What is a 'thunk'?
I've seen it used in programming (specifically in the C++ domain) and have no idea what it is. Presumably it is a design pattern, but I could be wrong. Can anyone give a good example of a thunk?
A thunk usually refers to a small piece of code that is called as a function, does some small thing, and then JUMPs to another location (usually a function) instead of returning to its caller. Assuming the JUMP target is a normal function, when it returns, it will return to the thunk's caller.
Thunks can be used to implement lots of useful things efficiently
protocol translation -- when calling from code that uses one calling convention to code that uses a different calling convention, a
thunkcan be used to translate the arguments appropriately. This only works if the return conventions are compatible, but that is often the casevirtual function handling -- when calling a virtual function of a multiply-inherited base class in C++, there needs to be a fix-up of the
thispointer to get it to point to the right place. Athunkcan do this.dynamic closures -- when you build a dynamic closure, the closure function needs to be able to get at the context where it was created. A small
thunkcan be built (usually on the stack) which sets up the context info in some register(s) and then jumps to a static piece of code that implements the closure's function. The thunk here is effectively supplying one or more hidden extra arguments to the function that are not provided by the call site.
The word thunk has at least three related meanings in computer science. A "thunk" may be:
- a piece of code to perform a delayed computation (similar to a closure)
- a feature of some virtual function table implementations (similar to a wrapper function)
- a mapping of machine data from one system-specific form to another, usually for compatibility reasons
I have usually seen it used in the third context.
http://en.wikipedia.org/wiki/Thunk
The term thunk originally referred to the mechanism used by the Royal Radar Establishment implementation of pass-by-name in their Algol60 compiler. In general it refers to any way to induce dynamic behavior when referencing an apparently static object. The term was invented by Brian Wichmann, who when asked to explain pass-by-name said "Well you go out to load the value from memory and then suddenly - thunk - there you are evaluating an expression."
Thunks have been put in hardware (cf. KDF9, Burroughs mainframes). There are several ways to implement them in software, all very machine, language and compiler specific.
The term has come to be generalized beyond pass-by-name, to include any situation in which an apparently or nominally static data reference induces dynamic behavior. Related terms include "trampoline" and "future".
Some compilers for object-oriented languages such as C++ generate functions called "thunks" as an optimization of virtual function calls in the presence of multiple or virtual inheritance.
Taken from: http://en.wikipedia.org/wiki/Thunk#Thunks_in_object-oriented_programming
This question has already been asked on SO, see:
What is a 'thunk', as used in Scheme or in general?
From what I can tell, it's akin to a lambda statement, where you may not want to return the value until you need to evaluate it; or it can also be compared to a property getter which by design executes some code in order to return a value while yet having the interface form that comes across more like a variable, but also has polymorphic behavior that can be swapped out whether by inheritance or by swapping out the function pointer that would evaluate and return a value at runtime based on compile-time or environmental characteristics.