43. ISR Callback Alias

In embedded systems, Interrupt Service Routines (ISRs) often notify the main firmware logic by calling a registered callback function. This design decouples low-level hardware events from higher-level application behavior and is commonly used in drivers and hardware abstraction layers (HALs).

Your task is to define a function pointer type alias using modern C++ syntax, select the correct callback based on a hardware mode input, and invoke the callback safely through the alias.

What You Must Do:

1. Create a function pointer type alias named ISRCallback using the using keyword.
2. The alias must represent a pointer to a function with the following signature:

   1. void callback(int32_t code);
      

3. Two callback functions are already provided:
   • onError(int32_t code) — prints ERROR <code>
   • onComplete(int32_t code) — prints COMPLETE <code>
4. Based on the input mode, register the appropriate callback.
5. Invoke the callback indirectly through the function pointer alias.

Program Input:

Read two space-separated integers from standard input:

mode eventCode

• mode determines which callback to register
• eventCode is passed to the callback

Selection Logic:

• If mode == 1 → register onError
• If mode == 2 → register onComplete

After registration, invoke the callback using eventCode as the argument.

Example 1

Input:

1 42

Output:

ERROR 42 

Example 2

Input:

2 -5

Output:

COMPLETE -5 

Constraints:

• mode will always be exactly 1 or 2
• eventCode is a 32-bit signed integer
  Range: -2,147,483,648 to 2,147,483,647
• You must use the using keyword to define the alias
• You must call the function through the function pointer variable (not directly)