Re: "Concepts" were removed from C++0x

From:

tni <nobody@example.com>

Newsgroups:

comp.lang.c++

Date:

Thu, 23 Jul 2009 09:30:21 +0200

Message-ID:

<h493iq$op1$00$1@news.t-online.com>

Stephen Horne wrote:

    Also, how exactly do you alias a variable without causing a
    run-time overhead? What I tend to do is use a reference
    variable...

      thing_t& shortname (longname);

    But in doing that, I'm risking a significant run-time overhead.
    Often I don't care (not an inner loop etc), and very likely the
    optimiser can eliminate that overhead anyway, but it's still a
    potential issue that IMO should never exist.

I would argue that it's the other way around. Using the reference will
give the compiler the option to generate faster code in some cases.

E.g.

    std::vector<size_t> blah;
    blah.resize(20);
    for(size_t i = 0; i < 50; i++) {
        size_t& ref = blah[10];
        ref += std::rand();
        ref += std::rand();
    }

Having the reference gives the compiler the guarantee that the address
of vector[10] won't change, so it doesn't have to call operator[] again
(the call is actually inlined, but not eliminated). Of course, semantics
are different, with the reference you have the danger of it going stale
(which can lead to very nice obscure bugs).

The loop with reference is 15 instructions with VS 2005 (release build),
the loop without reference is 24 instructions. With GCC 4.4 (-O2), it's
17 vs. 19 instructions.

(With only one 'ref += std::rand();' in the loop both generate the same
code with or without reference.)