In this article we will discuss what is a callback and what types of callbacks we can have in C++ and how to Design Callbacks as function pointers.

Now what is a callback?

Callback is a function that we pass to another APIs as argument while calling them. Now these APIs are expected to use our provided callback at some point.

Callbacks in C++ can be of 3 types,

1.) Function Pointer
2.) Function Objects / Functors
3.) Lambda functions

Behaviour or result of the API is dependent on the callback we provide i.e. if we keep the input to API same and just change the callback then the output of API will change. Let’s understand this by,

Using Function Pointer as callback:

From a framework we got an API that can build complete message from provided raw data. This API will perform following steps,

1.) Add header and footer in raw Data to make the message.
2.) Encrypt the complete message.
3.) Return the message

Now this API knows what header and footer to apply but don’t know about exact encryption logic, because that is not fixed to Framework, that can change from application to application. As, Encryption Logic is application dependent, therefore in this API provides a provision to pass encryption logic as a callback function pointer. This API will call back the code of application by calling this passed function pointer.

Callback Mechanism Exmplained
Callback Mechanism

Framework’s API that accepts the function pointer callback as an argument is as follows,

Now when an application calls this API of framework to build the message, it passes a pointer to its local function as an argument.


In this case this a function Pointer is a Callback,

This encrypt function increment all letters in string by 1.

Now lets call build message API will this function as callback,

Output:

[IFBEFS]TbnqmfTusjoh[GppUFS]

Now, lets design a new type of encryption function,

This encrypt function decrement all letters in string by 1. Now lets call build message API will this function as callback,

Output:

[GD@CDQ]R`lokdRsqhmf[EnnSDQ]

Now output of this call  is different to earlier.
We kept the API and passed data same but changed the passed callback, as a result behaviour or result of API changed.

This shows, that behaviour or result of the API is dependent on the callback passed as an argument.

Till now we have seen how to use function pointer as a callback. But,

What is a Function Object Callback?
Why do we need function objects?

We will discuss this in next article.