C Pass-by-Reference (Pointer) Basics & Value-Pass Differences

1. Why You Should Learn “Pass-by-Reference”?

2. Differences Between Pass-by-Value and Pass-by-Reference

What Is Pass‑by‑Value

void func(int x) {
    x = 10;
}

int main() {
    int a = 5;
    func(a);
    printf("%dn", a); // output is 5
}

What Is Pass‑by‑Reference (Pointer Passing)

void func(int *x) {
    *x = 10;
}

int main() {
    int a = 5;
    func(&a);
    printf("%dn", a); // output is 10
}

Summary of Differences Between Pass‑by‑Value and Pass‑by‑Reference

3. Understanding Through Code! Three Practical Examples

1. Basic Pass-by-Reference Example

void increment(int *p) {
    (*p)++;
}

int main() {
    int a = 0;
    increment(&a);
    printf("%dn", a); // Output: 1
    return 0;
}

2. swap Function Implementation Example

void swap(int *x, int *y) {
    int temp = *x;
    *x = *y;
    *y = temp;
}

int main() {
    int a = 3, b = 7;
    swap(&a, &b);
    printf("a = %d, b = %dn", a, b); // Output: a = 7, b = 3
    return 0;
}

3. Arrays and Pass-by-Reference

void fill_zero(int arr[], int size) {
    for (int i = 0; i < size; i++) {
        arr[i] = 0;
    }
}

int main() {
    int data[5] = {1, 2, 3, 4, 5};
    fill_zero(data, 5);
    for (int i = 0; i < 5; i++) {
        printf("%d ", data[i]);
    }
    // Output: 0 0 0 0 0
    return 0;
}

4. Why Use Pass-by-Reference? Benefits and Cautions

Benefits of Pass-by-Reference

1. You can directly modify caller’s value from a function With pass-by-value, changes to values inside a function do not affect the caller, but with pass-by-reference you can modify the variable itself that was passed as an argument. For example, it’s useful in situations like a swap function or adding/subtracting values where you want to return results in a form other than the return value.
2. Efficient handling of large data When passing large data such as arrays or structs to a function, pass-by-value copies everything, leading to unnecessary memory consumption and slower performance. With pass-by-reference (pointer passing), only the address (location) is passed, eliminating copy overhead and making it efficient.
3. Can modify multiple values simultaneously With pass-by-value you can return only one value, but using pass-by-reference you can change the values of multiple variables at once. This greatly expands a function’s flexibility.

Cautions and Drawbacks of Pass-by-Reference

1. Side effects are easy to occur Because you can directly modify the caller’s data, it can become hard to know where a value was changed in the program. In large codebases or team development, this can lead to unexpected bugs, so caution is required.
2. Bugs caused by mishandling pointers If you don’t manage addresses (pointers) correctly, unexpected problems arise. For example, accidentally dereferencing a NULL pointer or accessing freed memory can cause crashes—hazards that are unique to C.
3. Readability and comprehension difficulty Code that heavily uses pointers tends to be hard for beginners to understand. Also, misusing symbols like ‘&’ and ‘*’ can easily lead to unexpected behavior.

5. Differences with C++ (Supplement)

Pass‑by‑reference in C = Pass‑by‑pointer

void func(int *x) {
    *x = 100;
}
int main() {
    int a = 0;
    func(&a);
    // a becomes 100
    return 0;
}

Pass‑by‑reference in C++ is simpler

void func(int &x) {
    x = 100;
}
int main() {
    int a = 0;
    func(a);
    // a becomes 100
    return 0;
}

Summary: Differences in Usage

• C: Pass‑by‑reference = using pointers (* and & required)
• C++: Pass‑by‑reference = using reference types (& only, no pointer manipulation needed)

6. FAQ Frequently Asked Questions

Question 1: Why does passing an array to a function in C result in pass‑by‑reference?

Question 2: What happens if you modify the pointer’s value (the address) itself inside a function?

Question 3: Can structs also be passed by reference?

struct Data {
    int value;
};

void update(struct Data *d) {
    d->value = 123;
}

Question 4: What safety measures are needed when using pass‑by‑reference?

Question 5: When choosing between C and C++ for pass‑by‑reference, which should you pick?

Question 6: I’m not good with pointers. Any tips?