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Add RTC_EXPORT_TEMPLATE_{DECLARE,DEFINE} macros.
This CL adds macros to correctly export template instantiation declarations and definitions. These macros have been borrowed from Chromium's //base/export_template.h [1] and are supposed to be used together with RTC_EXPORT [2]. The goal is to start using explicit template instatiation declarations (introduced in C++11) [3] and remove workarounds that are not compatible with all the compilers. An example can be found in [4], where in order to workaround crbug.com/1018579, another workaround was almost created before being stopped at code review time. [1] - https://cs.chromium.org/chromium/src/base/export_template.h [2] - https://cs.chromium.org/chromium/src/third_party/webrtc/rtc_base/system/rtc_export.h [3] - https://en.cppreference.com/w/cpp/language/class_template#Explicit_instantiation [4] - https://webrtc-review.googlesource.com/c/src/+/158674 Bug: webrtc:9419 Change-Id: I2e9287a15e28f619462e0b9a5deb0b672be248c6 Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/158742 Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org> Reviewed-by: Karl Wiberg <kwiberg@webrtc.org> Cr-Commit-Position: refs/heads/master@{#29661}
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Mirko Bonadei
Commit Bot
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2 changed files with 198 additions and 0 deletions
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@ -63,6 +63,7 @@ rtc_source_set("unused") {
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rtc_source_set("rtc_export") {
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sources = [
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"rtc_export.h",
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"rtc_export_template.h",
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]
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}
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197
rtc_base/system/rtc_export_template.h
Normal file
197
rtc_base/system/rtc_export_template.h
Normal file
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@ -0,0 +1,197 @@
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/*
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* Copyright (c) 2019 The WebRTC project authors. All Rights Reserved.
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*
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* Use of this source code is governed by a BSD-style license
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* that can be found in the LICENSE file in the root of the source
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* tree. An additional intellectual property rights grant can be found
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* in the file PATENTS. All contributing project authors may
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* be found in the AUTHORS file in the root of the source tree.
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*/
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#ifndef RTC_BASE_SYSTEM_RTC_EXPORT_TEMPLATE_H_
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#define RTC_BASE_SYSTEM_RTC_EXPORT_TEMPLATE_H_
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// clang-format off
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// clang formating would cause cpplint errors in the macros below.
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// Most of this was borrowed (with minor modifications) from Chromium's
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// base/export_template.h.
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// Synopsis
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//
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// This header provides macros for using RTC_EXPORT macros with explicit
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// template instantiation declarations and definitions.
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// Generally, the RTC_EXPORT macros are used at declarations,
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// and GCC requires them to be used at explicit instantiation declarations,
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// but MSVC requires __declspec(dllexport) to be used at the explicit
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// instantiation definitions instead.
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// Usage
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//
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// In a header file, write:
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//
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// extern template class RTC_EXPORT_TEMPLATE_DECLARE(RTC_EXPORT) foo<bar>;
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//
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// In a source file, write:
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//
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// template class RTC_EXPORT_TEMPLATE_DEFINE(RTC_EXPORT) foo<bar>;
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// Implementation notes
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//
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// On Windows, when building when RTC_EXPORT expands to __declspec(dllexport)),
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// we want the two lines to expand to:
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//
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// extern template class foo<bar>;
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// template class RTC_EXPORT foo<bar>;
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//
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// In all other cases (non-Windows, and Windows when RTC_EXPORT expands to
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// __declspec(dllimport)), we want:
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//
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// extern template class RTC_EXPORT foo<bar>;
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// template class foo<bar>;
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//
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// The implementation of this header uses some subtle macro semantics to
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// detect what the provided RTC_EXPORT value was defined as and then
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// to dispatch to appropriate macro definitions. Unfortunately,
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// MSVC's C preprocessor is rather non-compliant and requires special
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// care to make it work.
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//
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// Issue 1.
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//
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// #define F(x)
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// F()
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//
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// MSVC emits warning C4003 ("not enough actual parameters for macro
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// 'F'), even though it's a valid macro invocation. This affects the
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// macros below that take just an "export" parameter, because export
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// may be empty.
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//
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// As a workaround, we can add a dummy parameter and arguments:
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//
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// #define F(x,_)
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// F(,)
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//
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// Issue 2.
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//
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// #define F(x) G##x
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// #define Gj() ok
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// F(j())
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//
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// The correct replacement for "F(j())" is "ok", but MSVC replaces it
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// with "Gj()". As a workaround, we can pass the result to an
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// identity macro to force MSVC to look for replacements again. (This
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// is why RTC_EXPORT_TEMPLATE_STYLE_3 exists.)
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#define RTC_EXPORT_TEMPLATE_DECLARE(export) \
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RTC_EXPORT_TEMPLATE_INVOKE( \
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DECLARE, \
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RTC_EXPORT_TEMPLATE_STYLE(export, ), export) // NOLINT
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#define RTC_EXPORT_TEMPLATE_DEFINE(export) \
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RTC_EXPORT_TEMPLATE_INVOKE( \
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DEFINE, \
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RTC_EXPORT_TEMPLATE_STYLE(export, ), export) // NOLINT
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// INVOKE is an internal helper macro to perform parameter replacements
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// and token pasting to chain invoke another macro. E.g.,
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// RTC_EXPORT_TEMPLATE_INVOKE(DECLARE, DEFAULT, RTC_EXPORT)
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// will export to call
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// RTC_EXPORT_TEMPLATE_DECLARE_DEFAULT(RTC_EXPORT, )
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// (but with RTC_EXPORT expanded too).
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#define RTC_EXPORT_TEMPLATE_INVOKE(which, style, export) \
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RTC_EXPORT_TEMPLATE_INVOKE_2(which, style, export)
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#define RTC_EXPORT_TEMPLATE_INVOKE_2(which, style, export) \
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RTC_EXPORT_TEMPLATE_##which##_##style(export, )
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// Default style is to apply the RTC_EXPORT macro at declaration sites.
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#define RTC_EXPORT_TEMPLATE_DECLARE_DEFAULT(export, _) export
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#define RTC_EXPORT_TEMPLATE_DEFINE_DEFAULT(export, _)
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// The "MSVC hack" style is used when RTC_EXPORT is defined
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// as __declspec(dllexport), which MSVC requires to be used at
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// definition sites instead.
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#define RTC_EXPORT_TEMPLATE_DECLARE_MSVC_HACK(export, _)
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#define RTC_EXPORT_TEMPLATE_DEFINE_MSVC_HACK(export, _) export
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// RTC_EXPORT_TEMPLATE_STYLE is an internal helper macro that identifies which
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// export style needs to be used for the provided RTC_EXPORT macro definition.
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// "", "__attribute__(...)", and "__declspec(dllimport)" are mapped
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// to "DEFAULT"; while "__declspec(dllexport)" is mapped to "MSVC_HACK".
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//
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// It's implemented with token pasting to transform the __attribute__ and
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// __declspec annotations into macro invocations. E.g., if RTC_EXPORT is
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// defined as "__declspec(dllimport)", it undergoes the following sequence of
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// macro substitutions:
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// RTC_EXPORT_TEMPLATE_STYLE(RTC_EXPORT,)
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// RTC_EXPORT_TEMPLATE_STYLE_2(__declspec(dllimport),)
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// RTC_EXPORT_TEMPLATE_STYLE_3(
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// RTC_EXPORT_TEMPLATE_STYLE_MATCH__declspec(dllimport))
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// RTC_EXPORT_TEMPLATE_STYLE_MATCH__declspec(dllimport)
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// RTC_EXPORT_TEMPLATE_STYLE_MATCH_DECLSPEC_dllimport
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// DEFAULT
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#define RTC_EXPORT_TEMPLATE_STYLE(export, _) \
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RTC_EXPORT_TEMPLATE_STYLE_2(export, )
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#define RTC_EXPORT_TEMPLATE_STYLE_2(export, _) \
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RTC_EXPORT_TEMPLATE_STYLE_3( \
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RTC_EXPORT_TEMPLATE_STYLE_MATCH_foj3FJo5StF0OvIzl7oMxA##export)
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#define RTC_EXPORT_TEMPLATE_STYLE_3(style) style
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// Internal helper macros for RTC_EXPORT_TEMPLATE_STYLE.
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//
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// XXX: C++ reserves all identifiers containing "__" for the implementation,
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// but "__attribute__" and "__declspec" already contain "__" and the token-paste
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// operator can only add characters; not remove them. To minimize the risk of
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// conflict with implementations, we include "foj3FJo5StF0OvIzl7oMxA" (a random
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// 128-bit string, encoded in Base64) in the macro name.
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#define RTC_EXPORT_TEMPLATE_STYLE_MATCH_foj3FJo5StF0OvIzl7oMxA DEFAULT
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#define RTC_EXPORT_TEMPLATE_STYLE_MATCH_foj3FJo5StF0OvIzl7oMxA__attribute__( \
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...) \
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DEFAULT
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#define RTC_EXPORT_TEMPLATE_STYLE_MATCH_foj3FJo5StF0OvIzl7oMxA__declspec(arg) \
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RTC_EXPORT_TEMPLATE_STYLE_MATCH_DECLSPEC_##arg
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// Internal helper macros for RTC_EXPORT_TEMPLATE_STYLE.
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#define RTC_EXPORT_TEMPLATE_STYLE_MATCH_DECLSPEC_dllexport MSVC_HACK
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#define RTC_EXPORT_TEMPLATE_STYLE_MATCH_DECLSPEC_dllimport DEFAULT
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// Sanity checks.
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//
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// RTC_EXPORT_TEMPLATE_TEST uses the same macro invocation pattern as
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// RTC_EXPORT_TEMPLATE_DECLARE and RTC_EXPORT_TEMPLATE_DEFINE do to check that
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// they're working correctly. When they're working correctly, the sequence of
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// macro replacements should go something like:
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//
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// RTC_EXPORT_TEMPLATE_TEST(DEFAULT, __declspec(dllimport));
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//
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// static_assert(RTC_EXPORT_TEMPLATE_INVOKE(TEST_DEFAULT,
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// RTC_EXPORT_TEMPLATE_STYLE(__declspec(dllimport), ),
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// __declspec(dllimport)), "__declspec(dllimport)");
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//
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// static_assert(RTC_EXPORT_TEMPLATE_INVOKE(TEST_DEFAULT,
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// DEFAULT, __declspec(dllimport)), "__declspec(dllimport)");
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//
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// static_assert(RTC_EXPORT_TEMPLATE_TEST_DEFAULT_DEFAULT(
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// __declspec(dllimport)), "__declspec(dllimport)");
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//
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// static_assert(true, "__declspec(dllimport)");
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//
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// When they're not working correctly, a syntax error should occur instead.
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#define RTC_EXPORT_TEMPLATE_TEST(want, export) \
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static_assert( \
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RTC_EXPORT_TEMPLATE_INVOKE( \
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TEST_##want, \
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RTC_EXPORT_TEMPLATE_STYLE(export, ), export), #export) // NOLINT
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#define RTC_EXPORT_TEMPLATE_TEST_DEFAULT_DEFAULT(...) true
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#define RTC_EXPORT_TEMPLATE_TEST_MSVC_HACK_MSVC_HACK(...) true
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RTC_EXPORT_TEMPLATE_TEST(DEFAULT, ); // NOLINT
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RTC_EXPORT_TEMPLATE_TEST(DEFAULT, __attribute__((visibility("default"))));
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RTC_EXPORT_TEMPLATE_TEST(MSVC_HACK, __declspec(dllexport));
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RTC_EXPORT_TEMPLATE_TEST(DEFAULT, __declspec(dllimport));
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#undef RTC_EXPORT_TEMPLATE_TEST
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#undef RTC_EXPORT_TEMPLATE_TEST_DEFAULT_DEFAULT
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#undef RTC_EXPORT_TEMPLATE_TEST_MSVC_HACK_MSVC_HACK
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// clang-format on
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#endif // RTC_BASE_SYSTEM_RTC_EXPORT_TEMPLATE_H_
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