Re: [Boost-Users] Re: UBLAS: copy on write for matrix & vector?

Hi Ivan, you've been faster again ;-). You wrote:

wrote in message news:b2nq2e+jht8@eGroups.com... | I tried to find out what is happening in the case: | | matrix<double> foo() | { | matrix<double> r(3); | return r; | } ... | r1 = foo(); | | The matrix (that was constructed on the stack) is copied twice when | used in this manner. One copy for the returned temporary, a second | copy for the assignment. (I am using MSVC debug mode) And this | definitely is not a cheap operation. Both in terms of memory _and_ | execution time.

The first copy indicates that MSVC (6?) could not apply the NRVO optimization. It may support the RVO optimization though: you could try using direct returns when possible: return matrix<double>(3);

The second copy will be avoided by the compiler if you use construction of a new variable instead of assignment: matrix<double> r1 = foo(); // shall avoid 2nd copy or: matrix<double> const& r2 = foo(); // always optimized // but can trigger a VC7 bug

I agree these alternatives are not fully equivalent, but they are coding style tricks that can be useful to improve performance.

Besides the question, when which compiler uses RVO: if Roland needs uttermost performance using similar interfaces, we'll either have to add the mojo protocol or he should try to extend the expression template machinery.

| Also a second question comes to me now: | Can I define a LHS that can be the target of an arbitrary size matrix? | I.e.: | matrix<double> lhs; | matrix<doubel> rhs(3,3); | | lhs = rhs; | | will give a runtime exeception because of different size. Can I do | anything about this?

Yes. Four ways: 1. matrix<double> lhs(3,3); matrix<double> rhs(3,3); lhs = rhs; 2. matrix<double> rhs(3,3); matrix<double> lhs(rhs); 3. matrix<double> lhs; matrix<double> rhs(3,3); lhs.resize(3,3); lhs = rhs; 4. matrix<double> lhs; matrix<double> rhs(3,3); lhs.reset(rhs);

I'm not a UBLAS expert. But here also, declaring variables where they are initialized would solve the problem, and provide best performance: matrix<double> const lhs = rhs; // drop the 'const' if required

This is the usually recommended C++ style. And to go back to your original question, it requires that toutines *return* their result instead of taking output parameters. ( but I cannot certify that boost's UBLAS was developed with this in mind )

| I have the feeling, that I still do not well understand the overall | semantics of the matrix and vector classes.

uBLAS assumes that vectors and matrices in an assignment have conforming dimensions. Copy semantics are deep.

|Are there any | books/examples which I can study to get a better understanding?

There probably are no books about this. For examples one could look into Michael Stevens' Bayesian Filters at www.sf.net/projects/bayesclasses or Paul C. Leopardi's Clifford Algebras at www.sf.net/projects/glucat

I won't be of much help here. I admit I don't use UBLAS personally. ( I use small fixed-size matrices in my current applications ). But, I home some of the suggestions above may still help...

Thanks, Joerg [Non-text portions of this message have been removed]