On 8/11/2011 10:50 AM, John Maddock wrote:
But I'm figuring that the occurrence of temporaries on the RHS of an expression, while not out of the question, is rare enough not to be too concerned about.
You can use Proto to eliminate the need for temporaries without rvalue reference support. What you describe above is a textbook application of expression templates. If it will save you 1 dynamic allocation every now and then, it's probably worth considering.
Can you explain again why rvalue references are important for you? I'm not following.
OK, lets say we have an expression:
a = x * y + w * z;
The order of evaluation might be:
*assign x to a. * multiply a by y * create one temporary as a copy of w * multiple temp by z * add temp to a
This part I don't understand, and it's probably because I don't know the particulars of the bigint domain. But I know if these things were vectors, it would go something like this: * walk the expression x*y + w*z (at runtime) and calculate the size of the resulting vector * allocate space for the result vector * walk the expression x*y + w*z and evaluate it (also at runtime), filling in the result vector directly (no temporaries). * swap the result vector with a's I'm ignorant of how bigint arithmetic works, but I wonder if a similar scheme might work here.
So we need one temporary to evaluate the expression.
Now imagine that one of the variables isn't a variable at all, but a temporary, lets say the result of a (non-protoized) function call:
a = x * y + foo() * z
Now we don't need to create a temporary for foo() * z since *we have one already*. So if one of the terminals is an rvalue reference, we know that we can reuse that value as a temporary and trash it's value with impunity - as long as we do so after it's value has been used of course :-)
OK, this much I understand. Thanks. -- Eric Niebler BoostPro Computing http://www.boostpro.com