C++ register Keyword

The register keyword in C++ is a storage class specifier that suggests to the compiler that the variable should be stored in a CPU register for faster access. Variables declared with register are typically used in performance-critical sections of code where speed is important, such as loops.

Modern compilers often optimize variable storage automatically, so the register keyword has become less relevant. It is retained in C++ for compatibility with older code but is rarely needed now a days.

Syntax

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register data_type variable_name;

register: The keyword suggesting that the variable should be stored in a CPU register.
data_type: The type of the variable (e.g., int, char, etc.).
variable_name: The name of the variable.

Examples

Example 1: Using `register` for Loop Optimization

In this example, you will learn how the register keyword can be used to suggest faster access to a loop counter.

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#include <iostream>
using namespace std;

int main() {
    register int counter; // Suggest storing the counter in a CPU register

    for (counter = 0; counter < 5; ++counter) {
        cout << "Counter: " << counter << endl;
    }

    return 0;
}

Output:

Counter: 0
Counter: 1
Counter: 2
Counter: 3
Counter: 4

Explanation:

The register keyword is used to suggest that the variable counter be stored in a CPU register for faster access.
The loop iterates from 0 to 4, printing the value of counter at each step.
Modern compilers may ignore the register keyword and optimize variable storage automatically.

Example 2: Using `register` with a Local Variable

In this example, you will learn how to use the register keyword for a local variable in a function.

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#include <iostream>
using namespace std;

int sumOfN(int n) {
    register int sum = 0; // Suggest storing 'sum' in a register
    for (int i = 1; i <= n; ++i) {
        sum += i;
    }
    return sum;
}

int main() {
    int n = 10;
    cout << "Sum of first " << n << " numbers: " << sumOfN(n) << endl;
    return 0;
}

Output:

Sum of first 10 numbers: 55

Explanation:

The register keyword suggests faster storage for the sum variable in the function.
The loop calculates the sum of the first n natural numbers by adding each integer to sum.
The result is returned and printed in the main function.

Key Points about `register` Keyword

The register keyword suggests that the variable be stored in a CPU register for faster access.
Modern compilers often ignore the register keyword and optimize variable storage automatically.
Register storage is only a suggestion to the compiler and is not guaranteed.
A variable declared with register cannot have its address taken using the & operator.

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C++ register Keyword