henrike@webrtc.org | 47be73b | 2014-05-13 18:00:26 +0000 | [diff] [blame] | 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) |
| 33 | template <bool> |
| 34 | struct 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_ |