diff options
Diffstat (limited to 'third_party/macros.h')
| -rw-r--r-- | third_party/macros.h | 98 |
1 files changed, 98 insertions, 0 deletions
diff --git a/third_party/macros.h b/third_party/macros.h new file mode 100644 index 0000000000..84c4366b16 --- /dev/null +++ b/third_party/macros.h @@ -0,0 +1,98 @@ +// 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_ |