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      1 /*
      2  * Copyright (C) 2011 The Android Open Source Project
      3  *
      4  * Licensed under the Apache License, Version 2.0 (the "License");
      5  * you may not use this file except in compliance with the License.
      6  * You may obtain a copy of the License at
      7  *
      8  *      http://www.apache.org/licenses/LICENSE-2.0
      9  *
     10  * Unless required by applicable law or agreed to in writing, software
     11  * distributed under the License is distributed on an "AS IS" BASIS,
     12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     13  * See the License for the specific language governing permissions and
     14  * limitations under the License.
     15  */
     16 
     17 #ifndef ART_RUNTIME_BASE_CASTS_H_
     18 #define ART_RUNTIME_BASE_CASTS_H_
     19 
     20 #include <assert.h>
     21 #include <limits>
     22 #include <stdint.h>
     23 #include <string.h>
     24 #include <type_traits>
     25 
     26 #include "base/logging.h"
     27 #include "base/macros.h"
     28 
     29 namespace art {
     30 
     31 // Use implicit_cast as a safe version of static_cast or const_cast
     32 // for upcasting in the type hierarchy (i.e. casting a pointer to Foo
     33 // to a pointer to SuperclassOfFoo or casting a pointer to Foo to
     34 // a const pointer to Foo).
     35 // When you use implicit_cast, the compiler checks that the cast is safe.
     36 // Such explicit implicit_casts are necessary in surprisingly many
     37 // situations where C++ demands an exact type match instead of an
     38 // argument type convertible to a target type.
     39 //
     40 // The From type can be inferred, so the preferred syntax for using
     41 // implicit_cast is the same as for static_cast etc.:
     42 //
     43 //   implicit_cast<ToType>(expr)
     44 //
     45 // implicit_cast would have been part of the C++ standard library,
     46 // but the proposal was submitted too late.  It will probably make
     47 // its way into the language in the future.
     48 template<typename To, typename From>
     49 inline To implicit_cast(From const &f) {
     50   return f;
     51 }
     52 
     53 // When you upcast (that is, cast a pointer from type Foo to type
     54 // SuperclassOfFoo), it's fine to use implicit_cast<>, since upcasts
     55 // always succeed.  When you downcast (that is, cast a pointer from
     56 // type Foo to type SubclassOfFoo), static_cast<> isn't safe, because
     57 // how do you know the pointer is really of type SubclassOfFoo?  It
     58 // could be a bare Foo, or of type DifferentSubclassOfFoo.  Thus,
     59 // when you downcast, you should use this macro.  In debug mode, we
     60 // use dynamic_cast<> to double-check the downcast is legal (we die
     61 // if it's not).  In normal mode, we do the efficient static_cast<>
     62 // instead.  Thus, it's important to test in debug mode to make sure
     63 // the cast is legal!
     64 //    This is the only place in the code we should use dynamic_cast<>.
     65 // In particular, you SHOULDN'T be using dynamic_cast<> in order to
     66 // do RTTI (eg code like this:
     67 //    if (dynamic_cast<Subclass1>(foo)) HandleASubclass1Object(foo);
     68 //    if (dynamic_cast<Subclass2>(foo)) HandleASubclass2Object(foo);
     69 // You should design the code some other way not to need this.
     70 
     71 template<typename To, typename From>     // use like this: down_cast<T*>(foo);
     72 inline To down_cast(From* f) {                   // so we only accept pointers
     73   static_assert(std::is_base_of<From, typename std::remove_pointer<To>::type>::value,
     74                 "down_cast unsafe as To is not a subtype of From");
     75 
     76   return static_cast<To>(f);
     77 }
     78 
     79 template <class Dest, class Source>
     80 inline Dest bit_cast(const Source& source) {
     81   // Compile time assertion: sizeof(Dest) == sizeof(Source)
     82   // A compile error here means your Dest and Source have different sizes.
     83   static_assert(sizeof(Dest) == sizeof(Source), "sizes should be equal");
     84   Dest dest;
     85   memcpy(&dest, &source, sizeof(dest));
     86   return dest;
     87 }
     88 
     89 // A version of static_cast that DCHECKs that the value can be precisely represented
     90 // when converting to Dest.
     91 template <typename Dest, typename Source>
     92 inline Dest dchecked_integral_cast(const Source source) {
     93   DCHECK(
     94       // Check that the value is within the lower limit of Dest.
     95       (static_cast<intmax_t>(std::numeric_limits<Dest>::min()) <=
     96           static_cast<intmax_t>(std::numeric_limits<Source>::min()) ||
     97           source >= static_cast<Source>(std::numeric_limits<Dest>::min())) &&
     98       // Check that the value is within the upper limit of Dest.
     99       (static_cast<uintmax_t>(std::numeric_limits<Dest>::max()) >=
    100           static_cast<uintmax_t>(std::numeric_limits<Source>::max()) ||
    101           source <= static_cast<Source>(std::numeric_limits<Dest>::max())));
    102 
    103   return static_cast<Dest>(source);
    104 }
    105 
    106 // A version of reinterpret_cast<>() between pointers and int64_t/uint64_t
    107 // that goes through uintptr_t to avoid treating the pointer as "signed."
    108 
    109 template <typename Dest, typename Source>
    110 inline Dest reinterpret_cast64(Source source) {
    111   // This is the overload for casting from int64_t/uint64_t to a pointer.
    112   static_assert(std::is_same<Source, int64_t>::value || std::is_same<Source, uint64_t>::value,
    113                 "Source must be int64_t or uint64_t.");
    114   static_assert(std::is_pointer<Dest>::value, "Dest must be a pointer.");
    115   // Check that we don't lose any non-0 bits here.
    116   DCHECK_EQ(static_cast<Source>(static_cast<uintptr_t>(source)), source);
    117   return reinterpret_cast<Dest>(static_cast<uintptr_t>(source));
    118 }
    119 
    120 template <typename Dest, typename Source>
    121 inline Dest reinterpret_cast64(Source* ptr) {
    122   // This is the overload for casting from a pointer to int64_t/uint64_t.
    123   static_assert(std::is_same<Dest, int64_t>::value || std::is_same<Dest, uint64_t>::value,
    124                 "Dest must be int64_t or uint64_t.");
    125   static_assert(sizeof(uintptr_t) <= sizeof(Dest), "Expecting at most 64-bit pointers.");
    126   return static_cast<Dest>(reinterpret_cast<uintptr_t>(ptr));
    127 }
    128 
    129 }  // namespace art
    130 
    131 #endif  // ART_RUNTIME_BASE_CASTS_H_
    132