rtc_base/system/rtc_export_template.h - src/ - Git at Google

 /*
  *  Copyright (c) 2019 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.
  */

 #ifndef RTC_BASE_SYSTEM_RTC_EXPORT_TEMPLATE_H_
 #define RTC_BASE_SYSTEM_RTC_EXPORT_TEMPLATE_H_

 // clang-format off
 // clang formating would cause cpplint errors in the macros below.

 // Most of this was borrowed (with minor modifications) from Chromium's
 // base/export_template.h.

 // Synopsis
 //
 // This header provides macros for using RTC_EXPORT macros with explicit
 // template instantiation declarations and definitions.
 // Generally, the RTC_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 RTC_EXPORT_TEMPLATE_DECLARE(RTC_EXPORT) foo<bar>;
 //
 // In a source file, write:
 //
 //   template class RTC_EXPORT_TEMPLATE_DEFINE(RTC_EXPORT) foo<bar>;

 // Implementation notes
 //
 // On Windows, when building when RTC_EXPORT expands to __declspec(dllexport)),
 // we want the two lines to expand to:
 //
 //     extern template class foo<bar>;
 //     template class RTC_EXPORT foo<bar>;
 //
 // In all other cases (non-Windows, and Windows when RTC_EXPORT expands to
 // __declspec(dllimport)), we want:
 //
 //     extern template class RTC_EXPORT foo<bar>;
 //     template class foo<bar>;
 //
 // The implementation of this header uses some subtle macro semantics to
 // detect what the provided RTC_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 RTC_EXPORT_TEMPLATE_STYLE_3 exists.)

 #define RTC_EXPORT_TEMPLATE_DECLARE(export)         \
   RTC_EXPORT_TEMPLATE_INVOKE(                       \
       DECLARE,                                      \
       RTC_EXPORT_TEMPLATE_STYLE(export, ), export)  // NOLINT
 #define RTC_EXPORT_TEMPLATE_DEFINE(export)          \
   RTC_EXPORT_TEMPLATE_INVOKE(                       \
       DEFINE,                                       \
       RTC_EXPORT_TEMPLATE_STYLE(export, ), export)  // NOLINT

 // INVOKE is an internal helper macro to perform parameter replacements
 // and token pasting to chain invoke another macro.  E.g.,
 //     RTC_EXPORT_TEMPLATE_INVOKE(DECLARE, DEFAULT, RTC_EXPORT)
 // will export to call
 //     RTC_EXPORT_TEMPLATE_DECLARE_DEFAULT(RTC_EXPORT, )
 // (but with RTC_EXPORT expanded too).
 #define RTC_EXPORT_TEMPLATE_INVOKE(which, style, export) \
   RTC_EXPORT_TEMPLATE_INVOKE_2(which, style, export)
 #define RTC_EXPORT_TEMPLATE_INVOKE_2(which, style, export) \
   RTC_EXPORT_TEMPLATE_##which##_##style(export, )

 // Default style is to apply the RTC_EXPORT macro at declaration sites.
 #define RTC_EXPORT_TEMPLATE_DECLARE_DEFAULT(export, _) export
 #define RTC_EXPORT_TEMPLATE_DEFINE_DEFAULT(export, _)

 // The "MSVC hack" style is used when RTC_EXPORT is defined
 // as __declspec(dllexport), which MSVC requires to be used at
 // definition sites instead.
 #define RTC_EXPORT_TEMPLATE_DECLARE_MSVC_HACK(export, _)
 #define RTC_EXPORT_TEMPLATE_DEFINE_MSVC_HACK(export, _) export

 // RTC_EXPORT_TEMPLATE_STYLE is an internal helper macro that identifies which
 // export style needs to be used for the provided RTC_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 RTC_EXPORT is
 // defined as "__declspec(dllimport)", it undergoes the following sequence of
 // macro substitutions:
 //     RTC_EXPORT_TEMPLATE_STYLE(RTC_EXPORT,)
 //     RTC_EXPORT_TEMPLATE_STYLE_2(__declspec(dllimport),)
 //     RTC_EXPORT_TEMPLATE_STYLE_3(
 //         RTC_EXPORT_TEMPLATE_STYLE_MATCH__declspec(dllimport))
 //     RTC_EXPORT_TEMPLATE_STYLE_MATCH__declspec(dllimport)
 //     RTC_EXPORT_TEMPLATE_STYLE_MATCH_DECLSPEC_dllimport
 //     DEFAULT
 #define RTC_EXPORT_TEMPLATE_STYLE(export, _) \
   RTC_EXPORT_TEMPLATE_STYLE_2(export, )
 #define RTC_EXPORT_TEMPLATE_STYLE_2(export, _) \
   RTC_EXPORT_TEMPLATE_STYLE_3(                 \
       RTC_EXPORT_TEMPLATE_STYLE_MATCH_foj3FJo5StF0OvIzl7oMxA##export)
 #define RTC_EXPORT_TEMPLATE_STYLE_3(style) style

 // Internal helper macros for RTC_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 RTC_EXPORT_TEMPLATE_STYLE_MATCH_foj3FJo5StF0OvIzl7oMxA DEFAULT
 #define RTC_EXPORT_TEMPLATE_STYLE_MATCH_foj3FJo5StF0OvIzl7oMxA__attribute__( \
     ...)                                                                     \
   DEFAULT
 #define RTC_EXPORT_TEMPLATE_STYLE_MATCH_foj3FJo5StF0OvIzl7oMxA__declspec(arg) \
   RTC_EXPORT_TEMPLATE_STYLE_MATCH_DECLSPEC_##arg

 // Internal helper macros for RTC_EXPORT_TEMPLATE_STYLE.
 #define RTC_EXPORT_TEMPLATE_STYLE_MATCH_DECLSPEC_dllexport MSVC_HACK
 #define RTC_EXPORT_TEMPLATE_STYLE_MATCH_DECLSPEC_dllimport DEFAULT

 // Sanity checks.
 //
 // RTC_EXPORT_TEMPLATE_TEST uses the same macro invocation pattern as
 // RTC_EXPORT_TEMPLATE_DECLARE and RTC_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:
 //
 //     RTC_EXPORT_TEMPLATE_TEST(DEFAULT, __declspec(dllimport));
 //
 //     static_assert(RTC_EXPORT_TEMPLATE_INVOKE(TEST_DEFAULT,
 //         RTC_EXPORT_TEMPLATE_STYLE(__declspec(dllimport), ),
 //         __declspec(dllimport)), "__declspec(dllimport)");
 //
 //     static_assert(RTC_EXPORT_TEMPLATE_INVOKE(TEST_DEFAULT,
 //         DEFAULT, __declspec(dllimport)), "__declspec(dllimport)");
 //
 //     static_assert(RTC_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 RTC_EXPORT_TEMPLATE_TEST(want, export)                    \
   static_assert(                                                  \
       RTC_EXPORT_TEMPLATE_INVOKE(                                 \
           TEST_##want,                                            \
           RTC_EXPORT_TEMPLATE_STYLE(export, ), export), #export)  // NOLINT
 #define RTC_EXPORT_TEMPLATE_TEST_DEFAULT_DEFAULT(...) true
 #define RTC_EXPORT_TEMPLATE_TEST_MSVC_HACK_MSVC_HACK(...) true

 RTC_EXPORT_TEMPLATE_TEST(DEFAULT, );  // NOLINT
 RTC_EXPORT_TEMPLATE_TEST(DEFAULT, __attribute__((visibility("default"))));
 RTC_EXPORT_TEMPLATE_TEST(MSVC_HACK, __declspec(dllexport));
 RTC_EXPORT_TEMPLATE_TEST(DEFAULT, __declspec(dllimport));

 #undef RTC_EXPORT_TEMPLATE_TEST
 #undef RTC_EXPORT_TEMPLATE_TEST_DEFAULT_DEFAULT
 #undef RTC_EXPORT_TEMPLATE_TEST_MSVC_HACK_MSVC_HACK

 // clang-format on

 #endif  // RTC_BASE_SYSTEM_RTC_EXPORT_TEMPLATE_H_
	/*
	* Copyright (c) 2019 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.
	*/

	#ifndef RTC_BASE_SYSTEM_RTC_EXPORT_TEMPLATE_H_
	#define RTC_BASE_SYSTEM_RTC_EXPORT_TEMPLATE_H_

	// clang-format off
	// clang formating would cause cpplint errors in the macros below.

	// Most of this was borrowed (with minor modifications) from Chromium's
	// base/export_template.h.

	// Synopsis
	//
	// This header provides macros for using RTC_EXPORT macros with explicit
	// template instantiation declarations and definitions.
	// Generally, the RTC_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 RTC_EXPORT_TEMPLATE_DECLARE(RTC_EXPORT) foo<bar>;
	//
	// In a source file, write:
	//
	// template class RTC_EXPORT_TEMPLATE_DEFINE(RTC_EXPORT) foo<bar>;

	// Implementation notes
	//
	// On Windows, when building when RTC_EXPORT expands to __declspec(dllexport)),
	// we want the two lines to expand to:
	//
	// extern template class foo<bar>;
	// template class RTC_EXPORT foo<bar>;
	//
	// In all other cases (non-Windows, and Windows when RTC_EXPORT expands to
	// __declspec(dllimport)), we want:
	//
	// extern template class RTC_EXPORT foo<bar>;
	// template class foo<bar>;
	//
	// The implementation of this header uses some subtle macro semantics to
	// detect what the provided RTC_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 RTC_EXPORT_TEMPLATE_STYLE_3 exists.)

	#define RTC_EXPORT_TEMPLATE_DECLARE(export) \
	RTC_EXPORT_TEMPLATE_INVOKE( \
	DECLARE, \
	RTC_EXPORT_TEMPLATE_STYLE(export, ), export) // NOLINT
	#define RTC_EXPORT_TEMPLATE_DEFINE(export) \
	RTC_EXPORT_TEMPLATE_INVOKE( \
	DEFINE, \
	RTC_EXPORT_TEMPLATE_STYLE(export, ), export) // NOLINT

	// INVOKE is an internal helper macro to perform parameter replacements
	// and token pasting to chain invoke another macro. E.g.,
	// RTC_EXPORT_TEMPLATE_INVOKE(DECLARE, DEFAULT, RTC_EXPORT)
	// will export to call
	// RTC_EXPORT_TEMPLATE_DECLARE_DEFAULT(RTC_EXPORT, )
	// (but with RTC_EXPORT expanded too).
	#define RTC_EXPORT_TEMPLATE_INVOKE(which, style, export) \
	RTC_EXPORT_TEMPLATE_INVOKE_2(which, style, export)
	#define RTC_EXPORT_TEMPLATE_INVOKE_2(which, style, export) \
	RTC_EXPORT_TEMPLATE_##which##_##style(export, )

	// Default style is to apply the RTC_EXPORT macro at declaration sites.
	#define RTC_EXPORT_TEMPLATE_DECLARE_DEFAULT(export, _) export
	#define RTC_EXPORT_TEMPLATE_DEFINE_DEFAULT(export, _)

	// The "MSVC hack" style is used when RTC_EXPORT is defined
	// as __declspec(dllexport), which MSVC requires to be used at
	// definition sites instead.
	#define RTC_EXPORT_TEMPLATE_DECLARE_MSVC_HACK(export, _)
	#define RTC_EXPORT_TEMPLATE_DEFINE_MSVC_HACK(export, _) export

	// RTC_EXPORT_TEMPLATE_STYLE is an internal helper macro that identifies which
	// export style needs to be used for the provided RTC_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 RTC_EXPORT is
	// defined as "__declspec(dllimport)", it undergoes the following sequence of
	// macro substitutions:
	// RTC_EXPORT_TEMPLATE_STYLE(RTC_EXPORT,)
	// RTC_EXPORT_TEMPLATE_STYLE_2(__declspec(dllimport),)
	// RTC_EXPORT_TEMPLATE_STYLE_3(
	// RTC_EXPORT_TEMPLATE_STYLE_MATCH__declspec(dllimport))
	// RTC_EXPORT_TEMPLATE_STYLE_MATCH__declspec(dllimport)
	// RTC_EXPORT_TEMPLATE_STYLE_MATCH_DECLSPEC_dllimport
	// DEFAULT
	#define RTC_EXPORT_TEMPLATE_STYLE(export, _) \
	RTC_EXPORT_TEMPLATE_STYLE_2(export, )
	#define RTC_EXPORT_TEMPLATE_STYLE_2(export, _) \
	RTC_EXPORT_TEMPLATE_STYLE_3( \
	RTC_EXPORT_TEMPLATE_STYLE_MATCH_foj3FJo5StF0OvIzl7oMxA##export)
	#define RTC_EXPORT_TEMPLATE_STYLE_3(style) style

	// Internal helper macros for RTC_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 RTC_EXPORT_TEMPLATE_STYLE_MATCH_foj3FJo5StF0OvIzl7oMxA DEFAULT
	#define RTC_EXPORT_TEMPLATE_STYLE_MATCH_foj3FJo5StF0OvIzl7oMxA__attribute__( \
	...) \
	DEFAULT
	#define RTC_EXPORT_TEMPLATE_STYLE_MATCH_foj3FJo5StF0OvIzl7oMxA__declspec(arg) \
	RTC_EXPORT_TEMPLATE_STYLE_MATCH_DECLSPEC_##arg

	// Internal helper macros for RTC_EXPORT_TEMPLATE_STYLE.
	#define RTC_EXPORT_TEMPLATE_STYLE_MATCH_DECLSPEC_dllexport MSVC_HACK
	#define RTC_EXPORT_TEMPLATE_STYLE_MATCH_DECLSPEC_dllimport DEFAULT

	// Sanity checks.
	//
	// RTC_EXPORT_TEMPLATE_TEST uses the same macro invocation pattern as
	// RTC_EXPORT_TEMPLATE_DECLARE and RTC_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:
	//
	// RTC_EXPORT_TEMPLATE_TEST(DEFAULT, __declspec(dllimport));
	//
	// static_assert(RTC_EXPORT_TEMPLATE_INVOKE(TEST_DEFAULT,
	// RTC_EXPORT_TEMPLATE_STYLE(__declspec(dllimport), ),
	// __declspec(dllimport)), "__declspec(dllimport)");
	//
	// static_assert(RTC_EXPORT_TEMPLATE_INVOKE(TEST_DEFAULT,
	// DEFAULT, __declspec(dllimport)), "__declspec(dllimport)");
	//
	// static_assert(RTC_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 RTC_EXPORT_TEMPLATE_TEST(want, export) \
	static_assert( \
	RTC_EXPORT_TEMPLATE_INVOKE( \
	TEST_##want, \
	RTC_EXPORT_TEMPLATE_STYLE(export, ), export), #export) // NOLINT
	#define RTC_EXPORT_TEMPLATE_TEST_DEFAULT_DEFAULT(...) true
	#define RTC_EXPORT_TEMPLATE_TEST_MSVC_HACK_MSVC_HACK(...) true

	RTC_EXPORT_TEMPLATE_TEST(DEFAULT, ); // NOLINT
	RTC_EXPORT_TEMPLATE_TEST(DEFAULT, __attribute__((visibility("default"))));
	RTC_EXPORT_TEMPLATE_TEST(MSVC_HACK, __declspec(dllexport));
	RTC_EXPORT_TEMPLATE_TEST(DEFAULT, __declspec(dllimport));

	#undef RTC_EXPORT_TEMPLATE_TEST
	#undef RTC_EXPORT_TEMPLATE_TEST_DEFAULT_DEFAULT
	#undef RTC_EXPORT_TEMPLATE_TEST_MSVC_HACK_MSVC_HACK

	// clang-format on

	#endif // RTC_BASE_SYSTEM_RTC_EXPORT_TEMPLATE_H_