| // Copyright 2014 The Chromium Authors. All rights reserved. | |
| // Use of this source code is governed by a BSD-style license that can be | |
| // found in the LICENSE file. | |
| // This file contains macros and macro-like constructs (e.g., templates) that | |
| // are commonly used throughout Chromium source. (It may also contain things | |
| // that are closely related to things that are commonly used that belong in this | |
| // file.) | |
| #ifndef BASE_MACROS_H_ | |
| #define BASE_MACROS_H_ | |
| // The COMPILE_ASSERT macro can be used to verify that a compile time | |
| // expression is true. For example, you could use it to verify the | |
| // size of a static array: | |
| // | |
| // COMPILE_ASSERT(ARRAYSIZE_UNSAFE(content_type_names) == CONTENT_NUM_TYPES, | |
| // content_type_names_incorrect_size); | |
| // | |
| // or to make sure a struct is smaller than a certain size: | |
| // | |
| // COMPILE_ASSERT(sizeof(foo) < 128, foo_too_large); | |
| // | |
| // The second argument to the macro is the name of the variable. If | |
| // the expression is false, most compilers will issue a warning/error | |
| // containing the name of the variable. | |
| #undef COMPILE_ASSERT | |
| #if __cplusplus >= 201103L | |
| // Under C++11, just use static_assert. | |
| #define COMPILE_ASSERT(expr, msg) static_assert(expr, #msg) | |
| #else | |
| template <bool> | |
| struct CompileAssert { | |
| }; | |
| // Annotate a variable indicating it's ok if the variable is not used. | |
| // (Typically used to silence a compiler warning when the assignment | |
| // is important for some other reason.) | |
| // Use like: | |
| // int x ALLOW_UNUSED = ...; | |
| #if defined(COMPILER_GCC) | |
| #define ALLOW_UNUSED __attribute__((unused)) | |
| #else | |
| #define ALLOW_UNUSED | |
| #endif | |
| #define COMPILE_ASSERT(expr, msg) \ | |
| typedef CompileAssert<(bool(expr))> msg[bool(expr) ? 1 : -1] ALLOW_UNUSED | |
| // Implementation details of COMPILE_ASSERT: | |
| // | |
| // - COMPILE_ASSERT works by defining an array type that has -1 | |
| // elements (and thus is invalid) when the expression is false. | |
| // | |
| // - The simpler definition | |
| // | |
| // #define COMPILE_ASSERT(expr, msg) typedef char msg[(expr) ? 1 : -1] | |
| // | |
| // does not work, as gcc supports variable-length arrays whose sizes | |
| // are determined at run-time (this is gcc's extension and not part | |
| // of the C++ standard). As a result, gcc fails to reject the | |
| // following code with the simple definition: | |
| // | |
| // int foo; | |
| // COMPILE_ASSERT(foo, msg); // not supposed to compile as foo is | |
| // // not a compile-time constant. | |
| // | |
| // - By using the type CompileAssert<(bool(expr))>, we ensures that | |
| // expr is a compile-time constant. (Template arguments must be | |
| // determined at compile-time.) | |
| // | |
| // - The outer parentheses in CompileAssert<(bool(expr))> are necessary | |
| // to work around a bug in gcc 3.4.4 and 4.0.1. If we had written | |
| // | |
| // CompileAssert<bool(expr)> | |
| // | |
| // instead, these compilers will refuse to compile | |
| // | |
| // COMPILE_ASSERT(5 > 0, some_message); | |
| // | |
| // (They seem to think the ">" in "5 > 0" marks the end of the | |
| // template argument list.) | |
| // | |
| // - The array size is (bool(expr) ? 1 : -1), instead of simply | |
| // | |
| // ((expr) ? 1 : -1). | |
| // | |
| // This is to avoid running into a bug in MS VC 7.1, which | |
| // causes ((0.0) ? 1 : -1) to incorrectly evaluate to 1. | |
| #endif | |
| #endif // BASE_MACROS_H_ |