What is a Callback in C++?

A callback is:

A function passed to another function so it can be called later.

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Simple Real-Life Analogy

Suppose you order food online:

"Call me when food arrives"

Your phone number is the callback.

Restaurant calls you later.

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In Programming

You give a function to another function.

That function gets executed later.

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Simple Callback Example

#include <iostream>
#include <functional>

using namespace std;

void finished()
{
    cout << "Task Finished!" << endl;
}

void process(function<void()> callback)
{
    cout << "Processing..." << endl;

    callback();
}

int main()
{
    process(finished);

    return 0;
}

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Output

Processing...
Task Finished!

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How it works

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1. Callback function

void finished()

This function will be called later.

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2. process() accepts callback

function<void()> callback

means:

Accept any function:
- no arguments
- returns void

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3. Execute callback

callback();

calls:

finished();

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Callback Flow

main()
   |
   v
process(finished)
   |
   v
callback()
   |
   v
finished()

---

Why callbacks are useful

Callbacks are heavily used in:

• GUI buttons
• networking
• threads
• timers
• event systems
• async programming

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Example: Button Click

When button clicked
→ call this function

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Example: Download Finished

Download complete
→ call callback

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Callback with Lambda

Modern C++ usually uses lambdas.

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Example

#include <iostream>
#include <functional>

using namespace std;

void process(function<void()> callback)
{
    cout << "Working..." << endl;

    callback();
}

int main()
{
    process([]()
    {
        cout << "Lambda Callback!" << endl;
    });

    return 0;
}

---

Output

Working...
Lambda Callback!

---

Callback with Parameters

#include <iostream>
#include <functional>

using namespace std;

void calculate(int a, int b,
               function<void(int)> callback)
{
    int result = a + b;

    callback(result);
}

int main()
{
    calculate(10, 20,
        [](int result)
        {
            cout << "Result: "
                 << result << endl;
        });

    return 0;
}

---

Output

Result: 30

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Your Earlier bind Example

You had:

function<void()> func =
bind(&ReadFile::replaceAndSave1, &rf);

This creates a callback.

Later:

func();

calls:

rf.replaceAndSave1();

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Real-World Thread Callback Example

thread t(callbackFunction);

Thread executes callback later.

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Event System Example

Mouse Clicked
→ callback function runs

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Timer Example

Wait 5 seconds
→ callback runs

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Modern C++ Preferred Style

Today developers prefer:

• lambdas
• std::function

instead of old C-style function pointers.

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Old Style Callback

void (*callback)();

---

Modern Style

function<void()>

Cleaner and safer.

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Final Understanding

A callback is simply:

A function you give to another function
to execute later.

It is fundamental in:

• async programming
• GUI applications
• event-driven systems
• multithreading
• networking
• robotics systems