In object-oriented programming, a programming language is said to have first-class messages or dynamic messages if in a method call not only the receiving object and parameter list can be varied dynamically (i.e. bound to a variable or computed as an expression) but also the specific method invoked. Typed object-oriented programming languages, such as Java and C++, often do not support first-class methods. Smalltalk only support them in an untyped way. In Objective-C (Cocoa), you can use NSInvocation to represent first-class messages in a way that is aware of the types at runtime; however, safe use still relies on the programmer. Some theoretical progress has been made to support first-class messages in a type-safe manner, but none of the proposed systems has been implemented in a programming language, possibly due to their complexity.
See also Delegate (object-oriented programming) First-class function
Notes
References Susumu Nishimura (1998). "Static Typing for Dynamic Messages". POPL '98. Michelle Bugliesi & Silvia Crafa (1999). "Object Calculi for Dynamic Messages". FOOL 6. Martin Müller & Susumu Nishimura (2000). "Type Inference for First-Class Messages with Feature Constraints". International Journal of Foundations of Computer Science 11:1. François Pottier (2000). "A versatile Constraint-based Type Inference System". Nordic Journal of Computing. Paritosh Shroff & Scott F. Smith. "Type Inference for First-Class Messages with Match-Functions".