1/* Dynamic testing for abstract is-a relationships. 2 Copyright (C) 2012-2013 Free Software Foundation, Inc. 3 Contributed by Lawrence Crowl. 4 5This file is part of GCC. 6 7GCC is free software; you can redistribute it and/or modify it under 8the terms of the GNU General Public License as published by the Free 9Software Foundation; either version 3, or (at your option) any later 10version. 11 12GCC is distributed in the hope that it will be useful, but WITHOUT ANY 13WARRANTY; without even the implied warranty of MERCHANTABILITY or 14FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 15for more details. 16 17You should have received a copy of the GNU General Public License 18along with GCC; see the file COPYING3. If not see 19<http://www.gnu.org/licenses/>. */ 20 21 22/* This header generic type query and conversion functions. 23 24 25USING THE GENERIC TYPE FACILITY 26 27 28The user functions are: 29 30bool is_a <TYPE> (pointer) 31 32 Tests whether the pointer actually points to a more derived TYPE. 33 34 Suppose you have a symtab_node_def *ptr, AKA symtab_node ptr. You can test 35 whether it points to a 'derived' cgraph_node as follows. 36 37 if (is_a <cgraph_node> (ptr)) 38 .... 39 40 41TYPE *as_a <TYPE> (pointer) 42 43 Converts pointer to a TYPE*. 44 45 You can just assume that it is such a node. 46 47 do_something_with (as_a <cgraph_node> *ptr); 48 49TYPE *dyn_cast <TYPE> (pointer) 50 51 Converts pointer to TYPE* if and only if "is_a <TYPE> pointer". Otherwise, 52 returns NULL. This function is essentially a checked down cast. 53 54 This functions reduce compile time and increase type safety when treating a 55 generic item as a more specific item. 56 57 You can test and obtain a pointer to the 'derived' type in one indivisible 58 operation. 59 60 if (cgraph_node *cptr = dyn_cast <cgraph_node> (ptr)) 61 .... 62 63 As an example, the code change is from 64 65 if (symtab_function_p (node)) 66 { 67 struct cgraph_node *cnode = cgraph (node); 68 .... 69 } 70 71 to 72 73 if (cgraph_node *cnode = dyn_cast <cgraph_node> (node)) 74 { 75 .... 76 } 77 78 The necessary conditional test defines a variable that holds a known good 79 pointer to the specific item and avoids subsequent conversion calls and 80 the assertion checks that may come with them. 81 82 When, the property test is embedded within a larger condition, the 83 variable declaration gets pulled out of the condition. (This approach 84 leaves some room for using the variable inappropriately.) 85 86 if (symtab_variable_p (node) && varpool (node)->finalized) 87 varpool_analyze_node (varpool (node)); 88 89 becomes 90 91 varpool_node *vnode = dyn_cast <varpool_node> (node); 92 if (vnode && vnode->finalized) 93 varpool_analyze_node (vnode); 94 95 Note that we have converted two sets of assertions in the calls to varpool 96 into safe and efficient use of a variable. 97 98 99If you use these functions and get a 'inline function not defined' or a 100'missing symbol' error message for 'is_a_helper<....>::test', it means that 101the connection between the types has not been made. See below. 102 103 104EXTENDING THE GENERIC TYPE FACILITY 105 106Each connection between types must be made by defining a specialization of the 107template member function 'test' of the template class 'is_a_helper'. For 108example, 109 110 template <> 111 template <> 112 inline bool 113 is_a_helper <cgraph_node>::test (symtab_node_def *p) 114 { 115 return p->symbol.type == SYMTAB_FUNCTION; 116 } 117 118If a simple reinterpret_cast between the pointer types is incorrect, then you 119must also specialize the template member function 'cast'. Failure to do so 120when needed may result in a crash. For example, 121 122 template <> 123 template <> 124 inline bool 125 is_a_helper <cgraph_node>::cast (symtab_node_def *p) 126 { 127 return &p->x_function; 128 } 129 130*/ 131 132#ifndef GCC_IS_A_H 133#define GCC_IS_A_H 134 135/* A generic type conversion internal helper class. */ 136 137template <typename T> 138struct is_a_helper 139{ 140 template <typename U> 141 static inline bool test (U *p); 142 template <typename U> 143 static inline T *cast (U *p); 144}; 145 146/* Note that we deliberately do not define the 'test' member template. Not 147 doing so will result in a build-time error for type relationships that have 148 not been defined, rather than a run-time error. See the discussion above 149 for when to define this member. */ 150 151/* This is the generic implementation for casting from one type to another. 152 Do not use this routine directly; it is an internal function. See the 153 discussion above for when to define this member. */ 154 155template <typename T> 156template <typename U> 157inline T * 158is_a_helper <T>::cast (U *p) 159{ 160 return reinterpret_cast <T *> (p); 161} 162 163 164/* The public interface. */ 165 166/* A generic test for a type relationship. See the discussion above for when 167 to use this function. The question answered is "Is type T a derived type of 168 type U?". */ 169 170template <typename T, typename U> 171inline bool 172is_a (U *p) 173{ 174 return is_a_helper<T>::test (p); 175} 176 177/* A generic conversion from a base type U to a derived type T. See the 178 discussion above for when to use this function. */ 179 180template <typename T, typename U> 181inline T * 182as_a (U *p) 183{ 184 gcc_assert (is_a <T> (p)); 185 return is_a_helper <T>::cast (p); 186} 187 188/* A generic checked conversion from a base type U to a derived type T. See 189 the discussion above for when to use this function. */ 190 191template <typename T, typename U> 192inline T * 193dyn_cast (U *p) 194{ 195 if (is_a <T> (p)) 196 return is_a_helper <T>::cast (p); 197 else 198 return static_cast <T *> (0); 199} 200 201#endif /* GCC_IS_A_H */ 202