My parting gift to you.

R. Clayton (rclayton@monmouth.edu)
(no date)

During last night's lecture I talked about relational type traits for
determining various relations between types.

I was cleaning out my 509 directory and found the little gem shown below. It
implements the "is parent of" relational type-trait, which is a boolean
relation between two types. The code is not written in the style of the
proposed type-traits standard, but it does use the various mechanisms exploited
by type traits. This code brakes the g++ compiler; it does compile correctly
on solaris CC and (oddly enough) Microsoft C++ 6.0.

I'll leave it up to you, during those lazy summertime hours, to figure out what
this code does and how it works. 

--

$ cat parentof.cc
#include <iostream>

template < class p, class c >
struct is_parent_of {

  struct yes { };
  struct no  { int i; };

  static yes Test(p *);
  static no  Test(...);

  public:

    enum {
      isparentof = sizeof(Test(reinterpret_cast<c *>(0))) == sizeof(yes)
      };
  };

struct parent { };

struct child : parent { };

int main() {
  typedef is_parent_of<parent, child> pc;
  typedef is_parent_of<child, parent> cp;

  std::cout << pc::isparentof << "\n";
  std::cout << cp::isparentof << "\n";
  }

$ CC parentof.cc

$ ./a.out
1
0

$ g++ ~/parentof.cc
parentof.cc: In instantiation of `is_parent_of<parent, child>':
parentof.cc:29:   instantiated from here
parentof.cc:29: invalid use of undefined type `struct is_parent_of<parent, child>'
parentof.cc:5: forward declaration of `struct is_parent_of<parent, child>'
parentof.cc: In instantiation of `is_parent_of<child, parent>':
parentof.cc:30:   instantiated from here
parentof.cc:30: invalid use of undefined type `struct is_parent_of<child, parent>'
parentof.cc:5: forward declaration of `struct is_parent_of<child, parent>'

$

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