Re: [Boost-users] Compilation Error related to 'range'

Subject: Re: [Boost-users] Compilation Error related to 'range'
From: Neil Groves (neil_at_[hidden])
Date: 2012-07-03 16:31:52

On 03/07/12 16:37, VimalMathew_at_[hidden] wrote:
>
> Hi,
>
> I have this line of code in my header file: project(C, range(0, 
> A_rows), range(0, A_cols)) = A;
>
> I've specified:
>
> #include<boost/numeric/ublas/storage.hpp>
>
> usingnamespace boost::numeric::ublas; at the beginning of the header file
>

It is generally regarded as bad form in most cases to have using 
namespace within a header file. There are some exceptions when having a 
very specific nested namespace with few symbols to protect against 
accidental argument-dependent lookup, but generally it shouldn't be done.
>
> When I compile my .cpp file, I get an error message saying: 
> "c:\users\c9995799\desktop\level 1 problem for mpi\dmatrix.hpp(34): 
> error C2784: 'const boost::numeric::ublas::matrix_range<M> 
> boost::numeric::ublas::project(const 
> boost::numeric::ublas::matrix_range<M> &,const 
> matrix_range<M>::range_type &,const matrix_range<M>::range_type &)' : 
> could not deduce template argument for 'const 
> boost::numeric::ublas::matrix_range<M> &' from 
> 'boost::numeric::ublas::matrix<T>'"
>
> This error goes away if I substitute boost::numeric::ublas::range 
> instead of justrange here:project(C, range(0, A_rows), range(0, 
> A_cols)) = A;
>
> How do I get this to work the way it currently is?
>

You don't! This is why the first paragraph is the general conclusion. 
This is the current state of C++ namespaces. I have always found this 
rather disappointing since one is frequently forced to qualify the 
namespace upon each use. This is therefore little better than adding a 
prefix and not having namespaces.

Seriously, it is best that you don't do it that way. If you insist then 
depending on your use of namespaces you could add "using 
boost::numeric::ublas::range;" This might work, but might have 
ambiguities. Otherwise in the function where you use ublas you could 
move the using namespace statement. My experience is that even if this 
does compile on GCC that the Microsoft compilers often go completely 
insane as a result of the using directive/declarations. Additionally 
someone else may define another range function in another namespace that 
is a better match within header files included directly or indirectly. 
It's excessively fragile and un-portable to have such widespread using 
clauses.

> Thanks,
>
> Vimal
>
I agree with your expectation. If namespaces and argument-dependent 
lookup were specified as I would have hoped it wouldn't be an issue.

I hope this helps,

Neil Groves