1 // Copyright 2008 Google Inc. 2 // Authors: Zhanyong Wan, Lincoln Smith 3 // 4 // Licensed under the Apache License, Version 2.0 (the "License"); 5 // you may not use this file except in compliance with the License. 6 // You may obtain a copy of the License at 7 // 8 // http://www.apache.org/licenses/LICENSE-2.0 9 // 10 // Unless required by applicable law or agreed to in writing, software 11 // distributed under the License is distributed on an "AS IS" BASIS, 12 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 13 // See the License for the specific language governing permissions and 14 // limitations under the License. 15 16 #ifndef OPEN_VCDIFF_COMPILE_ASSERT_H_ 17 #define OPEN_VCDIFF_COMPILE_ASSERT_H_ 18 19 #include <config.h> 20 21 // The COMPILE_ASSERT macro can be used to verify that a compile-time 22 // expression is true. For example, you could use it to verify the 23 // size of a static array: 24 // 25 // COMPILE_ASSERT(ARRAYSIZE(content_type_names) == CONTENT_NUM_TYPES, 26 // content_type_names_incorrect_size); 27 // 28 // or to make sure a struct is smaller than a certain size: 29 // 30 // COMPILE_ASSERT(sizeof(foo) < 128, foo_too_large); 31 // 32 // For the second argument to COMPILE_ASSERT, the programmer should supply 33 // a variable name that meets C++ naming rules, but that provides 34 // a description of the compile-time rule that has been violated. 35 // (In the example above, the name used is "foo_too_large".) 36 // If the expression is false, most compilers will issue a warning/error 37 // containing the name of the variable. 38 // This refinement (adding a descriptive variable name argument) 39 // is what differentiates COMPILE_ASSERT from Boost static asserts. 40 41 template <bool> 42 struct CompileAssert { 43 }; 44 45 #define COMPILE_ASSERT(expr, msg) \ 46 typedef CompileAssert<static_cast<bool>(expr)> \ 47 msg[static_cast<bool>(expr) ? 1 : -1] 48 49 // Implementation details of COMPILE_ASSERT: 50 // 51 // - COMPILE_ASSERT works by defining an array type that has -1 52 // elements (and thus is invalid) when the expression is false. 53 // 54 // - The simpler definition 55 // 56 // #define COMPILE_ASSERT(expr, msg) typedef char msg[(expr) ? 1 : -1] 57 // 58 // does not work, as gcc supports variable-length arrays whose sizes 59 // are determined at run-time (this is gcc's extension and not part 60 // of the C++ standard). As a result, gcc fails to reject the 61 // following code with the simple definition: 62 // 63 // int foo; 64 // COMPILE_ASSERT(foo, msg); // not supposed to compile as foo is 65 // // not a compile-time constant. 66 // 67 // - By using the type CompileAssert<(static_cast<bool>(expr))>, we ensure that 68 // expr is a compile-time constant. (Template arguments must be 69 // determined at compile-time.) 70 // 71 // - The array size is (static_cast<bool>(expr) ? 1 : -1), instead of simply 72 // 73 // ((expr) ? 1 : -1). 74 // 75 // This is to avoid running into a bug in MS VC 7.1, which 76 // causes ((0.0) ? 1 : -1) to incorrectly evaluate to 1. 77 78 #endif // OPEN_VCDIFF_COMPILE_ASSERT_H_ 79
