[Rcpp-devel] use of auxiliary functions

[Romain Francois]

Le 10/08/10 07:35, baptiste auguie a écrit :
> I see –– thanks for the clarification (it is indeed what Dirk really
> had suggested, the accidental shortcut was mine). This passing by
> reference seems very interesting (I pass several large complex arma
> matrices in other functions), but I need to work out how it works
> inside the functions.

It should make no difference in how you use the object between :

void foo( arma:::cx_mat m ) ;
void foo( const arma::cx_mat& m ) ;

inside foo, you use m exactly the same way. Although the const here is 
there to protect you from yourself. Having the const here means that 
inside foo you are only allowed to call const methods of the 
arma::cx_mat class, so you can't modify the object.

If you want to be able to modify the object, you could use

void foo( arma::cx_mat& m ) ;

> Thanks again for the precious comments,
>
> baptiste
>
> PS: my package is now on r-forge, hoping to see if it compiles there too:
>
> https://r-forge.r-project.org/scm/viewvc.php/pkg/cda/?root=photonics

cool. I'll make a few targetted comments when I have time to read it 
more carefully, but so far it looks good.

> On 10 August 2010 01:38, Davor Cubranic<cubranic at stat.ubc.ca>  wrote:
>> On 2010-08-07, at 7:03 PM, baptiste auguie wrote:
>>
>>> Thanks Davor, Douglas and Dirk –– all your comments have been very
>>> helpful and I used them to improve my code (along with getting a
>>> better understanding of C++). The only thing I haven't tried yet is
>>> avoid duplicating the objects in memory using const (seems tricky from
>>> what I read on the net).
>>
>> Using 'const' won't save you from duplicating objects: all it does is mark a variable/parameter as a constant, so that you can't accidentally change it. What does save you from duplication is passing arguments by reference. And passing arguments by reference is often combined with marking them 'const' for safety, because accidentally changing them would introduce wide-ranging bugs into your code that could be difficult to find.
>>
>> Also, note that passing by const reference only saves space for complex objects, not primitive types like doubles and consts. So for the 'euler' function you used in your example, it would be recommended to make its signature:
>>
>> arma::mat euler(const double phi, const double theta, const double psi);
>> In other words, 'phi', 'theta', and 'psi' are doubles, which I'm passing as constants.
>>
>> While something like matrix multiply could be:
>>
>> arma::mat mul(const arma::mat&  a, const arma::mat&  b);
>> Here, 'a' and 'b' are matrix objects, which I'm passing by constant reference.
>>
>> Hope this helps.
>>
>> Davor

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Romain Francois
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