use variadic macros/templates in IPC message implementation

ipc/export_template.h

Index: ipc/export_template.h
diff --git a/ipc/export_template.h b/ipc/export_template.h
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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>;
+
+// 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.)
+
+#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
+
+// 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 MSVC_HACK
+#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, 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));
+
+#undef EXPORT_TEMPLATE_TEST
+#undef EXPORT_TEMPLATE_TEST_DEFAULT_DEFAULT
+#undef EXPORT_TEMPLATE_TEST_MSVC_HACK_MSVC_HACK
+
+#endif  // IPC_EXPORT_TEMPLATE_H_