New build flag system, convert Google Now flag

build/build_header.h

+// 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.
+

[Mark Mentovai, 2015/11/24 14:33:33]

Optional, do it if you like it to prevent direct #include, since there doesn’t
ever seem to be a valid use case for that:

#ifndef BUILD_BUILD_HEADER_H_INCLUDE_OK
#error Do not #include this file directly
#endif

and in each generated .h file

#define BUILD_BUILD_HEADER_H_INCLUDE_OK
#include "build/build_header.h"
#undef BUILD_BUILD_HEADER_H_INCLUDE_OK

[brettw, 2015/11/24 23:36:01]

I think directly including this file is both useless and harmless enough that
it's not worth worrying about.

+#ifndef BUILD_BUILD_HEADER_H_
+#define BUILD_BUILD_HEADER_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.

[Mark Mentovai, 2015/11/24 14:33:33]

Since the former BUILDVAR is now covered by this, the comment should have an
example where a flag is used in a C++ expression.

+//
+// 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.
+//
+// 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)())

[Mark Mentovai, 2015/11/24 14:33:33]

I think that this can be buildflag.h now that the only macro exposed is
BUILDFLAG(). .gypi and .gni naming can follow.

[brettw, 2015/11/24 23:36:01]

Done. I renamed the build scripts buildflag_header*

+
+#endif  // BUILD_BUILD_HEADER_H_