Re: move, instead of copy, assignment

From:
SG <s.gesemann@gmail.com>
Newsgroups:
comp.lang.c++
Date:
Tue, 11 Aug 2009 10:36:29 -0700 (PDT)
Message-ID:
<9e9551aa-7323-41ab-bfe2-c6c2c887ede8@b15g2000yqd.googlegroups.com>
On 11 Aug., 02:06, Jonathan Lee <cho...@shaw.ca> wrote:

  I was wondering if there is a good way to detect an assignment from
an anonymous variable, so that the assignment might be accomplished by
a move, instead of a copy. For example:

      MatrixClass x, y, z
      // stuff happens to x, y, and z
      // ...
      z = x + y;

Suppose MatrixClass keeps a pointer to its data, which will be deleted
on destruction. Rather than copy (x + y) to z, it would be cheaper to
simply exchange the pointers inside the classes and allow the old z
data be deleted when (x + y) is destroyed.


Yes, this is what you will be able to do with a "move assignment
operator" in C++0x:

   class MatrixClass
   {
     std::vector<double> elements;
     ...
     // copy assignment operator
     MatrixClass& operator=(MatrixClass const& x) & = default;

     // move assignment operator
     MatrixClass& operator=(MatrixClass && x) &
     {
       this->clear();
       this->swap(x);
       return *this;
     }
     ...
   };

[Actually, I'm not sure whether the ref qualifier is legal in
combination with the =default; syntax. I hope that it is.]

Still, if operator+ returns a MatrixClass Objekt memory is allocated
(for the temporary object) and other memory is released again (after
swapping with the target object).

You could probably do even better with "expression templates" where
operator+ returns a "lazy matrix" that doesn't require a memory
allocation. The matrix class could have an additional assignment
operator that takes this "matrix expression" object and actually
evaluates it and stores the result directly. You can do this with C+
+98 already and I think this is what Boost.uBLAS does already.

Cheers!
SG

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