[Tbb-develop] parallel_do cannot change items in its feeding container

[Kukanov, A. <Ale...@in...>]

Hi everyone,

There was a problem reported recently in the TBB forum (see that post
below). The user wanted to change the data item in Body::operator(), but
passing the item by reference caused compilation errors. The user was
misled by an error in TBB documentation that said Body::operator() takes
items to process by reference.

That post made me thinking if such usage model is something TBB should
be able to handle, and if so, what the best way is to handle it. For
example, should parallel_do be extended or reworked for it, or should it
better be implemented in a special algorithm?

What do you think? 

Regards, 
- Alexey Kukanov
TBB developer @ Intel

________________________________ 

Original forum post:
Posted By: devbisme in Intel(r) Threading Building Blocks 
Subject: problems using parallel_do 

View the complete topic at: 
http://softwarecommunity.intel.com/isn/Community/en-US/forums/thread/302
50244.aspx 


I'm trying to use parallel_do (using tbb20_20080226oss), but I'm getting
an error
from the compiler (MS VC++ 2005).  I am trying to process some data in a
simple
structure maintained in a list:

typedef struct {
    double v1, v2, v3;
} my_item_t;

typedef list<my_item_t> vec_list;

class Body{
public:
    typedef my_item_t argument_type;
    Body() {};
    // overload () so it does a vector multiply
    void operator() (argument_type& it) const {
        it.v3 = it.v1 * it.v2;
    }
};

The () operator takes a reference to the simple structure as shown in
the documentation
for parallel_do.  However, the compiler cannot find a () operator that
matches
the prototype it wants.
If I change the operator by removing the reference, then it compiles
with no errors:

    void operator() (argument_type it) const {
        it.v3 = it.v1 * it.v2;
    }

But this doesn't work because any changes made by the operator only
occur in 
the local copy and don't get passed back into the main list.

I can get around this problem by making a list of pointers that point 
to the data structures, but it seems like the code shown above should
work 
and I want to understand where I'm making my mistake.

Thanks for any enlightenment anyone can give.


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