+= Assignment Operator in C++

In C++, the += operator is a compound assignment operator that adds the right-hand operand to the left-hand operand and assigns the result to the left-hand operand. This operator helps to write more concise and readable code.

This guide is aimed at complete beginners and will walk you through the usage of += with different data types, complete with step-by-step explanations.

Syntax

a += b; // Equivalent to: a = a + b;

How It Works

When you use a += b, it means:

1. Take the current value of a.
2. Add b to it.
3. Store the result back into a.

Example with Integers

#include <iostream>
using namespace std;
 
int main() {
    int a = 10;
    int b = 5;
    a += b;
    cout << "a = " << a << endl;
    return 0;
}

Explanation:

• a = 10, b = 5
• a += b is the same as a = a + b → a = 10 + 5
• So a becomes 15

Example with Floating-Point Numbers (float/double)

#include <iostream>
using namespace std;
 
int main() {
    float x = 3.5f;
    float y = 1.2f;
    x += y;
    cout << "x = " << x << endl;
    return 0;
}

Explanation:

• x = 3.5, y = 1.2
• x += y → x = x + y → x = 3.5 + 1.2
• x becomes 4.7

Example with Characters

#include <iostream>
using namespace std;
 
int main() {
    char ch = 'A'; // ASCII 65
    ch += 1;
    cout << "ch = " << ch << endl;
    return 0;
}

Explanation:

• The character 'A' has ASCII value 65.
• ch += 1 → ch = ch + 1 → ASCII value becomes 66
• ASCII 66 is 'B', so ch becomes 'B'

Example with Strings (Using std::string)

#include <iostream>
#include <string>
using namespace std;
 
int main() {
    string greeting = "Hello, ";
    string name = "Alice";
    greeting += name;
    cout << "greeting = " << greeting << endl;
    return 0;
}

Explanation:

• greeting = "Hello, ", name = "Alice"
• greeting += name → greeting = greeting + name → "Hello, " + "Alice"
• So greeting becomes "Hello, Alice"

Example with Arrays (Not Directly Supported)

The += operator does not work directly with arrays in C++. Here’s an example showing incorrect usage:

int arr1[3] = {1, 2, 3};
int arr2[3] = {4, 5, 6};
// arr1 += arr2; //  Not allowed

Why?

In C++, arrays are not objects; they are raw blocks of memory. You must use loops to perform element-wise addition:

#include <iostream>
using namespace std;
 
int main() {
    int arr1[3] = {1, 2, 3};
    int arr2[3] = {4, 5, 6};
 
    for (int i = 0; i < 3; i++) {
        arr1[i] += arr2[i];
    }
 
    for (int i = 0; i < 3; i++) {
        cout << "arr1[" << i << "] = " << arr1[i] << endl;
    }
 
    return 0;
}

Example with Pointers

The += operator can be used with pointers to move the pointer forward by a number of elements.

#include <iostream>
using namespace std;
 
int main() {
    int arr[] = {10, 20, 30};
    int* ptr = arr;
    ptr += 1;
    cout << "*ptr = " << *ptr << endl;
    return 0;
}

Explanation:

• ptr initially points to arr[0] (10)
• ptr += 1 moves the pointer to arr[1] (20)
• So *ptr prints 20

❗ Things to Remember

1. The += operator modifies the left-hand variable.
2. It can be used with most built-in types and with custom classes that overload the + and += operators.
3. It improves code readability and reduces redundancy.

🧪 Summary Table

Custom Types

If you’re creating your own class, you can overload the += operator to define how it should behave:

class Counter {
    int count;
public:
    Counter(int c = 0) : count(c) {}
    Counter& operator+=(int val) {
        count += val;
        return *this;
    }
    void show() {
        cout << "count = " << count << endl;
    }
};
 
int main() {
    Counter c(5);
    c += 10;
    c.show(); // count = 15
    return 0;
}

Conclusion

The += operator is a powerful shorthand in C++ that improves code clarity. It behaves consistently across most primitive types and is also extendable to custom types. Understanding it thoroughly can make your code cleaner and easier to maintain.

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