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
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