To ensure interoperability between Delphi and C++, there are restrictions on the type names mentioned in the parameter lists of published closure types. The parameter types have to be simple type names with optional const modifier and pointer or reference notation. 

So when declaring a closure type, the arguments passed to that closure must be of a simple type. For example, templates are not accepted. To pass a reference to an object of template type to a closure, you must declare a typedef, which counts as a simple type name.