[Insight-developers] templates and friends

[Luis Ibanez]

Hi,

There is a potential conflict with the use
of friend function in templated classes.

It happens with code like this
---------------------------------------
template <class T>
class myClass {
public:
  friend  myFunction( myClass<T> );
};
---------------------------------------

(This is usually done for ostream & operator<<
and istream & operator>>)

Currently we have this conflict in itkIndex.h (line 122)

The friend function is implicitly templated because
it receives a templated class as parameter.

Compilers have different reactions to this.

Visual C++ accept it as this, and interpret the
implicit parametrization of the function.

Gcc on the other hand, raise a warning because
it expects to find a explicit indication that the
function is templated.  This should take the form of

friend myFunction <> (myClass<T>);

the "<>" indicating that the function is templated.

----

This have been solved in VXL code by using a macro
named

     VCL_STL_NULL_TMPL_ARGS

(ie,    in vxl/vnl_matrix.h lines 136-146,
 and  in vxl/vnl_vector.h lines 117-137 )

This macro is defined as   "<>"    or  as    "/* <> */
depending on the compiler, in the header file

vxl/vcl/vcl_compiler.h

------

Other options are just to turn off the warning in
gcc by using

-Wno-non-template-friend

or using the gcc compiler option

-fguiding-decls

as is done in vxl makefiles.

------



What are your preferences about the
way in which that should be done ?





Luis