wrote:
"James Kanze" <james.ka...@gmail.com> wrote in message
news:cf223765-b6e6-4aa4-9cc8-8acfe67457de@e1g2000yqe.googlegroups.com...
I'm talking about actual use. I know that both g++ and VC++
do RVO---I found a bug in the code generation for RVO in
VC++, remember. I also know, from actual measurements, that
RVO doesn't address all of the issues, and can't be used all
of the time. (VC++ will not do it if there is more than one
return in the function, for example.)
Not sure that is correct, the following program uses RVO on VC++:
It could easily depend on the context, yes. In the case I
observed, it was a question of NRVO (which is a special case of
RVO). My only point is that you can't necessarily count on RVO
for everything. If you have a performance problem, measure, and
experiment, to find out what works (and be aware that it might
not work with a different compiler). If you don't have a
performance problem, of course, just return the value. (Whether
the compiler does RVO in any particular case may determine
whether you have a performance problem or not, of course.)
struct foo
{
foo(const std::string& type) : type(type) { std::cout << type << "
ctor\n"; }
foo(const foo& other) : type(other.type) { std::cout << type << "
cctor\n"; }
~foo() { std::cout << type << " dtor\n"; }
std::string type;
};
foo func()
{
int n;
std::cin >> n;
if (n % 2 == 0)
return foo("even");
else
return foo("odd");
}
int main()
{
foo o = func();
std::cout << "type was " << o.type << std::endl;
}
It outputs:
even ctor
type was even
even dtor
which implies RVO is happening when there is more than one
return in a function. There may exist optimization corner
cases though yes.
That's an interesting example: I would have written func:
foo func()
{
int n;
std::cin >> n;
return foo(n % 2 == 0 ? "even" : "odd");
}
It would be interesting to see if the compiler is transforming
your code into something like this. Another alternative is
return n % 2 == 0 ? foo("even") : foo("odd");
(Thinking about it---I think in the case I observed, different
constructors were being used: in one case, the default
constructor, a constructor with arguments. Something along the
lines of:
if (initial criteria not met)
return Toto();
// lots of work...
return Toto(calculated data);
except that it was written:
Toto retval;
if (initial criteria not met)
return retval;
// lots of work, setting values in retval.
return retval;
.. It would be interesting to see what happens in those two
cases. As well as the way I would have actually written it:
Toto retval;
if (initial criteria met) {
// lots of work, setting values in retval
}
return retval;
All three forms are, I would think, fairly frequent idioms.
..)
--
James Kanze