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_ 83
