Hi Romain,
thanks for refreshing my C++ basics
As I am regrettably not privileged to spare the amount of time on C++
skill improvement I would like to, but have the assignment to
construct stochastic models and implement them, I focus here on a
solution and maybe someone can help me, with his experience in C++
(at this point I'd like to apologize for maybe confusing list members
with my wrong statements below).
Back to the origin of my problem I intend to solve:
I have a variable STOREPOST, that indicates, if additional
information should be stored during the processing of my algorithm. In
the case
STOREPOST is not true I would like to avoid the construction of an
arma::mat object and in the case that STOREPOST is true an arma::mat
object should be constructed with auxiliary memory from R.
From your corrections below I understand, that the way I showed is
not appropriate to solve my problem, as the temporary object in the
if-clause is not known anymore to the compiler
from the line on I close the bracket ('}') of the if-condition. I
also assume, that using a pointer '*arma::mat maybeM' does not change
this,
as the object inside the if-clause is still a temporary rvalue
(please correct me here if I am wrong).
I would like to ask, if a list member has maybe an advice for me? Should
I construct two
different algorithms (that would be a very easy solution - maybe not
the
way a C++ programmer would use). Or would you construct a base class
and one that inherits from the base class containing additional
members? I think there are a lot
of pitfalls I do not see yet.
Best
Simon
On 06/11/2013 12:02 AM, [email protected] wrote:
Some C++ 101 insights below.
Le 2013-06-10 23:07, Simon Zehnder a écrit :
Dear Rcpp::Devels and Rcpp::Users,
I am temporarily experiencing with a delayed assignment of auxiliary
memory in RcppArmadillo:
Declare a matrix from which you know you may need it
arma::mat maybeM;
(In C++ a declaration makes only known the type and name but allocates
no storage yet).
SO WRONG.
here the default constructor or arma::mat is called. What it does is
entirely up to the implementation of the class.
Now you check a condition and if it is true, you will need the matrix,
so you initialize it
if(mycondition) {
maybeM = arma::mat(aux_mem*, n_rows, n_cols, copy_aux_mem = false,
strict = true)
}
(for simplicity I only printed here the default constructor for using
auxiliary memory; please assume that all values are presented)
wrong again.
what happens here is in two steps.
- first, an arma::mat is constructed, using the so called advanced
constructor. this does use the auxiliary memory to create a temporary.
- second, the assignment operator is used to copy the temporary into
"maybeM"
So indeed maybeM does not use the auxiliary memory, I hope now you
understand why.
The resulting matrix maybeM does not use the auxiliary memory. I tried
to access in some way the 'mem' member in the Armadillo class, but
this member is constant and can not be
manipulated after first construction. A primer declaration and later
initialization is a very common pattern in C++, but it seems not be
possible in case of reusing memory.
Everything is possible, it is a decision made by the implementation.
e.g. for Rcpp classes copy construction means just cheap pointer
copies, not data copies.
armadillo is implemented with more traditional copy semantics.
I conclude, that a delayed assignment of auxiliary memory is not
possible.
wrong conclusion. what you demonstrated here is that you need to study
basic c++ (assignment operators, copy construction, ...) :)
now, things would be different with move semantics in c++11. Not sure
armadillo has implemented them already, but I'm pretty sure that is
not what you were talking about.
Romain
Best
Simon
_______________________________________________
Rcpp-devel mailing list
[email protected]
https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/rcpp-devel
_______________________________________________
Rcpp-devel mailing list
[email protected]
https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/rcpp-devel
