libcamera: utils: Add overloaded visitor helpers

std::visit() allows quite elegant type-matching implementation of the
visitor pattern.

The 'overloaded' type helpers allow to define a hierarchy of overloaded
operator() implementations which can be used by std::visit().

Currently only the Virtual pipeline handler uses this type-matching
implementation of std::visit(). To prepare to add another user in the
Mali C55 pipeline handler move the 'overloaded' helper type to
libcamera::utils for easier re-use.

Signed-off-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Reviewed-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>

include/libcamera/base/utils.h

@@ -37,6 +37,15 @@ namespace libcamera {
 
 namespace utils {
 
+template<class... Ts>
+struct overloaded : Ts... {
+	using Ts::operator()...;
+};
+#ifndef __DOXYGEN__
+template<class... Ts>
+overloaded(Ts...) -> overloaded<Ts...>;
+#endif
+
 const char *basename(const char *path);
 
 char *secure_getenv(const char *name);