apply_visitor
A method that allows you to access the metadata item value in its original type through a visitor pattern.
To access the value, create a visitor class derived from MetadataVisitorBase, with methods that can handle the possible item value types. Then pass an instance of that class to this function, which establishes the correct type and calls the applicable member function on your visitor.
Syntax
template< typename ReturnT > ReturnT apply_visitor(MetadataVisitorBase< ReturnT >& visitor) const