BZSUPPORT: Passing arrays by value

[Todd V. <tve...@ex...>]

Matthias Troyer writes:
> Today's question regards call by value of functions with array arguments
> like in:
> 
> #include <iostream.h>
> #include <complex.h>
> #include <blitz/array.h>
> 
> using namespace blitz;
> using namespace std;
> 
> Array<double,1> kolu(Array<double,1>);
> 
> int main()
> {
>   Array<double,1> BB(1);
>   Array<double,1> CC(1);
>   BB=1;
> 
>   cout<<BB<<endl;
>   CC=kolu(BB);
>   cout<<CC<<endl;
>   cout<<BB<<endl;
> }
> 
> Array<double,1> kolu(Array<double,1> B)
> {
>   B*=2;
>   return B;
> }
> 
> Assuming normal C++ semantics the argument of the kolu function should not
> be changed by the function, but it is changed as can be seen from the
> following output:
> 
> [          1 ]
> [          2 ]
> [          2 ]
> 
> My question to you: is this a bug or a feature of Blits++?

I'd hesitate to call it a "feature," because obviously it creates
some problems like the one you describe.  It's a result of
reference-counting.  Basically you have to think of array
objects as being "handles" to underlying arrays.  The function
kolu(..) doesn't receive a copy of the array data, but rather
a copy of the handle.  So kolu::B is the same array data as
main::BB.  The alternative would be to copy the array data
when passing by value, which would be grossly inefficient.

Cheers,
Todd

--------------------- blitz-support list --------------------------------
* To subscribe/unsubscribe: mail to maj...@oo..., with
"subscribe blitz-support" or "unsubscribe blitz-support" in the body of the message
* Blitz++ web page: http://oonumerics.org/blitz/