The type must be a model of . A PrimitiveTransform is a class type that has a nested class template called impl<> that takes three template parameters representing an expression type, a state type and a data type. Specializations of the nested impl template are ternary monomorphic function objects that accept expression, state, and data parameters. A PrimitiveTransform is also a PolymorphicFunctionObject implemented in terms of the nested impl<> template. The return type of the overloaded function call operator. Applies the transform. Applies the transform. Applies the transform. Applies the transform.