140 lines
5.2 KiB
C++
140 lines
5.2 KiB
C++
//
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// Copyright 2017 The Abseil Authors.
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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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// -----------------------------------------------------------------------------
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// File: casts.h
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// -----------------------------------------------------------------------------
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//
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// This header file defines casting templates to fit use cases not covered by
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// the standard casts provided in the C++ standard. As with all cast operations,
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// use these with caution and only if alternatives do not exist.
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#ifndef ABSL_BASE_CASTS_H_
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#define ABSL_BASE_CASTS_H_
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#include <cstring>
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#include <type_traits>
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#include "absl/base/internal/identity.h"
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namespace absl {
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// implicit_cast()
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//
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// Performs an implicit conversion between types following the language
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// rules for implicit conversion; if an implicit conversion is otherwise
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// allowed by the language in the given context, this function performs such an
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// implicit conversion.
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//
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// Example:
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//
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// // If the context allows implicit conversion:
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// From from;
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// To to = from;
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//
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// // Such code can be replaced by:
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// implicit_cast<To>(from);
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//
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// An `implicit_cast()` may also be used to annotate numeric type conversions
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// that, although safe, may produce compiler warnings (such as `long` to `int`).
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// Additionally, an `implicit_cast()` is also useful within return statements to
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// indicate a specific implicit conversion is being undertaken.
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//
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// Example:
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//
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// return implicit_cast<double>(size_in_bytes) / capacity_;
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//
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// Annotating code with `implicit_cast()` allows you to explicitly select
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// particular overloads and template instantiations, while providing a safer
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// cast than `reinterpret_cast()` or `static_cast()`.
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//
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// Additionally, an `implicit_cast()` can be used to allow upcasting within a
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// type hierarchy where incorrect use of `static_cast()` could accidentally
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// allow downcasting.
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//
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// Finally, an `implicit_cast()` can be used to perform implicit conversions
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// from unrelated types that otherwise couldn't be implicitly cast directly;
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// C++ will normally only implicitly cast "one step" in such conversions.
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//
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// That is, if C is a type which can be implicitly converted to B, with B being
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// a type that can be implicitly converted to A, an `implicit_cast()` can be
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// used to convert C to B (which the compiler can then implicitly convert to A
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// using language rules).
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//
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// Example:
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//
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// // Assume an object C is convertible to B, which is implicitly convertible
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// // to A
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// A a = implicit_cast<B>(C);
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//
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// Such implicit cast chaining may be useful within template logic.
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template <typename To>
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inline To implicit_cast(typename absl::internal::identity_t<To> to) {
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return to;
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}
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// bit_cast()
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//
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// Performs a bitwise cast on a type without changing the underlying bit
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// representation of that type's value. The two types must be of the same size
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// and both types must be trivially copyable. As with most casts, use with
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// caution. A `bit_cast()` might be needed when you need to temporarily treat a
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// type as some other type, such as in the following cases:
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//
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// * Serialization (casting temporarily to `char *` for those purposes is
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// always allowed by the C++ standard)
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// * Managing the individual bits of a type within mathematical operations
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// that are not normally accessible through that type
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// * Casting non-pointer types to pointer types (casting the other way is
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// allowed by `reinterpret_cast()` but round-trips cannot occur the other
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// way).
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//
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// Example:
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//
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// float f = 3.14159265358979;
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// int i = bit_cast<int32_t>(f);
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// // i = 0x40490fdb
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//
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// Casting non-pointer types to pointer types and then dereferencing them
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// traditionally produces undefined behavior.
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//
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// Example:
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//
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// // WRONG
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// float f = 3.14159265358979; // WRONG
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// int i = * reinterpret_cast<int*>(&f); // WRONG
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//
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// The address-casting method produces undefined behavior according to the ISO
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// C++ specification section [basic.lval]. Roughly, this section says: if an
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// object in memory has one type, and a program accesses it with a different
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// type, the result is undefined behavior for most values of "different type".
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//
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// Such casting results in type punning: holding an object in memory of one type
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// and reading its bits back using a different type. A `bit_cast()` avoids this
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// issue by implementing its casts using `memcpy()`, which avoids introducing
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// this undefined behavior.
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template <typename Dest, typename Source>
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inline Dest bit_cast(const Source& source) {
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static_assert(sizeof(Dest) == sizeof(Source),
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"Source and destination types should have equal sizes.");
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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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} // namespace absl
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#endif // ABSL_BASE_CASTS_H_
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