On Sun, 06 Mar 2011 22:28:46 -0800 "Jeffrey Lee Hellrung, Jr." <jhellrung@ucla.edu> wrote:
This whole argument misses the point of optimizing code that uses a bigint class. You want to reuse the memory allocated in a bigint variable over and over instead of reallocating every time.
That requires writing at least a minimal memory manager. [...]
I don't think that's what Luke's talking about. He's saying, if x has some value, say [x0, x1, x2], and you assign some expression to it, you want the result of that expression to directly replace the digits [x0, x1, x2], rather than allocating a new block, compute the result in this new block, and swap this block in place of the block holding [x0, x1, x2].
Okay, I don't see the mechanism, but more on that below.
Whether you make one unneeded allocation or two is a less important consideration than whether you make one or zero since 2/1 is a lot smaller than 1/0. If a temporary is generated by the user's code they have already pessimized their code. I am sorry, but you cannot avoid allocation with a return by value semantic, only expression templates allows you to reuse the storage on the left hand side of the assignment operator. [...]
I'm not familiar with a use of expression templates that would allow that. Can you point me to something explaining it?
Expression templates would allow you to tell the compiler, when it sees
x = a + b,
to generate code that replaces x's digits with the sum of a's and b's digits, one at a time. [...]
How would the expression's code know anything about x, or have access to its allocated memory? If the template is operator+, you'd have to pass x to it somehow. If it's not, operator+ would have to allocate some memory for a temporary, wouldn't it?
google "c++ expression templates" and/or look at Boost.Proto and libraries that use Boost.Proto.
I haven't looked at Boost.Proto yet, but I did look at a couple hits from Google on the subject in the last couple days. Maybe I'm missing it, but I didn't see anything that would allow that. -- Chad Nelson Oak Circle Software, Inc. * * *