libcamera: base: utils: Provide defopt to simplify std::optional::value_or() usage

The std::optional<T>::value_or(U &&default_value) function returns the
contained value if available, or default_value if the std::optional has
no value. If the desired default value is a default-constructed T, the
obvious option is to call std::optional<T>::value_or(T{}). This approach
has two drawbacks:

- The \a default_value T{} is constructed even if the std::optional
  instance has a value, which impacts efficiency.
- The T{} default constructor needs to be spelled out explicitly in the
  value_or() call, leading to long lines if the type is complex.

Introduce a defopt variable that solves these issues by providing a
value that can be passed to std::optional<T>::value_or() and get
implicitly converted to a default-constructed T.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Umang Jain <umang.jain@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>

1 files changed, 14 insertions, 0 deletions

diff --git a/include/libcamera/base/utils.h b/include/libcamera/base/utils.h
index cfff0583..889bb4a2 100644
--- a/include/libcamera/base/utils.h
+++ b/include/libcamera/base/utils.h
@@ -361,6 +361,20 @@ decltype(auto) abs_diff(const T &a, const T &b)
 		return a - b;
 }
 
+namespace details {
+
+struct defopt_t {
+	template<typename T>
+	operator T() const
+	{
+		return T{};
+	}
+};
+
+} /* namespace details */
+
+constexpr details::defopt_t defopt;
+
 } /* namespace utils */
 
 #ifndef __DOXYGEN__