| // 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_ |
