diff options
Diffstat (limited to 'gcc')
| -rw-r--r-- | gcc/is-a.h | 81 |
1 files changed, 63 insertions, 18 deletions
@@ -116,9 +116,30 @@ 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, +Method 1 +-------- + +If DERIVED is derived from BASE, and if BASE contains enough information +to determine whether an object is actually an instance of DERIVED, +then you can make the above routines work for DERIVED by defining +a specialization of is_a_helper such as: + + template<> + struct is_a_helper<DERIVED *> : static_is_a_helper<DERIVED *> + { + static inline bool test (const BASE *p) { return ...; } + }; + +This test function should return true if P is an instanced of DERIVED. +This on its own is enough; the comments below for method 2 do not apply. + +Method 2 +-------- + +Alternatively, if two types are connected in ways other than C++ +inheritance, each connection between them must be made by defining a +specialization of the template member function 'test' of the template +class 'is_a_helper'. For example, template <> template <> @@ -145,15 +166,52 @@ when needed may result in a crash. For example, #ifndef GCC_IS_A_H #define GCC_IS_A_H +/* A base class that specializations of is_a_helper can use if casting + U * to T is simply a reinterpret_cast. */ + +template <typename T> +struct reinterpret_is_a_helper +{ + template <typename U> + static inline T cast (U *p) { return reinterpret_cast <T> (p); } +}; + +/* A base class that specializations of is_a_helper can use if casting + U * to T is simply a static_cast. This is more type-safe than + reinterpret_is_a_helper. */ + +template <typename T> +struct static_is_a_helper +{ + template <typename U> + static inline T cast (U *p) { return static_cast <T> (p); } +}; + /* A generic type conversion internal helper class. */ template <typename T> -struct is_a_helper +struct is_a_helper : reinterpret_is_a_helper<T> { template <typename U> static inline bool test (U *p); +}; + +/* Reuse the definition of is_a_helper<T *> to implement + is_a_helper<const T *>. */ + +template <typename T> +struct is_a_helper<const T *> +{ template <typename U> - static inline T cast (U *p); + static inline const T *cast (const U *p) + { + return is_a_helper<T *>::cast (const_cast <U *> (p)); + } + template <typename U> + static inline bool test (const U *p) + { + return is_a_helper<T *>::test (p); + } }; /* Note that we deliberately do not define the 'test' member template. Not @@ -161,19 +219,6 @@ struct is_a_helper 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 |