Re: [Boost-users] Does shared_array's get() method introduceruntimeoverhead?

From: Peng Yu (pengyu.ut_at_[hidden])
Date: 2006-10-24 20:32:57

On 10/24/06, Sohail Somani <s.somani_at_[hidden]> wrote:
> > -----Original Message-----
> > From: boost-users-bounces_at_[hidden]
> > [mailto:boost-users-bounces_at_[hidden]] On Behalf Of
> > Sohail Somani
> > > I have the following two code segments. I'm wondering if
> > there is any
> > > run time overhead of the get() method. Shall I use the latter one
> > > instead of the former one?
> > >
> > >
> > >
> > > boost::shared_array a(new int[100]);
> > > for(int i = 0; i < 100; ++ i)
> > >   a[i] = i;
> > >
> > >
> > >
> > > boost::shared_array a(new int[100]);
> > > int a_ptr = a.get();
> > > for(int i = 0; i < 100; ++ i)
> > >   a_ptr[i] = i;
> >
> > I'd guess not (from shared_array.hpp):
> >
> > template<class T> class shared_array
> > {
> > ...
> >     T * get() const // never throws
> >     {
> >         return px;
> >     }
> > ...
> >
> > Unless that doesn't get inlined of course.
> >
> [snip smart-ass comment]
>
> Argh, of course I meant to paste:
>
>    T & operator[] (std::ptrdiff_t i) const // never throws
>     {
>         BOOST_ASSERT(px != 0);
>         BOOST_ASSERT(i >= 0);
>         return px[i];
>     }

Does it mean that code segment 1 is as fast as segment 2, but code
segment 3 is the slowest?

///////1
boost::shared_array a(new int[100]);
for(int i = 0; i < 100; ++ i)
 a.get()[i] = i;

////////2
boost::shared_array a(new int[100]);
int a_ptr = a.get();
for(int i = 0; i < 100; ++ i)
 a_ptr[i] = i;

////////3
boost::shared_array a(new int[100]);
for(int i = 0; i < 100; ++ i)
 a[i] = i;