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 41template <bool> 42struct 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