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193 lines
8.5 KiB
193 lines
8.5 KiB
/*
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* Copyright (C) 2011 The Android Open Source Project
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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#ifndef ART_LIBARTBASE_BASE_CASTS_H_
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#define ART_LIBARTBASE_BASE_CASTS_H_
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#include <stdint.h>
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#include <string.h>
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#include <limits>
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#include <type_traits>
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#include <android-base/logging.h>
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#include "stl_util_identity.h"
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namespace art {
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// Use implicit_cast as a safe version of static_cast or const_cast
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// for upcasting in the type hierarchy (i.e. casting a pointer to Foo
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// to a pointer to SuperclassOfFoo or casting a pointer to Foo to
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// a const pointer to Foo).
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// When you use implicit_cast, the compiler checks that the cast is safe.
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// Such explicit implicit_casts are necessary in surprisingly many
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// situations where C++ demands an exact type match instead of an
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// argument type convertible to a target type.
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//
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// The From type can be inferred, so the preferred syntax for using
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// implicit_cast is the same as for static_cast etc.:
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//
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// implicit_cast<ToType>(expr)
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//
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// implicit_cast would have been part of the C++ standard library,
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// but the proposal was submitted too late. It will probably make
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// its way into the language in the future.
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template<typename To, typename From>
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inline To implicit_cast(From const &f) {
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return f;
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}
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// When you upcast (that is, cast a pointer from type Foo to type
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// SuperclassOfFoo), it's fine to use implicit_cast<>, since upcasts
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// always succeed. When you downcast (that is, cast a pointer from
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// type Foo to type SubclassOfFoo), static_cast<> isn't safe, because
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// how do you know the pointer is really of type SubclassOfFoo? It
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// could be a bare Foo, or of type DifferentSubclassOfFoo. Thus,
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// when you downcast, you should use this macro. In debug mode, we
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// use dynamic_cast<> to double-check the downcast is legal (we die
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// if it's not). In normal mode, we do the efficient static_cast<>
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// instead. Thus, it's important to test in debug mode to make sure
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// the cast is legal!
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// This is the only place in the code we should use dynamic_cast<>.
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// In particular, you SHOULDN'T be using dynamic_cast<> in order to
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// do RTTI (eg code like this:
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// if (dynamic_cast<Subclass1>(foo)) HandleASubclass1Object(foo);
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// if (dynamic_cast<Subclass2>(foo)) HandleASubclass2Object(foo);
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// You should design the code some other way not to need this.
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template<typename To, typename From> // use like this: down_cast<T*>(foo);
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inline To down_cast(From* f) { // so we only accept pointers
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static_assert(std::is_base_of<From, typename std::remove_pointer<To>::type>::value,
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"down_cast unsafe as To is not a subtype of From");
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return static_cast<To>(f);
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}
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template<typename To, typename From> // use like this: down_cast<T&>(foo);
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inline To down_cast(From& f) { // so we only accept references
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static_assert(std::is_base_of<From, typename std::remove_reference<To>::type>::value,
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"down_cast unsafe as To is not a subtype of From");
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return static_cast<To>(f);
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}
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template <class Dest, class Source>
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inline Dest bit_cast(const Source& source) {
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// Compile time assertion: sizeof(Dest) == sizeof(Source)
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// A compile error here means your Dest and Source have different sizes.
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static_assert(sizeof(Dest) == sizeof(Source), "sizes should be equal");
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Dest dest;
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memcpy(&dest, &source, sizeof(dest));
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return dest;
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}
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// A version of static_cast that DCHECKs that the value can be precisely represented
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// when converting to Dest.
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template <typename Dest, typename Source>
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constexpr Dest dchecked_integral_cast(Source source) {
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DCHECK(
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// Check that the value is within the lower limit of Dest.
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(static_cast<intmax_t>(std::numeric_limits<Dest>::min()) <=
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static_cast<intmax_t>(std::numeric_limits<Source>::min()) ||
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source >= static_cast<Source>(std::numeric_limits<Dest>::min())) &&
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// Check that the value is within the upper limit of Dest.
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(static_cast<uintmax_t>(std::numeric_limits<Dest>::max()) >=
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static_cast<uintmax_t>(std::numeric_limits<Source>::max()) ||
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source <= static_cast<Source>(std::numeric_limits<Dest>::max())))
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<< "dchecked_integral_cast failed for " << source
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<< " (would be " << static_cast<Dest>(source) << ")";
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return static_cast<Dest>(source);
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}
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// A version of dchecked_integral_cast casting between an integral type and an enum type.
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// When casting to an enum type, the cast does not check if the value corresponds to an enumerator.
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// When casting from an enum type, the target type can be omitted and the enum's underlying type
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// shall be used.
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template <typename Dest, typename Source>
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constexpr
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typename std::enable_if<!std::is_enum<Source>::value, Dest>::type
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enum_cast(Source value) {
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return static_cast<Dest>(
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dchecked_integral_cast<typename std::underlying_type<Dest>::type>(value));
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}
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template <typename Dest = void, typename Source>
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constexpr
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typename std::enable_if<std::is_enum<Source>::value,
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typename std::conditional<std::is_same<Dest, void>::value,
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std::underlying_type<Source>,
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Identity<Dest>>::type>::type::type
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enum_cast(Source value) {
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using return_type = typename std::conditional<std::is_same<Dest, void>::value,
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std::underlying_type<Source>,
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Identity<Dest>>::type::type;
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return dchecked_integral_cast<return_type>(
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static_cast<typename std::underlying_type<Source>::type>(value));
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}
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// A version of reinterpret_cast<>() between pointers and int64_t/uint64_t
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// that goes through uintptr_t to avoid treating the pointer as "signed."
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template <typename Dest, typename Source>
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inline Dest reinterpret_cast64(Source source) {
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// This is the overload for casting from int64_t/uint64_t to a pointer.
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static_assert(std::is_same<Source, int64_t>::value || std::is_same<Source, uint64_t>::value,
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"Source must be int64_t or uint64_t.");
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static_assert(std::is_pointer<Dest>::value, "Dest must be a pointer.");
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// Check that we don't lose any non-0 bits here.
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DCHECK_EQ(static_cast<Source>(static_cast<uintptr_t>(source)), source);
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return reinterpret_cast<Dest>(static_cast<uintptr_t>(source));
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}
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template <typename Dest, typename Source>
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inline Dest reinterpret_cast64(Source* ptr) {
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// This is the overload for casting from a pointer to int64_t/uint64_t.
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static_assert(std::is_same<Dest, int64_t>::value || std::is_same<Dest, uint64_t>::value,
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"Dest must be int64_t or uint64_t.");
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static_assert(sizeof(uintptr_t) <= sizeof(Dest), "Expecting at most 64-bit pointers.");
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return static_cast<Dest>(reinterpret_cast<uintptr_t>(ptr));
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}
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// A version of reinterpret_cast<>() between pointers and int32_t/uint32_t that enforces
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// zero-extension and checks that the values are converted without loss of precision.
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template <typename Dest, typename Source>
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inline Dest reinterpret_cast32(Source source) {
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// This is the overload for casting from int32_t/uint32_t to a pointer.
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static_assert(std::is_same<Source, int32_t>::value || std::is_same<Source, uint32_t>::value,
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"Source must be int32_t or uint32_t.");
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static_assert(std::is_pointer<Dest>::value, "Dest must be a pointer.");
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// Check that we don't lose any non-0 bits here.
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static_assert(sizeof(uintptr_t) >= sizeof(Source), "Expecting at least 32-bit pointers.");
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return reinterpret_cast<Dest>(static_cast<uintptr_t>(static_cast<uint32_t>(source)));
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}
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template <typename Dest, typename Source>
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inline Dest reinterpret_cast32(Source* ptr) {
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// This is the overload for casting from a pointer to int32_t/uint32_t.
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static_assert(std::is_same<Dest, int32_t>::value || std::is_same<Dest, uint32_t>::value,
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"Dest must be int32_t or uint32_t.");
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static_assert(sizeof(uintptr_t) >= sizeof(Dest), "Expecting at least 32-bit pointers.");
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return static_cast<Dest>(dchecked_integral_cast<uint32_t>(reinterpret_cast<uintptr_t>(ptr)));
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}
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} // namespace art
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#endif // ART_LIBARTBASE_BASE_CASTS_H_
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