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 21namespace 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 43template <bool> 44struct 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