[libcamera-devel] [PATCH] libcamera: span: Make constructors explicit as required by C++20

[Laurent Pinchart]

The C++20 std::span class, after which Span is modelled, specifies four
constructors as explicit when extent is not dynamic_extent. Align our
implementation with those requirements.

A careful reviewer may notice that this change addresses five
constructors, not four. The reason is that the two constructors taking
Container and const Container parameters are not specified in C++20,
which uses a single constructor taking a range parameter instead. As
ranges are not available in C++17, the Container constructors are our
best effort at providing a similar feature.

Signed-off-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
---
 include/libcamera/span.h | 62 ++++++++++++++++++++++++----------------
 1 file changed, 37 insertions(+), 25 deletions(-)

diff --git a/include/libcamera/span.h b/include/libcamera/span.h
index c0e439923933..b0dec77c6b1b 100644
--- a/include/libcamera/span.h
+++ b/include/libcamera/span.h
@@ -113,12 +113,12 @@ public:
 	{
 	}
 
-	constexpr Span(pointer ptr, [[maybe_unused]] size_type count)
+	explicit constexpr Span(pointer ptr, [[maybe_unused]] size_type count)
 		: data_(ptr)
 	{
 	}
 
-	constexpr Span(pointer first, [[maybe_unused]] pointer last)
+	explicit constexpr Span(pointer first, [[maybe_unused]] pointer last)
 		: data_(first)
 	{
 	}
@@ -154,7 +154,7 @@ public:
 	}
 
 	template<class Container>
-	constexpr Span(Container &cont,
+	explicit constexpr Span(Container &cont,
 		       std::enable_if_t<!details::is_span<Container>::value &&
 					!details::is_array<Container>::value &&
 					!std::is_array<Container>::value &&
@@ -166,23 +166,23 @@ public:
 	}
 
 	template<class Container>
-	constexpr Span(const Container &cont,
-		       std::enable_if_t<!details::is_span<Container>::value &&
-					!details::is_array<Container>::value &&
-					!std::is_array<Container>::value &&
-					std::is_convertible<std::remove_pointer_t<decltype(utils::data(cont))> (*)[],
-							    element_type (*)[]>::value,
-					std::nullptr_t> = nullptr)
+	explicit constexpr Span(const Container &cont,
+				std::enable_if_t<!details::is_span<Container>::value &&
+						 !details::is_array<Container>::value &&
+						 !std::is_array<Container>::value &&
+						 std::is_convertible<std::remove_pointer_t<decltype(utils::data(cont))> (*)[],
+								     element_type (*)[]>::value,
+						 std::nullptr_t> = nullptr)
 		: data_(utils::data(cont))
 	{
 		static_assert(utils::size(cont) == Extent, "Size mismatch");
 	}
 
 	template<class U, std::size_t N>
-	constexpr Span(const Span<U, N> &s,
-		       std::enable_if_t<std::is_convertible<U (*)[], element_type (*)[]>::value &&
-					N == Extent,
-					std::nullptr_t> = nullptr) noexcept
+	explicit constexpr Span(const Span<U, N> &s,
+				std::enable_if_t<std::is_convertible<U (*)[], element_type (*)[]>::value &&
+						 N == Extent,
+						 std::nullptr_t> = nullptr) noexcept
 		: data_(s.data())
 	{
 	}
@@ -212,24 +212,24 @@ public:
 	constexpr Span<element_type, Count> first() const
 	{
 		static_assert(Count <= Extent, "Count larger than size");
-		return { data(), Count };
+		return Span<element_type, Count>{ data(), Count };
 	}
 
 	constexpr Span<element_type, dynamic_extent> first(std::size_t Count) const
 	{
-		return { data(), Count };
+		return Span<element_type, dynamic_extent>{ data(), Count };
 	}
 
 	template<std::size_t Count>
 	constexpr Span<element_type, Count> last() const
 	{
 		static_assert(Count <= Extent, "Count larger than size");
-		return { data() + size() - Count, Count };
+		return Span<element_type, Count>{ data() + size() - Count, Count };
 	}
 
 	constexpr Span<element_type, dynamic_extent> last(std::size_t Count) const
 	{
-		return { data() + size() - Count, Count };
+		return Span<element_type, dynamic_extent>{ data() + size() - Count, Count };
 	}
 
 	template<std::size_t Offset, std::size_t Count = dynamic_extent>
@@ -238,13 +238,19 @@ public:
 		static_assert(Offset <= Extent, "Offset larger than size");
 		static_assert(Count == dynamic_extent || Count + Offset <= Extent,
 			      "Offset + Count larger than size");
-		return { data() + Offset, Count == dynamic_extent ? size() - Offset : Count };
+		return Span<element_type, Count != dynamic_extent ? Count : Extent - Offset>{
+			data() + Offset,
+			Count == dynamic_extent ? size() - Offset : Count
+		};
 	}
 
 	constexpr Span<element_type, dynamic_extent>
 	subspan(std::size_t Offset, std::size_t Count = dynamic_extent) const
 	{
-		return { data() + Offset, Count == dynamic_extent ? size() - Offset : Count };
+		return Span<element_type, dynamic_extent>{
+			data() + Offset,
+			Count == dynamic_extent ? size() - Offset : Count
+		};
 	}
 
 private:
@@ -371,7 +377,7 @@ public:
 	template<std::size_t Count>
 	constexpr Span<element_type, Count> first() const
 	{
-		return { data(), Count };
+		return Span<element_type, Count>{ data(), Count };
 	}
 
 	constexpr Span<element_type, dynamic_extent> first(std::size_t Count) const
@@ -382,24 +388,30 @@ public:
 	template<std::size_t Count>
 	constexpr Span<element_type, Count> last() const
 	{
-		return { data() + size() - Count, Count };
+		return Span<element_type, Count>{ data() + size() - Count, Count };
 	}
 
 	constexpr Span<element_type, dynamic_extent> last(std::size_t Count) const
 	{
-		return { data() + size() - Count, Count };
+		return Span<element_type, dynamic_extent>{ data() + size() - Count, Count };
 	}
 
 	template<std::size_t Offset, std::size_t Count = dynamic_extent>
 	constexpr Span<element_type, Count> subspan() const
 	{
-		return { data() + Offset, Count == dynamic_extent ? size() - Offset : Count };
+		return Span<element_type, Count>{
+			data() + Offset,
+			Count == dynamic_extent ? size() - Offset : Count
+		};
 	}
 
 	constexpr Span<element_type, dynamic_extent>
 	subspan(std::size_t Offset, std::size_t Count = dynamic_extent) const
 	{
-		return { data() + Offset, Count == dynamic_extent ? size() - Offset : Count };
+		return Span<element_type, dynamic_extent>{
+			data() + Offset,
+			Count == dynamic_extent ? size() - Offset : Count
+		};
 	}
 
 private:
-- 
Regards,

Laurent Pinchart