// 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 BASE_EXPORT_TEMPLATE_H_ #define BASE_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; // // In a source file, write: // // template class EXPORT_TEMPLATE_DEFINE(FOO_EXPORT) foo; // Implementation notes // // On Windows, when building the FOO library (that is, when FOO_EXPORT expands // to __declspec(dllexport)), we want the two lines to expand to: // // extern template class foo; // template class FOO_EXPORT foo; // // In all other cases (non-Windows, and Windows when using the FOO library (that // is when FOO_EXPORT expands to __declspec(dllimport)), we want: // // extern template class FOO_EXPORT foo; // template class foo; // // 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. #define EXPORT_TEMPLATE_DECLARE(foo_export) \ EXPORT_TEMPLATE_INVOKE(DECLARE, EXPORT_TEMPLATE_STYLE(foo_export), foo_export) #define EXPORT_TEMPLATE_DEFINE(foo_export) \ EXPORT_TEMPLATE_INVOKE(DEFINE, EXPORT_TEMPLATE_STYLE(foo_export), foo_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 expand to call // EXPORT_TEMPLATE_DECLARE_DEFAULT(FOO_EXPORT) // (but with FOO_EXPORT expanded too). #define EXPORT_TEMPLATE_INVOKE(which, style, foo_export) \ EXPORT_TEMPLATE_INVOKE_2(which, style, foo_export) #define EXPORT_TEMPLATE_INVOKE_2(which, style, foo_export) \ EXPORT_TEMPLATE_##which##_##style(foo_export) // Default style is to apply the FOO_EXPORT macro at declaration sites. #define EXPORT_TEMPLATE_DECLARE_DEFAULT(foo_export) foo_export #define EXPORT_TEMPLATE_DEFINE_DEFAULT(foo_export) // The "declspec" 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_EXPORT_DLLEXPORT(foo_export) #define EXPORT_TEMPLATE_DEFINE_EXPORT_DLLEXPORT(foo_export) foo_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 "EXPORT_DLLEXPORT". // (NaCl headers define "DLLEXPORT" already, else we'd use that. // TODO(thakis): Rename once nacl is gone.) // // 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_MATCH__declspec(dllimport) // EXPORT_TEMPLATE_STYLE_MATCH_DECLSPEC_dllimport // DEFAULT #define EXPORT_TEMPLATE_STYLE(foo_export) EXPORT_TEMPLATE_STYLE_2(foo_export) #define EXPORT_TEMPLATE_STYLE_2(foo_export) \ EXPORT_TEMPLATE_STYLE_MATCH_foj3FJo5StF0OvIzl7oMxA##foo_export // 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 // Internal helper macros for EXPORT_TEMPLATE_STYLE. #define EXPORT_TEMPLATE_STYLE_MATCH_DECLSPEC_dllexport EXPORT_DLLEXPORT #define EXPORT_TEMPLATE_STYLE_MATCH_DECLSPEC_dllimport DEFAULT // 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, foo_export) \ static_assert( \ EXPORT_TEMPLATE_INVOKE(TEST_##want, EXPORT_TEMPLATE_STYLE(foo_export), \ foo_export), \ #foo_export) #define EXPORT_TEMPLATE_TEST_DEFAULT_DEFAULT(...) true #define EXPORT_TEMPLATE_TEST_EXPORT_DLLEXPORT_EXPORT_DLLEXPORT(...) true EXPORT_TEMPLATE_TEST(DEFAULT, ); EXPORT_TEMPLATE_TEST(DEFAULT, __attribute__((visibility("default")))); EXPORT_TEMPLATE_TEST(EXPORT_DLLEXPORT, __declspec(dllexport)); EXPORT_TEMPLATE_TEST(DEFAULT, __declspec(dllimport)); #undef EXPORT_TEMPLATE_TEST #undef EXPORT_TEMPLATE_TEST_DEFAULT_DEFAULT #undef EXPORT_TEMPLATE_TEST_EXPORT_DLLEXPORT_EXPORT_DLLEXPORT #endif // BASE_EXPORT_TEMPLATE_H_