Index: build/buildflag.h |
diff --git a/build/buildflag.h b/build/buildflag.h |
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index 0000000000000000000000000000000000000000..283f5bce41c23314cf3db7806fedbc3b55723f1b |
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+++ b/build/buildflag.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 BUILD_BUILDFLAG_H_ |
+#define BUILD_BUILDFLAG_H_ |
+ |
+// These macros un-mangle the names of the build flags in a way that looks |
+// natural, and gives errors if the flag is not defined. Normally in the |
+// preprocessor it's easy to make mistakes that interpret "you haven't done |
+// the setup to know what the flag is" as "flag is off". Normally you would |
+// include the generated header rather than include this file directly. |
+// |
+// This is for use with generated headers. See build/build_header.gni. |
+ |
+// This dance of two macros does a concatenation of two preprocessor args using |
+// ## doubly indirectly because using ## directly prevents macros in that |
+// parameter from being expanded. |
+#define BUILDFLAG_CAT_INDIRECT(a, b) a ## b |
+#define BUILDFLAG_CAT(a, b) BUILDFLAG_CAT_INDIRECT(a, b) |
+ |
+// Accessor for build flags. |
+// |
+// To test for a value, if the build file specifies: |
+// |
+// ENABLE_FOO=true |
+// |
+// Then you would check at build-time in source code with: |
+// |
+// #include "foo_flags.h" // The header the build file specified. |
+// |
+// #if BUILDFLAG(ENABLE_FOO) |
+// ... |
+// #endif |
+// |
+// There will no #define called ENABLE_FOO so if you accidentally test for |
+// whether that is defined, it will always be negative. You can also use |
+// the value in expressions: |
+// |
+// const char kSpamServerName[] = BUILDFLAG(SPAM_SERVER_NAME); |
+// |
+// Because the flag is accessed as a preprocessor macro with (), an error |
+// will be thrown if the proper header defining the internal flag value has |
+// not been included. |
+#define BUILDFLAG(flag) (BUILDFLAG_CAT(BUILDFLAG_INTERNAL_, flag)()) |
+ |
+#endif // BUILD_BUILDFLAG_H_ |