[PATCH v2] libcamera: base: Remove custom __nodiscard attribute

[Mattijs Korpershoek]

__nodiscard was introduced for compatibility with C++14.
In C++17, there is an official attribute: [[nodiscard]].
Moreover, some libc implementations (like bionic) already define the
__nodiscard macro [1].

Since:
- libcamera builds with cpp_std=c++17
- [[nodiscard]] is already used in the android HAL (exif)

We should replace all usage __nodiscard of by [[nodiscard]] for
consistency.

Do the replacement and remove the no longer used compiler.h.

[1] https://android-review.googlesource.com/c/platform/bionic/+/3254860

Signed-off-by: Mattijs Korpershoek <mkorpershoek at baylibre.com>
---
Hi, it's been a while. I've found a (trivial) build issue when
building against recent bionic versions.

After discussion in v1, dropping compiler.h seemed the right choice.
---
Changes in v2:
- Drop support of __nodiscard instead of adding another #ifdef (Laurent)
- Link to v1: https://lists.libcamera.org/pipermail/libcamera-devel/2025-January/047922.html
---
 include/libcamera/base/compiler.h  | 14 --------------
 include/libcamera/base/meson.build |  1 -
 include/libcamera/base/unique_fd.h |  3 +--
 include/libcamera/geometry.h       | 28 +++++++++++++---------------
 4 files changed, 14 insertions(+), 32 deletions(-)

diff --git a/include/libcamera/base/compiler.h b/include/libcamera/base/compiler.h
deleted file mode 100644
index fda8fdfdc543f86c5554e38ef790c00d72d60389..0000000000000000000000000000000000000000
--- a/include/libcamera/base/compiler.h
+++ /dev/null
@@ -1,14 +0,0 @@
-/* SPDX-License-Identifier: LGPL-2.1-or-later */
-/*
- * Copyright (C) 2021, Google Inc.
- *
- * Compiler support
- */
-
-#pragma once
-
-#if __cplusplus >= 201703L
-#define __nodiscard		[[nodiscard]]
-#else
-#define __nodiscard
-#endif
diff --git a/include/libcamera/base/meson.build b/include/libcamera/base/meson.build
index 2a0cee317204b0d6b44276703fac229bfd7973b9..f28ae4d42a69c755710b51ffc92976bb6fb6a0d8 100644
--- a/include/libcamera/base/meson.build
+++ b/include/libcamera/base/meson.build
@@ -5,7 +5,6 @@ libcamera_base_include_dir = libcamera_include_dir / 'base'
 libcamera_base_public_headers = files([
     'bound_method.h',
     'class.h',
-    'compiler.h',
     'flags.h',
     'object.h',
     'shared_fd.h',
diff --git a/include/libcamera/base/unique_fd.h b/include/libcamera/base/unique_fd.h
index c9a3b5d0e8628533bd18649e156ba78da40ca200..3066bf28f745df1d26a74c20d21b607dba3d41f4 100644
--- a/include/libcamera/base/unique_fd.h
+++ b/include/libcamera/base/unique_fd.h
@@ -10,7 +10,6 @@
 #include <utility>
 
 #include <libcamera/base/class.h>
-#include <libcamera/base/compiler.h>
 
 namespace libcamera {
 
@@ -43,7 +42,7 @@ public:
 		return *this;
 	}
 
-	__nodiscard int release()
+	[[nodiscard]] int release()
 	{
 		int fd = fd_;
 		fd_ = -1;
diff --git a/include/libcamera/geometry.h b/include/libcamera/geometry.h
index e5f0a843d3144d2086c42c11ab40b0a98438a7e2..0a7133f24c78d4b70075fdd02eabfa52857937dc 100644
--- a/include/libcamera/geometry.h
+++ b/include/libcamera/geometry.h
@@ -11,8 +11,6 @@
 #include <ostream>
 #include <string>
 
-#include <libcamera/base/compiler.h>
-
 namespace libcamera {
 
 class Rectangle;
@@ -110,7 +108,7 @@ public:
 		return *this;
 	}
 
-	__nodiscard constexpr Size alignedDownTo(unsigned int hAlignment,
+	[[nodiscard]] constexpr Size alignedDownTo(unsigned int hAlignment,
 						 unsigned int vAlignment) const
 	{
 		return {
@@ -119,7 +117,7 @@ public:
 		};
 	}
 
-	__nodiscard constexpr Size alignedUpTo(unsigned int hAlignment,
+	[[nodiscard]] constexpr Size alignedUpTo(unsigned int hAlignment,
 					       unsigned int vAlignment) const
 	{
 		return {
@@ -128,7 +126,7 @@ public:
 		};
 	}
 
-	__nodiscard constexpr Size boundedTo(const Size &bound) const
+	[[nodiscard]] constexpr Size boundedTo(const Size &bound) const
 	{
 		return {
 			std::min(width, bound.width),
@@ -136,7 +134,7 @@ public:
 		};
 	}
 
-	__nodiscard constexpr Size expandedTo(const Size &expand) const
+	[[nodiscard]] constexpr Size expandedTo(const Size &expand) const
 	{
 		return {
 			std::max(width, expand.width),
@@ -144,7 +142,7 @@ public:
 		};
 	}
 
-	__nodiscard constexpr Size grownBy(const Size &margins) const
+	[[nodiscard]] constexpr Size grownBy(const Size &margins) const
 	{
 		return {
 			width + margins.width,
@@ -152,7 +150,7 @@ public:
 		};
 	}
 
-	__nodiscard constexpr Size shrunkBy(const Size &margins) const
+	[[nodiscard]] constexpr Size shrunkBy(const Size &margins) const
 	{
 		return {
 			width > margins.width ? width - margins.width : 0,
@@ -160,10 +158,10 @@ public:
 		};
 	}
 
-	__nodiscard Size boundedToAspectRatio(const Size &ratio) const;
-	__nodiscard Size expandedToAspectRatio(const Size &ratio) const;
+	[[nodiscard]] Size boundedToAspectRatio(const Size &ratio) const;
+	[[nodiscard]] Size expandedToAspectRatio(const Size &ratio) const;
 
-	__nodiscard Rectangle centeredTo(const Point &center) const;
+	[[nodiscard]] Rectangle centeredTo(const Point &center) const;
 
 	Size operator*(float factor) const;
 	Size operator/(float factor) const;
@@ -294,11 +292,11 @@ public:
 	Rectangle &scaleBy(const Size &numerator, const Size &denominator);
 	Rectangle &translateBy(const Point &point);
 
-	__nodiscard Rectangle boundedTo(const Rectangle &bound) const;
-	__nodiscard Rectangle enclosedIn(const Rectangle &boundary) const;
-	__nodiscard Rectangle scaledBy(const Size &numerator,
+	[[nodiscard]] Rectangle boundedTo(const Rectangle &bound) const;
+	[[nodiscard]] Rectangle enclosedIn(const Rectangle &boundary) const;
+	[[nodiscard]] Rectangle scaledBy(const Size &numerator,
 				       const Size &denominator) const;
-	__nodiscard Rectangle translatedBy(const Point &point) const;
+	[[nodiscard]] Rectangle translatedBy(const Point &point) const;
 
 	Rectangle transformedBetween(const Rectangle &source,
 				     const Rectangle &target) const;

---
base-commit: 35ed4b91291d9f3d08e4b51acfb51163e65df8f8
change-id: 20250103-nodiscard-redef-9158e8fdc3f5

Best regards,
-- 
Mattijs Korpershoek <mkorpershoek at baylibre.com>