Re: Speed of passing a string by value vs. const reference

SG <>
Fri, 31 May 2013 10:18:30 -0700 (PDT)
On May 31, 3:50 pm, Andreas Dehmel wrote:

On Thu, 30 May 2013 17:55:04 -0700 (PDT) SG wrote:

On May 30, 3:28 pm, Andreas Dehmel wrote:

On Thu, 30 May 2013 02:06:37 -0700 (PDT) SG wrote:

On May 29, 11:46 pm, James Kanze wrote:

The ubiquitous rule is to pass class types by reference to
const, other types by value. Since std:;string is a class type=


convention says to pass it by reference to const, even if with a
well written string class, it won't make a significant

Some rules are now outdated in the light of move semantics.

Nonsense. For container types, a move still implies swapping at
least 2-3 pointer-sized members and while this is certainly orders
of magnitude faster than copying the whole thing, there's no way
this'll be faster than simply dereferencing a pointer.


  string flip(string const& cref) {
    string result = cref; // explicit copy,
                          // might be unneces=


    return result; // NRVO applicable


  string flip(string temp) { // temp might be move-constructed or
                             // even cons=

tructed directly via

                             // copy/move=


    return temp; // implicitly moved

You're not measuring the overhead of passing the argument one way
or another but the copy constructor in the first case, which is
a completely different issue that can easily be avoided by using
string result(cref.rbegin(), cref.rend()) rather than call reverse
on a copy. I'd call this a pathological textbook example with zero
practical relevance; one can always construct these, that doesn't
mean anything.

I don't think there is a big difference between

    string result = cref;


    string result (cref.rbegin(),cref.rend());

in terms of performance. In both cases you'll have a free store
allocation which is exactly what can be avoided in certain cases by --
oddly enough -- taking the argument by value.

Look Andreas, I simply objected to the simple rule of "always take
class types by ref-to-const" which you had to answer with "Nonsense".
I gave you an example where this rule would mean unnecessary heap
allocation, something your version of flip still does.

Plus moving the argument in the second case will
not be possible if you don't call the function with a temporary,

If it's not a temporary there will be almost no difference between all
the functions. They all create a new string object and make it contain
the reversed string value, something a string usually does by asking
the free store for a chunk of memory which one expects to be a lot
slower than swapping three pointers.

so performance will vary considerably for rather obscure reasons
(and still be slower when not calling with a temporary).

In the case of an Lvalue argument, it might be a little bit slower,
yes. But that's probably nothing compared to an unnecessary free store
allocation which it can save.

I'm actually surprized to see such resistance against rethinking the
"always take a ref-to-const for class types" idea. James made it sound
like removing a "const&" suddently makes the code less maintainable
and readable. I find such a claim rather ridiculous, to be honest.

I stand by what I said.


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