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 namespace open_vcdiff { 22 23 // The VCD_COMPILE_ASSERT macro can be used to verify that a compile-time 24 // expression is true. For example, you could use it to verify the 25 // size of a static array: 26 // 27 // VCD_COMPILE_ASSERT(ARRAYSIZE(content_type_names) == CONTENT_NUM_TYPES, 28 // content_type_names_incorrect_size); 29 // 30 // or to make sure a struct is smaller than a certain size: 31 // 32 // VCD_COMPILE_ASSERT(sizeof(foo) < 128, foo_too_large); 33 // 34 // For the second argument to VCD_COMPILE_ASSERT, the programmer should supply 35 // a variable name that meets C++ naming rules, but that provides 36 // a description of the compile-time rule that has been violated. 37 // (In the example above, the name used is "foo_too_large".) 38 // If the expression is false, most compilers will issue a warning/error 39 // containing the name of the variable. 40 // This refinement (adding a descriptive variable name argument) 41 // is what differentiates VCD_COMPILE_ASSERT from Boost static asserts. 42 43 template <bool> 44 struct CompileAssert { 45 }; 46 47 } // namespace open_vcdiff 48 49 #define VCD_COMPILE_ASSERT(expr, msg) \ 50 typedef open_vcdiff::CompileAssert<static_cast<bool>(expr)> \ 51 msg[static_cast<bool>(expr) ? 1 : -1] 52 53 // Implementation details of VCD_COMPILE_ASSERT: 54 // 55 // - VCD_COMPILE_ASSERT works by defining an array type that has -1 56 // elements (and thus is invalid) when the expression is false. 57 // 58 // - The simpler definition 59 // 60 // #define VCD_COMPILE_ASSERT(expr, msg) typedef char msg[(expr) ? 1 : -1] 61 // 62 // does not work, as gcc supports variable-length arrays whose sizes 63 // are determined at run-time (this is gcc's extension and not part 64 // of the C++ standard). As a result, gcc fails to reject the 65 // following code with the simple definition: 66 // 67 // int foo; 68 // VCD_COMPILE_ASSERT(foo, msg); // not supposed to compile as foo is 69 // // not a compile-time constant. 70 // 71 // - By using the type CompileAssert<(static_cast<bool>(expr))>, we ensure that 72 // expr is a compile-time constant. (Template arguments must be 73 // determined at compile-time.) 74 // 75 // - The array size is (static_cast<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 // OPEN_VCDIFF_COMPILE_ASSERT_H_ 83