allow AsyncSignalHandler to attach and detach from an EventBase

[folly.git] / folly / Preprocessor.h

diff --git a/folly/Preprocessor.h b/folly/Preprocessor.h
index 20fafabf9000369753312a6853fac00a5eb32da9..b94c082b6cc1af0a732ff58a5ba000fa26aebff7 100644 (file)
--- a/folly/Preprocessor.h
+++ b/folly/Preprocessor.h
@@ -1,5 +1,5 @@
 /*
- * Copyright 2013 Facebook, Inc.
+ * Copyright 2017 Facebook, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -16,19 +16,22 @@
 
 // @author: Andrei Alexandrescu
 
-#ifndef FOLLY_PREPROCESSOR_
-#define FOLLY_PREPROCESSOR_
+#pragma once
 
 /**
  * Necessarily evil preprocessor-related amenities.
  */
 
+// MSVC's preprocessor is a pain, so we have to
+// forcefully expand the VA args in some places.
+#define FB_VA_GLUE(a, b) a b
+
 /**
  * FB_ONE_OR_NONE(hello, world) expands to hello and
  * FB_ONE_OR_NONE(hello) expands to nothing. This macro is used to
  * insert or eliminate text based on the presence of another argument.
  */
-#define FB_ONE_OR_NONE(a, ...) FB_THIRD(a, ## __VA_ARGS__, a)
+#define FB_ONE_OR_NONE(a, ...) FB_VA_GLUE(FB_THIRD, (a, ## __VA_ARGS__, a))
 #define FB_THIRD(a, b, ...) __VA_ARGS__
 
 /**
@@ -42,10 +45,38 @@
  * number of arguments. If only one argument is given, it returns
  * that.
  */
+#ifdef _MSC_VER
+// GCC refuses to expand this correctly if this macro itself was
+// called with FB_VA_GLUE :(
+#define FB_ARG_2_OR_1(...) \
+  FB_VA_GLUE(FB_ARG_2_OR_1_IMPL, (__VA_ARGS__, __VA_ARGS__))
+#else
 #define FB_ARG_2_OR_1(...) FB_ARG_2_OR_1_IMPL(__VA_ARGS__, __VA_ARGS__)
+#endif
 // Support macro for the above
 #define FB_ARG_2_OR_1_IMPL(a, b, ...) b
 
+/**
+ * Helper macro that provides a way to pass argument with commas in it to
+ * some other macro whose syntax doesn't allow using extra parentheses.
+ * Example:
+ *
+ *   #define MACRO(type, name) type name
+ *   MACRO(FB_SINGLE_ARG(std::pair<size_t, size_t>), x);
+ *
+ */
+#define FB_SINGLE_ARG(...) __VA_ARGS__
+
+/**
+ * Helper macro that just ignores its parameters.
+ */
+#define FOLLY_IGNORE(...)
+
+/**
+ * Helper macro that just ignores its parameters and inserts a semicolon.
+ */
+#define FOLLY_SEMICOLON(...) ;
+
 /**
  * FB_ANONYMOUS_VARIABLE(str) introduces an identifier starting with
  * str and ending with a number that varies with the line.
@@ -61,10 +92,7 @@
 #endif
 
 /**
- * Use FB_STRINGIZE(name) when you'd want to do what #name does inside
+ * Use FB_STRINGIZE(x) when you'd want to do what #x does inside
  * another macro expansion.
  */
-#define FB_STRINGIZE(name) #name
-
-
-#endif // FOLLY_PREPROCESSOR_
+#define FB_STRINGIZE(x) #x