Re: [boost] Re: Zero Unnecessary Temporary Objects: A Revolution

From: David Abrahams (dave_at_[hidden])
Date: 2002-10-12 07:42:01

Terje Slettebø <tslettebo_at_[hidden]> writes:

> I think it would be good to get this patch applied. I looked at
> operators.hpp a while ago, and found the asymmetric parameters rather
> inelegant. I.e.:
> 
> friend const T operator+( T lhs, const T& rhs )
> {
>    return lhs += rhs;
> }
> 
> The patch uses:
> 
> friend T operator+( const T& lhs, const T& rhs )
> {
>    T nrv( lhs );
>    nrv += rhs;
>    return nrv;
> }

I don't think the fact that the current implementation clashes with
one person's aesthetics is really a good reason to change it.

> A couple of things I wonder about:
> 
> - The posting with the patch
> (http://lists.boost.org/MailArchives/boost/msg31458.php) says "See also
> Scott Meyers "More Efficient C++", Item 4.7.". There's no Item 4.7, and Item
> 4 doesn't seem to apply, so I wonder what is referred to, here?
> 
> - The same posting uses "friend const T operator+()" in the posting, but
> "friend T operator+()" in the patch. Which one will be used, and why?
> 
> "Effective C++", Item 21, "Use const whenever possible" argues for returning
> const UDTs, to avoid operating on temporaries, such as "(a * b)=c". This
> would also be illegal for built-in types. Why then not return "const T" in
> operators.hpp? The const is ignored for built-in types (as they have no
> "this"), but prevents the operation on temporaries, such as the above.

In a world of expression template metaprogramming there may be uses
for operating on the lhs result. Hoever, I'm willing to go with const
T returns for the next release to see if it raises any alarm bells
with users.

> - "More Effective C++", Item 20, "Facilitate the return value optimization"
> argues for using the constructor in the return statement, rather than
> creating a named temp. The argument is that although NRVO is also now
> possible, RVO (unnamed temporary) has been around longer, and may therefore
> be more widely implemented. Thus, compilers implementing NRVO almost
> certainly implements RVO, but not necessarily the other way around. Given
> this, why isn't the following used in operators.hpp:
> 
> friend T operator+( const T& lhs, const T& rhs )
> {
>    return T(lhs)+=rhs;
> }
> 

This is exactly the sort of good question which I felt was unresolved
before, and which kept me from applying the patch until we had solid
answers.

Thanks, Terje, for raising it again!

-- 
           David Abrahams * Boost Consulting
dave_at_[hidden] * http://www.boost-consulting.com