Safe Callback Invocation

#include <iostream>
using namespace std;

// Callback function that prints sensor data
void sensorCallback(int v) {
    cout << "DATA " << v;
}

int main() {
    int flag, value;
    cin >> flag >> value;

    void (*callback)(int) = nullptr;   // optional callback pointer

    if (flag == 1) {
       callback = sensorCallback;
       callback(value);
    }

    if (nullptr == callback) {
        cout << "NO CALLBACK" << endl;
    } 

    // Write nullptr check and callback invocation here
    callback = nullptr;
    
    return 0;
}

Solving Approach