C++ Pass Array to a Function

📦 C++ Pass Array to a Function

In C++, arrays are passed to functions by reference by default.
That means the function receives the address of the first element, so changes inside the function affect the original array.

🔹 1. Basic Syntax

void functionName(dataType arrayName[], int size);

or

void functionName(dataType *arrayName, int size);

👉 Both are equivalent.

🔹 2. Simple Example (Print Array)

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

int main() {
int a[] = {1, 2, 3, 4};
printArray(a, 4);
}

🔹 3. Modify Array Inside a Function

void updateArray(int arr[], int size) {
for (int i = 0; i < size; i++) {
arr[i] *= 2;
}
}

int main() {
int a[] = {1, 2, 3};
updateArray(a, 3);

for (int x : a)
cout << x << " ";
}

Output:

2 4 6

✔ Original array is modified.

🔹 4. Passing Array Using Pointer

void printArray(int *arr, int size) {
for (int i = 0; i < size; i++) {
cout << *(arr + i) << " ";
}
}

✔ Same behavior as arr[].

🔹 5. Passing 2D Array to a Function

⚠️ Column size must be specified.

void printMatrix(int mat[][3], int rows) {
for (int i = 0; i < rows; i++) {
for (int j = 0; j < 3; j++) {
cout << mat[i][j] << " ";
}
cout << endl;
}
}

int main() {
int matrix[2][3] = {{1,2,3},{4,5,6}};
printMatrix(matrix, 2);
}

🔹 6. Passing Array with const (Read-Only)

void show(const int arr[], int size) {
for (int i = 0; i < size; i++) {
cout << arr[i] << " ";
}
}

✔ Prevents accidental modification
✔ Best practice for safety

🔹 7. Array Size Is NOT Automatically Known

❌ This does NOT work:

void fun(int arr[]) {
int size = sizeof(arr); // wrong
}

✔ Correct approach:

void fun(int arr[], int size) {
// use size
}

🔹 8. Return Array from a Function (Using Dynamic Memory)

int* createArray() {
int *arr = new int[3]{1, 2, 3};
return arr;
}

int main() {
int *a = createArray();
delete[] a;
}

⚠️ Caller must free memory.

🔁 Array vs Vector in Functions

Array Vector
Fixed size Dynamic size
Size passed manually v.size()
Less safe Safer

📌 Summary

  • Arrays are passed by reference automatically

  • Function receives the base address

  • Size must be passed separately

  • Use const for read-only arrays

  • For dynamic needs, prefer vector

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