[Rcpp-devel] Delayed usage of auxiliary memory in RcppArmadillo

[Simon Zehnder]

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, romain at r-enthusiasts.com 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
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