I can't understand this expression?

Question

A typical use of a function object is in writing callback functions. A callback in procedural languages, such as C, may be performed by using function pointers. Here is the sentence that confuses me:>However it can be difficult or awkward to pass a state into or out of the callback function.In particular, I can't understand this expression: ***pass a state into or out of the callback function***. Can you explain it to me?

Answer 1

Hi: this phrase is pulled out of some text or instructional manual. Without knowing where this specific phrase came from, can't really answer the question. This almost sounds like some type of computer engineering instruction, but again, would need more specifics. Thanks!

Answer 2

This has to do with the concepts of stateful and stateless.

In computing we often deal with connections over a network and/or with certain pieces of information, whose age and therefore accuracy depends on some periodic refreshing. This creates stateless systems, i.e. data that at any time is not representative of the state of the object or data it represents, but representative of a certain point in time.

When we talk about functions, passing a state means that we need to find a way to pass not any data from a specific point in time into the function, but some way in which the data we can access inside of our function is "fresh" and changes as the state of the object or data outside of our function does.

In VB/VBA for example we can pass a variable to a function "ByVal" or ByRef", where by val would be stateless as just a disconnected value and "ByRef" would be a reference to the original object/data and allow us to retrieve the current state at any time.

HTH