Callback listeners, or callbacks, play a pivotal role in programming by facilitating dynamic function execution.In C programming, a callback is essentially a way to provide a function to another function so that it can be called at a later time. To effectively implement callback functions the following steps are essential:
Step 1: Defining the Callback
Define a function pointer type using a `typedef`. This type will describe the structure of functions that can be used as callbacks. In your case, you've defined a callback as a function that takes an `int` parameter and returns `void` (i.e., it doesn't return any value).
typedef void (*Callback)(int);
This line establishes a new type `Callback` that represents a function pointer with the specified signature. By using this `typedef`, we make the code more readable and allow ourselves to declare variables of type `Callback` instead of explicitly declaring function pointers every time.
Step 2 : Adding the Callback
Now that we have our callback type defined, we can proceed to create a global variable of this type. This global variable will act as a placeholder for the callback function that we want to attach later.
Callback gCallback;
In this line, `gCallback` is declared as a variable of type `Callback`. It's worth noting that at this point, `gCallback` is uninitialized, which means it doesn't currently point to any function.
Next, we create a function named `addListener` that takes a `Callback` parameter. This function's purpose is to assign the provided callback function to the global `gCallback` variable.
void addListener(Callback mCallback){
gCallback = mCallback;
}
Step 3 : Passing the Desired Function
In this step, we'll define a function named `eventListener`, which adheres to the callback signature we've established (accepts an `int` parameter and returns `void`). This function will serve as the function we want to call later using the callback mechanism.
void eventListener(int value){
std::cout<<"Calling from the listener ";
std::cout<<value;
}
Here, the `eventListener` function is designed to print a message along with the integer value it receives.
Now, we call the `addListener` function and pass our `eventListener` function as an argument. This action associates the `eventListener` function with the global `gCallback` variable.
addListener(eventListener);
At this point, the `gCallback` variable is effectively pointing to the `eventListener` function.
Finally, to demonstrate the callback mechanism, we call `gCallback(50)`. Since `gCallback` now points to `eventListener`, invoking `gCallback(50)` is equivalent to calling `eventListener(50)`.
gCallback(50);
Executing this line triggers the `eventListener` function, which in turn prints the specified message along with the value `50`.
>> Calling from the listener 50
By following these steps, you've successfully implemented a callback listener in C, enabling you to dynamically execute functions based on the context and requirements of your program.