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use variadic macros/templates in IPC message implementation
TBR=jam@chromium.org, mseaborn@chromium.org Review URL: https://codereview.chromium.org/1532053002 Cr-Commit-Position: refs/heads/master@{#374316}
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| // Copyright 2015 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. | ||
|
|
||
| #ifndef IPC_EXPORT_TEMPLATE_H_ | ||
| #define IPC_EXPORT_TEMPLATE_H_ | ||
|
|
||
| // Synopsis | ||
| // | ||
| // This header provides macros for using FOO_EXPORT macros with explicit | ||
| // template instantiation declarations and definitions. | ||
| // Generally, the FOO_EXPORT macros are used at declarations, | ||
| // and GCC requires them to be used at explicit instantiation declarations, | ||
| // but MSVC requires __declspec(dllexport) to be used at the explicit | ||
| // instantiation definitions instead. | ||
|
|
||
| // Usage | ||
| // | ||
| // In a header file, write: | ||
| // | ||
| // extern template class EXPORT_TEMPLATE_DECLARE(FOO_EXPORT) foo<bar>; | ||
| // | ||
| // In a source file, write: | ||
| // | ||
| // template class EXPORT_TEMPLATE_DEFINE(FOO_EXPORT) foo<bar>; | ||
|
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||
| // Implementation notes | ||
| // | ||
| // The implementation of this header uses some subtle macro semantics to | ||
| // detect what the provided FOO_EXPORT value was defined as and then | ||
| // to dispatch to appropriate macro definitions. Unfortunately, | ||
| // MSVC's C preprocessor is rather non-compliant and requires special | ||
| // care to make it work. | ||
| // | ||
| // Issue 1. | ||
| // | ||
| // #define F(x) | ||
| // F() | ||
| // | ||
| // MSVC emits warning C4003 ("not enough actual parameters for macro | ||
| // 'F'), even though it's a valid macro invocation. This affects the | ||
| // macros below that take just an "export" parameter, because export | ||
| // may be empty. | ||
| // | ||
| // As a workaround, we can add a dummy parameter and arguments: | ||
| // | ||
| // #define F(x,_) | ||
| // F(,) | ||
| // | ||
| // Issue 2. | ||
| // | ||
| // #define F(x) G##x | ||
| // #define Gj() ok | ||
| // F(j()) | ||
| // | ||
| // The correct replacement for "F(j())" is "ok", but MSVC replaces it | ||
| // with "Gj()". As a workaround, we can pass the result to an | ||
| // identity macro to force MSVC to look for replacements again. (This | ||
| // is why EXPORT_TEMPLATE_STYLE_3 exists.) | ||
|
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| #define EXPORT_TEMPLATE_DECLARE(export) \ | ||
| EXPORT_TEMPLATE_INVOKE(DECLARE, EXPORT_TEMPLATE_STYLE(export, ), export) | ||
| #define EXPORT_TEMPLATE_DEFINE(export) \ | ||
| EXPORT_TEMPLATE_INVOKE(DEFINE, EXPORT_TEMPLATE_STYLE(export, ), export) | ||
|
|
||
| // INVOKE is an internal helper macro to perform parameter replacements | ||
| // and token pasting to chain invoke another macro. E.g., | ||
| // EXPORT_TEMPLATE_INVOKE(DECLARE, DEFAULT, FOO_EXPORT) | ||
| // will export to call | ||
| // EXPORT_TEMPLATE_DECLARE_DEFAULT(FOO_EXPORT, ) | ||
| // (but with FOO_EXPORT expanded too). | ||
| #define EXPORT_TEMPLATE_INVOKE(which, style, export) \ | ||
| EXPORT_TEMPLATE_INVOKE_2(which, style, export) | ||
| #define EXPORT_TEMPLATE_INVOKE_2(which, style, export) \ | ||
| EXPORT_TEMPLATE_##which##_##style(export, ) | ||
|
|
||
| // Default style is to apply the FOO_EXPORT macro at declaration sites. | ||
| #define EXPORT_TEMPLATE_DECLARE_DEFAULT(export, _) export | ||
| #define EXPORT_TEMPLATE_DEFINE_DEFAULT(export, _) | ||
|
|
||
| // The "MSVC hack" style is used when FOO_EXPORT is defined | ||
| // as __declspec(dllexport), which MSVC requires to be used at | ||
| // definition sites instead. | ||
| #define EXPORT_TEMPLATE_DECLARE_MSVC_HACK(export, _) | ||
| #define EXPORT_TEMPLATE_DEFINE_MSVC_HACK(export, _) export | ||
|
|
||
| // EXPORT_TEMPLATE_STYLE is an internal helper macro that identifies which | ||
| // export style needs to be used for the provided FOO_EXPORT macro definition. | ||
| // "", "__attribute__(...)", and "__declspec(dllimport)" are mapped | ||
| // to "DEFAULT"; while "__declspec(dllexport)" is mapped to "MSVC_HACK". | ||
| // | ||
| // It's implemented with token pasting to transform the __attribute__ and | ||
| // __declspec annotations into macro invocations. E.g., if FOO_EXPORT is | ||
| // defined as "__declspec(dllimport)", it undergoes the following sequence of | ||
| // macro substitutions: | ||
| // EXPORT_TEMPLATE_STYLE(FOO_EXPORT, ) | ||
| // EXPORT_TEMPLATE_STYLE_2(__declspec(dllimport), ) | ||
| // EXPORT_TEMPLATE_STYLE_3(EXPORT_TEMPLATE_STYLE_MATCH__declspec(dllimport)) | ||
| // EXPORT_TEMPLATE_STYLE_MATCH__declspec(dllimport) | ||
| // EXPORT_TEMPLATE_STYLE_MATCH_DECLSPEC_dllimport | ||
| // DEFAULT | ||
| #define EXPORT_TEMPLATE_STYLE(export, _) EXPORT_TEMPLATE_STYLE_2(export, ) | ||
| #define EXPORT_TEMPLATE_STYLE_2(export, _) \ | ||
| EXPORT_TEMPLATE_STYLE_3( \ | ||
| EXPORT_TEMPLATE_STYLE_MATCH_foj3FJo5StF0OvIzl7oMxA##export) | ||
| #define EXPORT_TEMPLATE_STYLE_3(style) style | ||
|
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||
| // Internal helper macros for EXPORT_TEMPLATE_STYLE. | ||
| // | ||
| // XXX: C++ reserves all identifiers containing "__" for the implementation, | ||
| // but "__attribute__" and "__declspec" already contain "__" and the token-paste | ||
| // operator can only add characters; not remove them. To minimize the risk of | ||
| // conflict with implementations, we include "foj3FJo5StF0OvIzl7oMxA" (a random | ||
| // 128-bit string, encoded in Base64) in the macro name. | ||
| #define EXPORT_TEMPLATE_STYLE_MATCH_foj3FJo5StF0OvIzl7oMxA DEFAULT | ||
| #define EXPORT_TEMPLATE_STYLE_MATCH_foj3FJo5StF0OvIzl7oMxA__attribute__(...) \ | ||
| DEFAULT | ||
| #define EXPORT_TEMPLATE_STYLE_MATCH_foj3FJo5StF0OvIzl7oMxA__declspec(arg) \ | ||
| EXPORT_TEMPLATE_STYLE_MATCH_DECLSPEC_##arg | ||
|
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| // Internal helper macros for EXPORT_TEMPLATE_STYLE. | ||
| #define EXPORT_TEMPLATE_STYLE_MATCH_DECLSPEC_dllexport MSVC_HACK | ||
| #define EXPORT_TEMPLATE_STYLE_MATCH_DECLSPEC_dllimport DEFAULT | ||
|
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||
| // Sanity checks. | ||
| // | ||
| // EXPORT_TEMPLATE_TEST uses the same macro invocation pattern as | ||
| // EXPORT_TEMPLATE_DECLARE and EXPORT_TEMPLATE_DEFINE do to check that they're | ||
| // working correctly. When they're working correctly, the sequence of macro | ||
| // replacements should go something like: | ||
| // | ||
| // EXPORT_TEMPLATE_TEST(DEFAULT, __declspec(dllimport)); | ||
| // | ||
| // static_assert(EXPORT_TEMPLATE_INVOKE(TEST_DEFAULT, | ||
| // EXPORT_TEMPLATE_STYLE(__declspec(dllimport), ), | ||
| // __declspec(dllimport)), "__declspec(dllimport)"); | ||
| // | ||
| // static_assert(EXPORT_TEMPLATE_INVOKE(TEST_DEFAULT, | ||
| // DEFAULT, __declspec(dllimport)), "__declspec(dllimport)"); | ||
| // | ||
| // static_assert(EXPORT_TEMPLATE_TEST_DEFAULT_DEFAULT( | ||
| // __declspec(dllimport)), "__declspec(dllimport)"); | ||
| // | ||
| // static_assert(true, "__declspec(dllimport)"); | ||
| // | ||
| // When they're not working correctly, a syntax error should occur instead. | ||
| #define EXPORT_TEMPLATE_TEST(want, export) \ | ||
| static_assert(EXPORT_TEMPLATE_INVOKE( \ | ||
| TEST_##want, EXPORT_TEMPLATE_STYLE(export, ), export), \ | ||
| #export) | ||
| #define EXPORT_TEMPLATE_TEST_DEFAULT_DEFAULT(...) true | ||
| #define EXPORT_TEMPLATE_TEST_MSVC_HACK_MSVC_HACK(...) true | ||
|
|
||
| EXPORT_TEMPLATE_TEST(DEFAULT, ); | ||
| EXPORT_TEMPLATE_TEST(DEFAULT, __attribute__((visibility("default")))); | ||
| EXPORT_TEMPLATE_TEST(MSVC_HACK, __declspec(dllexport)); | ||
| EXPORT_TEMPLATE_TEST(DEFAULT, __declspec(dllimport)); | ||
|
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||
| #undef EXPORT_TEMPLATE_TEST | ||
| #undef EXPORT_TEMPLATE_TEST_DEFAULT_DEFAULT | ||
| #undef EXPORT_TEMPLATE_TEST_MSVC_HACK_MSVC_HACK | ||
|
|
||
| #endif // IPC_EXPORT_TEMPLATE_H_ |
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