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Unified Diff: base/bit_cast.h

Issue 399313006: Move bit_cast from base/macros.h to its own header (Closed) Base URL: svn://svn.chromium.org/chrome/trunk/src
Patch Set: bit_cast spruce up+fix typo Created 4 years, 12 months ago
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Index: base/bit_cast.h
diff --git a/base/bit_cast.h b/base/bit_cast.h
new file mode 100644
index 0000000000000000000000000000000000000000..5ff5e99fa1630851858f46eeec81604eb56a2e03
--- /dev/null
+++ b/base/bit_cast.h
@@ -0,0 +1,76 @@
+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef BASE_BIT_CAST_H_
+#define BASE_BIT_CAST_H_
+
+#include <string.h> // For memcpy.
+
+#include <type_traits>
+
+// bit_cast<Dest,Source> is a template function that implements the equivalent
+// of "*reinterpret_cast<Dest*>(&source)". We need this in very low-level
+// functions like the protobuf library and fast math support.
+//
+// float f = 3.14159265358979;
+// int i = bit_cast<int32_t>(f);
+// // i = 0x40490fdb
+//
+// The classical address-casting method is:
+//
+// // WRONG
+// float f = 3.14159265358979; // WRONG
+// int i = * reinterpret_cast<int*>(&f); // WRONG
+//
+// The address-casting method actually produces undefined behavior according to
+// the ISO C++98 specification, section 3.10 ("basic.lval"), paragraph 15.
+// (This did not substantially change in C++11.) Roughly, this section says: if
+// an object in memory has one type, and a program accesses it with a different
+// type, then the result is undefined behavior for most values of "different
+// type".
+//
+// This is true for any cast syntax, either *(int*)&f or
+// *reinterpret_cast<int*>(&f). And it is particularly true for conversions
+// between integral lvalues and floating-point lvalues.
+//
+// The purpose of this paragraph is to allow optimizing compilers to assume that
+// expressions with different types refer to different memory. Compilers are
+// known to take advantage of this. So a non-conforming program quietly
+// produces wildly incorrect output.
+//
+// The problem is not the use of reinterpret_cast. The problem is type punning:
+// holding an object in memory of one type and reading its bits back using a
+// different type.
+//
+// The C++ standard is more subtle and complex than this, but that is the basic
+// idea.
+//
+// Anyways ...
+//
+// bit_cast<> calls memcpy() which is blessed by the standard, especially by the
+// example in section 3.9 . Also, of course, bit_cast<> wraps up the nasty
+// logic in one place.
+//
+// Fortunately memcpy() is very fast. In optimized mode, compilers replace
+// calls to memcpy() with inline object code when the size argument is a
+// compile-time constant. On a 32-bit system, memcpy(d,s,4) compiles to one
+// load and one store, and memcpy(d,s,8) compiles to two loads and two stores.
+//
+// Note: Dest and Source must be POD types.
+//
+template <class Dest, class Source>
+inline Dest bit_cast(const Source& source) {
+ static_assert(sizeof(Dest) == sizeof(Source),
+ "bit_cast requires source and destination to be the same size");
+ static_assert(std::is_trivially_copyable<Source>::value,
+ "Source must be a POD type or trivially copyable");
+ static_assert(std::is_trivially_copyable<Dest>::value,
+ "Dest must be a POD type or trivially copyable");
Avi (use Gerrit) 2016/01/04 00:24:46 This is indeed a better choice than is_pod.
tapted 2016/01/04 00:59:30 Doh - bots are still reporting in, but it looks li
+
+ Dest dest;
+ memcpy(&dest, &source, sizeof(dest));
+ return dest;
+}
+
+#endif // BASE_BIT_CAST_H_
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