libcamera: Add macro to conditionally use [[nodiscard]]

The [[nodiscard]] attribute has been added to C++17. It can thus be used
inside libcamera, but would prevent applications compiled for C++14 to
use libcamera if the attribute was used in public headers.

To offer this feature when the application is compiled with a
recent-enough C++ version, as well as for compiling libcamera itself,
add a __nodiscard macro that expands as [[nodiscard]] when using C++17
or newer.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

include/libcamera/compiler.h

@@ -0,0 +1,16 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+/*
+ * Copyright (C) 2021, Google Inc.
+ *
+ * compiler.h - Compiler support
+ */
+#ifndef __LIBCAMERA_COMPILER_H__
+#define __LIBCAMERA_COMPILER_H__
+
+#if __cplusplus >= 201703L
+#define __nodiscard		[[nodiscard]]
+#else
+#define __nodiscard
+#endif
+
+#endif /* __LIBCAMERA_COMPILER_H__ */

include/libcamera/meson.build

@@ -5,6 +5,7 @@ libcamera_public_headers = files([
     'buffer.h',
     'camera.h',
     'camera_manager.h',
+    'compiler.h',
     'controls.h',
     'extensible.h',
     'file_descriptor.h',