1/*
2 *  Copyright 2013 The WebRTC Project Authors. All rights reserved.
3 *
4 *  Use of this source code is governed by a BSD-style license
5 *  that can be found in the LICENSE file in the root of the source
6 *  tree. An additional intellectual property rights grant can be found
7 *  in the file PATENTS.  All contributing project authors may
8 *  be found in the AUTHORS file in the root of the source tree.
9 */
10
11// COMPILE_ASSERT macro, borrowed from google3/base/macros.h.
12#ifndef WEBRTC_BASE_COMPILE_ASSERT_H_
13#define WEBRTC_BASE_COMPILE_ASSERT_H_
14
15// The COMPILE_ASSERT macro can be used to verify that a compile time
16// expression is true. For example, you could use it to verify the
17// size of a static array:
18//
19//   COMPILE_ASSERT(ARRAYSIZE(content_type_names) == CONTENT_NUM_TYPES,
20//                  content_type_names_incorrect_size);
21//
22// or to make sure a struct is smaller than a certain size:
23//
24//   COMPILE_ASSERT(sizeof(foo) < 128, foo_too_large);
25//
26// The second argument to the macro is the name of the variable. If
27// the expression is false, most compilers will issue a warning/error
28// containing the name of the variable.
29
30// TODO(ajm): Hack to avoid multiple definitions until the base/ of webrtc and
31// libjingle are merged.
32#if !defined(COMPILE_ASSERT)
33template <bool>
34struct CompileAssert {
35};
36
37#define COMPILE_ASSERT(expr, msg) \
38  typedef CompileAssert<(bool(expr))> msg[bool(expr) ? 1 : -1]  // NOLINT
39#endif  // COMPILE_ASSERT
40
41// Implementation details of COMPILE_ASSERT:
42//
43// - COMPILE_ASSERT works by defining an array type that has -1
44//   elements (and thus is invalid) when the expression is false.
45//
46// - The simpler definition
47//
48//     #define COMPILE_ASSERT(expr, msg) typedef char msg[(expr) ? 1 : -1]
49//
50//   does not work, as gcc supports variable-length arrays whose sizes
51//   are determined at run-time (this is gcc's extension and not part
52//   of the C++ standard).  As a result, gcc fails to reject the
53//   following code with the simple definition:
54//
55//     int foo;
56//     COMPILE_ASSERT(foo, msg); // not supposed to compile as foo is
57//                               // not a compile-time constant.
58//
59// - By using the type CompileAssert<(bool(expr))>, we ensures that
60//   expr is a compile-time constant.  (Template arguments must be
61//   determined at compile-time.)
62//
63// - The outer parentheses in CompileAssert<(bool(expr))> are necessary
64//   to work around a bug in gcc 3.4.4 and 4.0.1.  If we had written
65//
66//     CompileAssert<bool(expr)>
67//
68//   instead, these compilers will refuse to compile
69//
70//     COMPILE_ASSERT(5 > 0, some_message);
71//
72//   (They seem to think the ">" in "5 > 0" marks the end of the
73//   template argument list.)
74//
75// - The array size is (bool(expr) ? 1 : -1), instead of simply
76//
77//     ((expr) ? 1 : -1).
78//
79//   This is to avoid running into a bug in MS VC 7.1, which
80//   causes ((0.0) ? 1 : -1) to incorrectly evaluate to 1.
81
82#endif  // WEBRTC_BASE_COMPILE_ASSERT_H_
83