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/* 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 */
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