is-a.h

/* Dynamic testing for abstract is-a relationships.
   Copyright (C) 2012-2020 Free Software Foundation, Inc.
   Contributed by Lawrence Crowl.

This file is part of GCC.

GCC is free software; you can redistribute it and/or modify it under
the terms of the GNU General Public License as published by the Free
Software Foundation; either version 3, or (at your option) any later
version.

GCC is distributed in the hope that it will be useful, but WITHOUT ANY
WARRANTY; without even the implied warranty of MERCHANTABILITY or
FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
for more details.

You should have received a copy of the GNU General Public License
along with GCC; see the file COPYING3.  If not see
<http://www.gnu.org/licenses/>.  */


/* This header generic type query and conversion functions.


USING THE GENERIC TYPE FACILITY


The user functions are:

bool is_a <TYPE> (pointer)

    Tests whether the pointer actually points to a more derived TYPE.

    Suppose you have a symtab_node *ptr, AKA symtab_node *ptr.  You can test
    whether it points to a 'derived' cgraph_node as follows.

      if (is_a <cgraph_node *> (ptr))
        ....


TYPE as_a <TYPE> (pointer)

    Converts pointer to a TYPE.

    You can just assume that it is such a node.

      do_something_with (as_a <cgraph_node *> *ptr);

TYPE safe_as_a <TYPE> (pointer)

    Like as_a <TYPE> (pointer), but where pointer could be NULL.  This
    adds a check against NULL where the regular is_a_helper hook for TYPE
    assumes non-NULL.

      do_something_with (safe_as_a <cgraph_node *> *ptr);

TYPE dyn_cast <TYPE> (pointer)

    Converts pointer to TYPE if and only if "is_a <TYPE> pointer".  Otherwise,
    returns NULL.  This function is essentially a checked down cast.

    This functions reduce compile time and increase type safety when treating a
    generic item as a more specific item.

    You can test and obtain a pointer to the 'derived' type in one indivisible
    operation.

      if (cgraph_node *cptr = dyn_cast <cgraph_node *> (ptr))
        ....

    As an example, the code change is from

      if (symtab_function_p (node))
        {
          struct cgraph_node *cnode = cgraph (node);
          ....
        }

    to

      if (cgraph_node *cnode = dyn_cast <cgraph_node *> (node))
        {
          ....
        }

    The necessary conditional test defines a variable that holds a known good
    pointer to the specific item and avoids subsequent conversion calls and
    the assertion checks that may come with them.

    When, the property test is embedded within a larger condition, the
    variable declaration gets pulled out of the condition.  (This approach
    leaves some room for using the variable inappropriately.)

      if (symtab_variable_p (node) && varpool (node)->finalized)
        varpool_analyze_node (varpool (node));

    becomes

      varpool_node *vnode = dyn_cast <varpool_node *> (node);
      if (vnode && vnode->finalized)
        varpool_analyze_node (vnode);

    Note that we have converted two sets of assertions in the calls to varpool
    into safe and efficient use of a variable.

TYPE safe_dyn_cast <TYPE> (pointer)

    Like dyn_cast <TYPE> (pointer), except that it accepts null pointers
    and returns null results for them.


If you use these functions and get a 'inline function not defined' or a
'missing symbol' error message for 'is_a_helper<....>::test', it means that
the connection between the types has not been made.  See below.


EXTENDING THE GENERIC TYPE FACILITY

Each connection between types must be made by defining a specialization of the
template member function 'test' of the template class 'is_a_helper'.  For
example,

  template <>
  template <>
  inline bool
  is_a_helper <cgraph_node *>::test (symtab_node *p)
  {
    return p->type == SYMTAB_FUNCTION;
  }

If a simple reinterpret_cast between the pointer types is incorrect, then you
must also specialize the template member function 'cast'.  Failure to do so
when needed may result in a crash.  For example,

  template <>
  template <>
  inline bool
  is_a_helper <cgraph_node *>::cast (symtab_node *p)
  {
    return &p->x_function;
  }

*/

#ifndef GCC_IS_A_H
#define GCC_IS_A_H

/* A generic type conversion internal helper class.  */

template <typename T>
struct is_a_helper
{
  template <typename U>
  static inline bool test (U *p);
  template <typename U>
  static inline T cast (U *p);
};

/* Note that we deliberately do not define the 'test' member template.  Not
   doing so will result in a build-time error for type relationships that have
   not been defined, rather than a run-time error.  See the discussion above
   for when to define this member.  */

/* This is the generic implementation for casting from one type to another.
   Do not use this routine directly; it is an internal function.  See the
   discussion above for when to define this member.  */

template <typename T>
template <typename U>
inline T
is_a_helper <T>::cast (U *p)
{
  return reinterpret_cast <T> (p);
}


/* The public interface.  */

/* A generic test for a type relationship.  See the discussion above for when
   to use this function.  The question answered is "Is type T a derived type of
   type U?".  */

template <typename T, typename U>
inline bool
is_a (U *p)
{
  return is_a_helper<T>::test (p);
}

/* A generic conversion from a base type U to a derived type T.  See the
   discussion above for when to use this function.  */

template <typename T, typename U>
inline T
as_a (U *p)
{
  gcc_checking_assert (is_a <T> (p));
  return is_a_helper <T>::cast (p);
}

/* Similar to as_a<>, but where the pointer can be NULL, even if
   is_a_helper<T> doesn't check for NULL.  */

template <typename T, typename U>
inline T
safe_as_a (U *p)
{
  if (p)
    {
      gcc_checking_assert (is_a <T> (p));
      return is_a_helper <T>::cast (p);
    }
  else
    return NULL;
}

/* A generic checked conversion from a base type U to a derived type T.  See
   the discussion above for when to use this function.  */

template <typename T, typename U>
inline T
dyn_cast (U *p)
{
  if (is_a <T> (p))
    return is_a_helper <T>::cast (p);
  else
    return static_cast <T> (0);
}

/* Similar to dyn_cast, except that the pointer may be null.  */

template <typename T, typename U>
inline T
safe_dyn_cast (U *p)
{
  return p ? dyn_cast <T> (p) : 0;
}

#endif  /* GCC_IS_A_H  */