Re: [boost] Heads up - string_ref landing

Subject: Re: [boost] Heads up - string_ref landing
From: Marshall Clow (mclow.lists_at_[hidden])
Date: 2012-11-16 10:27:37

On Nov 16, 2012, at 7:00 AM, Maxim Yanchenko <MaximYanchenko_at_[hidden]> wrote:

> Olaf van der Spek <ml <at> vdspek.org> writes:
>> On Fri, Nov 16, 2012 at 3:25 PM, Mathias Gaunard
>> <mathias.gaunard <at> ens-lyon.org> wrote:
>>> void f(std::string const&);
>>> f("Olaf");
>> 
>> Are compilers/optimizers not smart enough to construct the temporary
>> object at compile time?
> 
> I'm afraid they are not smart enough to eliminate an unneeded temporary when
> it's something sophisticated like std::string…

Actually, this is a really good example.

        void f(std::string const&);
        void g(string_ref);
        f("Olaf");
        g("Olaf");

In the call to "f", the compiler will create a temporary std::string. This will involve a call to strlen (possibly done at compile time), a memory allocation (modulo the small-string optimization), and copying the data.

In the call to "g", the compiler will create a temporary string_ref. This will involve a call to strlen (possibly done at compile time).

And for most cases, the code internal to "f" and "g" won't care that they got a string_ref instead of a  const std::string.

-- Marshall

Marshall Clow     Idio Software   <mailto:mclow.lists_at_[hidden]>

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