Problems with the template implementation
=========================================

   g++ does not implement a separate pass to instantiate template
functions and classes at this point; for this reason, it will not work,
for the most part, to declare your template functions in one file and
define them in another.  The compiler will need to see the entire
definition of the function, and will generate a static copy of the
function in each file in which it is used.

   (The experimental template repository code (see See repository)
that can be added to 2.7.0 or later does implement a separate pass, but
there is still no searching of files that the compiler never saw).

   For version 2.6.0, however, a new switch `-fno-implicit-templates'
was added; with this switch, templates are expanded only under user
control.  I recommend that all g++ users that use templates read the
section "Template Instantiation" in the gcc manual (version 2.6.x and
newer).  g++ now supports explicit template expansion using the syntax
from the latest C++ working paper:

     template class A<int>;
     template ostream& operator << (ostream&, const A<int>&);

   As of version 2.6.3, there are still a few limitations in the
template implementation besides the above (thanks to Jason Merrill for
this info): These are still present in version 2.7.2, but a new
implementation of templates planned for version 2.8 will eliminate them.

  1. Static data member templates are not supported.  You can work
     around this by explicitly declaring the static variable for each
     template specialization:

          template <class T> struct A {
            static T t;
          };
          
          template <class T> T A<T>::t = 0; // gets bogus error
          int A<int>::t = 0;                // OK (workaround)

     (still a limitation in 2.7.2)

  2. Template member names are not available when defining member
     function templates.

          template <class T> struct A {
            typedef T foo;
            void f (foo);
            void g (foo arg) { ... }; // this works
          };
          
          template <class T> void A<T>::f (foo) { } // gets bogus error

  3. Templates are instantiated using the parser.  This results in two
     problems:

     a) Class templates are instantiated in some situations where such
     instantiation should not occur.

          template <class T> class A { };
          A<int> *aip = 0; // should not instantiate A<int> (but does)

     b) Function templates cannot be inlined at the site of their
     instantiation.

          template <class T> inline T min (T a, T b) { return a < b ? a : b; }
          
          void f () {
            int i = min (1, 0);           // not inlined
          }
          
          void g () {
            int j = min (1, 0);           // inlined
          }

     A workaround that works in version 2.6.1 and later is to specify

          extern template int min (int, int);

     before `f()'; this will force it to be instantiated (though not
     emitted).

  4. Member function templates are always instantiated when their
     containing class is.  This is wrong.