[Boost-users] Strange bug : own function conflicts with MPL::size

From: Joel FALCOU (joel.falcou_at_[hidden])
Date: 2008-02-27 08:32:08

Hi dear members,

I'm currently working on some template based DSL using MPL and proto and 
i came across
some really strange problem.I'll try to give as much details as 
possible, but I mus confess I'm really los so some important things may 
have slipped.

So, i have a class called matrix which is a simple end-user class for 
handling multidimensionnal array of data. It's declared as follow :

template<class T, class S = settings<> > class matrix : public 
process_settings<T,S>::type;

In this declaration, T is the type of the element stored in the matrix 
ans S is a setting list. A seting list is a static vector of types that 
defines various aspects of the matrix :
nbr of dimensions, 'shape' (aka dense, upper triangular etc), other 
optimisations settings like unroll factor etc. As i didn't want my user 
to be forced to register those options in
a linear fashion (and thus forcing them to remember a cumbersome list of 
predefined options), I did the folowing :

- each options type defines a type that tag them and discriminate them. 
For example, all shape related type are done likewise :

struct dense { typedef shape_tag type; };
struct upper_triangular { typedef shape_tag type; };

- The vector is turned into a mpl::map in which the key of each element 
of the vector is used as a key. This allow me to store any number of 
options in any order
- this map is then passed to process_settings which retrieves the shape 
tag options and returns another type called shape as the public base of 
matrix.

Now, this shape class is overloaded for each shape type by using 
tempalte argument. For ex., the dense shape overload is given by
shape<T,dense,options>, where T is the element of the matrix and options 
the processed map of options. This class
does all the grutn work of allocating, freeing and accessing elements. I 
also have a method called size() that returns the size of the shape in 
nbr of elment.
matrix::size then just call back this size() method.

Now I had the need to have a free function called size that take a 
matrix and call .size() on it. Alas, the compiler don't let me do it 
right :/
Using gcc 4.1 (on linux or using mingw) i got the following error :

C:/Boost/include/boost-1_34_1/boost/mpl/size_fwd.hpp: In function 'int 
main()':
C:/Boost/include/boost-1_34_1/boost/mpl/size_fwd.hpp:20: error: 
'template<class Sequence> struct boost::mpl::size' is not a function,
../../ophelia/nt2/container/matrix/matrix.hpp:499: error:   conflict 
with 'template<class T,class S> const typename matrix<T,S>::size_type& 
nt2::size(const matrix<T,S>&)'
I:\dev\nt2\ophelia\nt2\main.cpp:17: error:   in call to 'size'

Basically, instead of using my size() function, the compiler fetches 
boost::mpl::size ...
I checked and triple checked that i have NO using namespace boost 
anywhere and that, if I rename size into let's say gimme_size, it works.
It just fail at finding size cause he find the mpl structure instead. 
Same for any function name like at or stuff like this (ie names that are 
also a mpl class name).
Now, if matrix ends up with only ONE tempalte arguments (ie T), it works 
fine ...

I'm completely lost. Even if i know that i can just rename this, i don't 
want to just let it go. Either I did something stupid, either there is 
some bug (I think the former alas) and i don't want such errors 
resufraces later
in some unrelated code. If details on the class actual definition is 
needed, I'll post them.

Joel FALCOU
Research Engineer @ Institu d'Electronique Fondamentale
Université PARIS SUD XI
France