libcamera: yaml_parser: Take string keys in `std::string_view`

In many cases a static string literal is used as key. Thus
having the argument type be `const std::string&` is suboptimal
since an `std::string` object needs to be constructed before
the call.

C++17 introduced `std::string_view`, using which the call
can be done with less overhead, as the `std::string_view`
is non-owning and may be passed in registers entirely.

So make `YamlObject::{contains,operator[]}` take the string keys
in `std::string_view`s.

Unfortunately, that is not sufficient yet, because `std::map::find()`
takes an reference to `const key_type`, which would be `const std::string&`
in the case of `YamlParser`. However, with a transparent comparator
such as `std::less<>` `std::map::find()` is able to accept any
object as the argument, and it forwards it to the comparator.

So make `YamlParser::dictionary_` use `std::less<>` as the comparator
to enable the use of `std::map::find()` with any type of argument.

Signed-off-by: Barnabás Pőcze <pobrn@protonmail.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

2 files changed, 8 insertions, 10 deletions

diff --git a/include/libcamera/internal/yaml_parser.h b/include/libcamera/internal/yaml_parser.h
index 6211ff4a..de452844 100644
--- a/include/libcamera/internal/yaml_parser.h
+++ b/include/libcamera/internal/yaml_parser.h
@@ -12,6 +12,7 @@
 #include <optional>
 #include <stdint.h>
 #include <string>
+#include <string_view>
 #include <vector>
 
 #include <libcamera/base/class.h>
@@ -206,8 +207,8 @@ public:
 
 	const YamlObject &operator[](std::size_t index) const;
 
-	bool contains(const std::string &key) const;
-	const YamlObject &operator[](const std::string &key) const;
+	bool contains(std::string_view key) const;
+	const YamlObject &operator[](std::string_view key) const;
 
 private:
 	LIBCAMERA_DISABLE_COPY_AND_MOVE(YamlObject)
@@ -232,7 +233,7 @@ private:
 
 	std::string value_;
 	Container list_;
-	std::map<std::string, YamlObject *> dictionary_;
+	std::map<std::string, YamlObject *, std::less<>> dictionary_;
 };
 
 class YamlParser final
diff --git a/src/libcamera/yaml_parser.cpp b/src/libcamera/yaml_parser.cpp
index 4784f2dc..7c0b341a 100644
--- a/src/libcamera/yaml_parser.cpp
+++ b/src/libcamera/yaml_parser.cpp
@@ -481,16 +481,13 @@ const YamlObject &YamlObject::operator[](std::size_t index) const
  *
  * \return True if an element exists, false otherwise
  */
-bool YamlObject::contains(const std::string &key) const
+bool YamlObject::contains(std::string_view key) const
 {
-	if (dictionary_.find(std::ref(key)) == dictionary_.end())
-		return false;
-
-	return true;
+	return dictionary_.find(key) != dictionary_.end();
 }
 
 /**
- * \fn YamlObject::operator[](const std::string &key) const
+ * \fn YamlObject::operator[](std::string_view key) const
  * \brief Retrieve a member by name from the dictionary
  *
  * This function retrieve a member of a YamlObject by name. Only YamlObject
@@ -500,7 +497,7 @@ bool YamlObject::contains(const std::string &key) const
  *
  * \return The YamlObject corresponding to the \a key member
  */
-const YamlObject &YamlObject::operator[](const std::string &key) const
+const YamlObject &YamlObject::operator[](std::string_view key) const
 {
 	if (type_ != Type::Dictionary)
 		return empty;