diff --git a/webrtc/base/compile_assert.h b/webrtc/base/compile_assert.h new file mode 100644 index 0000000000..47d40a9bd0 --- /dev/null +++ b/webrtc/base/compile_assert.h @@ -0,0 +1,90 @@ +/* + * Copyright 2013 The WebRTC Project Authors. All rights reserved. + * + * Use of this source code is governed by a BSD-style license + * that can be found in the LICENSE file in the root of the source + * tree. An additional intellectual property rights grant can be found + * in the file PATENTS. All contributing project authors may + * be found in the AUTHORS file in the root of the source tree. + */ + +// Borrowed from Chromium's src/base/macros.h. + +#ifndef WEBRTC_BASE_COMPILE_ASSERT_H_ +#define WEBRTC_BASE_COMPILE_ASSERT_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. + +// TODO(ajm): Hack to avoid multiple definitions until the base/ of webrtc and +// libjingle are merged. +#if !defined(COMPILE_ASSERT) +#if __cplusplus >= 201103L +// Under C++11, just use static_assert. +#define COMPILE_ASSERT(expr, msg) static_assert(expr, #msg) + +#else +template +struct CompileAssert { +}; + +#define COMPILE_ASSERT(expr, msg) \ + typedef CompileAssert<(bool(expr))> msg[bool(expr) ? 1 : -1] + +#endif // __cplusplus >= 201103L +#endif // !defined(COMPILE_ASSERT) + +// 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 +// +// 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 // WEBRTC_BASE_COMPILE_ASSERT_H_