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-rw-r--r--include/libcamera/base/utils.h14
-rw-r--r--src/libcamera/base/utils.cpp21
-rw-r--r--test/utils.cpp54
3 files changed, 89 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__
diff --git a/src/libcamera/base/utils.cpp b/src/libcamera/base/utils.cpp
index 6a307940..9cd6cb19 100644
--- a/src/libcamera/base/utils.cpp
+++ b/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__
diff --git a/test/utils.cpp b/test/utils.cpp
index d65467b5..129807a6 100644
--- a/test/utils.cpp
+++ b/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;
}
};