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) {
        // Assign callback pointer here.
        callback = sensorCallback; 
    }

    // Write nullptr check and callback invocation here
    if (callback == nullptr) {
        cout << "NO CALLBACK";
    }
    else {
        callback(value);
    }

    return 0;
}

Solving Approach