f048872e91
migrating talk/base to webrtc/base. BUG=N/A R=niklas.enbom@webrtc.org Review URL: https://webrtc-codereview.appspot.com/17479005 git-svn-id: http://webrtc.googlecode.com/svn/trunk@6129 4adac7df-926f-26a2-2b94-8c16560cd09d
83 lines
2.9 KiB
C++
83 lines
2.9 KiB
C++
/*
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* Copyright 2013 The WebRTC Project Authors. All rights reserved.
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*
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* Use of this source code is governed by a BSD-style license
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* that can be found in the LICENSE file in the root of the source
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* tree. An additional intellectual property rights grant can be found
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* in the file PATENTS. All contributing project authors may
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* be found in the AUTHORS file in the root of the source tree.
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*/
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// COMPILE_ASSERT macro, borrowed from google3/base/macros.h.
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#ifndef WEBRTC_BASE_COMPILE_ASSERT_H_
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#define WEBRTC_BASE_COMPILE_ASSERT_H_
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// The COMPILE_ASSERT macro can be used to verify that a compile time
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// expression is true. For example, you could use it to verify the
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// size of a static array:
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//
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// COMPILE_ASSERT(ARRAYSIZE(content_type_names) == CONTENT_NUM_TYPES,
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// content_type_names_incorrect_size);
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//
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// or to make sure a struct is smaller than a certain size:
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//
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// COMPILE_ASSERT(sizeof(foo) < 128, foo_too_large);
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//
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// The second argument to the macro is the name of the variable. If
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// the expression is false, most compilers will issue a warning/error
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// containing the name of the variable.
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// TODO(ajm): Hack to avoid multiple definitions until the base/ of webrtc and
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// libjingle are merged.
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#if !defined(COMPILE_ASSERT)
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template <bool>
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struct CompileAssert {
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};
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#define COMPILE_ASSERT(expr, msg) \
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typedef CompileAssert<(bool(expr))> msg[bool(expr) ? 1 : -1] // NOLINT
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#endif // COMPILE_ASSERT
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// Implementation details of COMPILE_ASSERT:
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//
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// - COMPILE_ASSERT works by defining an array type that has -1
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// elements (and thus is invalid) when the expression is false.
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//
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// - The simpler definition
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//
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// #define COMPILE_ASSERT(expr, msg) typedef char msg[(expr) ? 1 : -1]
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//
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// does not work, as gcc supports variable-length arrays whose sizes
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// are determined at run-time (this is gcc's extension and not part
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// of the C++ standard). As a result, gcc fails to reject the
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// following code with the simple definition:
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//
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// int foo;
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// COMPILE_ASSERT(foo, msg); // not supposed to compile as foo is
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// // not a compile-time constant.
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//
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// - By using the type CompileAssert<(bool(expr))>, we ensures that
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// expr is a compile-time constant. (Template arguments must be
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// determined at compile-time.)
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//
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// - The outer parentheses in CompileAssert<(bool(expr))> are necessary
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// to work around a bug in gcc 3.4.4 and 4.0.1. If we had written
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//
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// CompileAssert<bool(expr)>
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//
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// instead, these compilers will refuse to compile
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//
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// COMPILE_ASSERT(5 > 0, some_message);
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//
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// (They seem to think the ">" in "5 > 0" marks the end of the
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// template argument list.)
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//
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// - The array size is (bool(expr) ? 1 : -1), instead of simply
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//
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// ((expr) ? 1 : -1).
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//
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// This is to avoid running into a bug in MS VC 7.1, which
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// causes ((0.0) ? 1 : -1) to incorrectly evaluate to 1.
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#endif // WEBRTC_BASE_COMPILE_ASSERT_H_
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