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>

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__

src/libcamera/base/utils.cpp

@@ -463,6 +463,27 @@ std::string toAscii(const std::string &str)
  * \a b
  */
 
+/**
+ * \var defopt
+ * \brief Constant used with std::optional::value_or() to create a
+ * default-constructed object
+ *
+ * The std::optional<T>::value_or(U &&default_value) function returns the
+ * contained value if available, or \a 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.
+ *
+ * The defopt variable 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.
+ */
+
 } /* namespace utils */
 
 #ifndef __DOXYGEN__

test/utils.cpp

@@ -7,6 +7,7 @@
 
 #include <iostream>
 #include <map>
+#include <optional>
 #include <sstream>
 #include <string>
 #include <vector>
@@ -169,6 +170,55 @@ protected:
 		return TestPass;
 	}
 
+	int testDefopt()
+	{
+		static bool defaultConstructed = false;
+
+		struct ValueType {
+			ValueType()
+				: value_(-1)
+			{
+				defaultConstructed = true;
+			}
+
+			ValueType(int value)
+				: value_(value)
+			{
+			}
+
+			int value_;
+		};
+
+		/*
+		 * Test that utils::defopt doesn't cause default-construction
+		 * of a ValueType instance when value_or(utils::defopt) is
+		 * called on a std::optional that has a value.
+		 */
+		std::optional<ValueType> opt = ValueType(0);
+		ValueType value = opt.value_or(utils::defopt);
+
+		if (defaultConstructed || value.value_ != 0) {
+			std::cerr << "utils::defopt didn't prevent default construction"
+				  << std::endl;
+			return TestFail;
+		}
+
+		/*
+		 * Then test that the ValueType is correctly default-constructed
+		 * when the std::optional has no value.
+		 */
+		opt = std::nullopt;
+		value = opt.value_or(utils::defopt);
+
+		if (!defaultConstructed || value.value_ != -1) {
+			std::cerr << "utils::defopt didn't cause default construction"
+				  << std::endl;
+			return TestFail;
+		}
+
+		return TestPass;
+	}
+
 	int run()
 	{
 		/* utils::hex() test. */
@@ -281,6 +331,10 @@ protected:
 		if (testDuration() != TestPass)
 			return TestFail;
 
+		/* utils::defopt test. */
+		if (testDefopt() != TestPass)
+			return TestFail;
+
 		return TestPass;
 	}
 };