[libcamera-devel] [PATCH/RFC] libcamera: geometry: Mark const functions with [[nodiscard]]

[Laurent Pinchart]

Geometry classes generally have two sets of functions, one that operates
on the object and modifies it (e.g. Rectangle::scaleBy()), and one that
performs the same operations by instead return a modified copy of the
object, leaving the original untouched (e.g.Rectangle::scaledBy()). As
the names are close, they can easily be mistaken, with the const version
used instead of the in-place version.

To catch these errors at compile time, mark the const versions with
[[nodiscard]], as there is no use case for calling them without using
the result of the call.

Signed-off-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
---
 include/libcamera/geometry.h | 27 ++++++++++++++-------------
 1 file changed, 14 insertions(+), 13 deletions(-)

This patch came as a result of catching one of the issues described in
the commit message during review of another patch. [[nodiscard]] is only
available in C++17, which would make it impossible to use libcamera in
C++14 code (this can for instance for a problem with Chromium - the web
browser, not the OS). An alternative would be to define a custom
attribute, for instance

#define __nodiscard [[nodiscard]]

in a public header (compile.h ? util.h ?), conditionally on the C++
version. The check would thus be disabled in application compiled with
C++14, but would be enabled for the compilation of libcamera itself.

Is this worth it ?

diff --git a/include/libcamera/geometry.h b/include/libcamera/geometry.h
index 2f3a82e2afc3..0a9d61f45874 100644
--- a/include/libcamera/geometry.h
+++ b/include/libcamera/geometry.h
@@ -92,8 +92,8 @@ public:
 		return *this;
 	}
 
-	constexpr Size alignedDownTo(unsigned int hAlignment,
-				     unsigned int vAlignment) const
+	[[nodiscard]] constexpr Size alignedDownTo(unsigned int hAlignment,
+						   unsigned int vAlignment) const
 	{
 		return {
 			width / hAlignment * hAlignment,
@@ -101,8 +101,8 @@ public:
 		};
 	}
 
-	constexpr Size alignedUpTo(unsigned int hAlignment,
-				   unsigned int vAlignment) const
+	[[nodiscard]] constexpr Size alignedUpTo(unsigned int hAlignment,
+						 unsigned int vAlignment) const
 	{
 		return {
 			(width + hAlignment - 1) / hAlignment * hAlignment,
@@ -110,7 +110,7 @@ public:
 		};
 	}
 
-	constexpr Size boundedTo(const Size &bound) const
+	[[nodiscard]] constexpr Size boundedTo(const Size &bound) const
 	{
 		return {
 			std::min(width, bound.width),
@@ -118,7 +118,7 @@ public:
 		};
 	}
 
-	constexpr Size expandedTo(const Size &expand) const
+	[[nodiscard]] constexpr Size expandedTo(const Size &expand) const
 	{
 		return {
 			std::max(width, expand.width),
@@ -126,10 +126,10 @@ public:
 		};
 	}
 
-	Size boundedToAspectRatio(const Size &ratio) const;
-	Size expandedToAspectRatio(const Size &ratio) const;
+	[[nodiscard]] Size boundedToAspectRatio(const Size &ratio) const;
+	[[nodiscard]] Size expandedToAspectRatio(const Size &ratio) const;
 
-	Rectangle centeredTo(const Point &center) const;
+	[[nodiscard]] Rectangle centeredTo(const Point &center) const;
 
 	Size operator*(float factor) const;
 	Size operator/(float factor) const;
@@ -247,10 +247,11 @@ public:
 	Rectangle &scaleBy(const Size &numerator, const Size &denominator);
 	Rectangle &translateBy(const Point &point);
 
-	Rectangle boundedTo(const Rectangle &bound) const;
-	Rectangle enclosedIn(const Rectangle &boundary) const;
-	Rectangle scaledBy(const Size &numerator, const Size &denominator) const;
-	Rectangle translatedBy(const Point &point) const;
+	[[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;
 };
 
 bool operator==(const Rectangle &lhs, const Rectangle &rhs);
-- 
Regards,

Laurent Pinchart