TITLE: restricting template arguments to only built-in types (part II)

(Newsgroups: comp.lang.c++.moderated, 13 Feb 97)


ODOWD: "Robert O'Dowd" <robert.odowd@dsto.defence.gov.au>

> > We need a cheap persistent store container, (a map) which only
> > takes primitive types. [...]  Is there any way to cause a compiler
> > error or at least a deliberate run time error if the class is not
> > primitive?

[ Terminology note: "primitive" == "built-in" -adc ]


GLICKSTEIN: Bob Glickstein <bobg@hiro.zanshin.com>

I can think of an indirect way to do this.  It involves the fact that
only types with trivial constructors and destructors are allowed in
unions.  I believe this permits all the types you want (including
simple structs and arrays) and excludes all the types you don't.

  template <class T>
  class CheapPersistentStore {
   ...
   private:
    union only_works_with_primitive_T {
     T val;
    };
    ...
  };

You don't need to use the only_works_with_primitive_T union anywhere.
Just include it in the template definition, and it should prevent the
definition from compiling for any instantiation in which T is
non-"primitive".