What is the difference between pass by value, pass by reference, and pass by pointer?

Question

Accepted Answer

**Pass by value** — copy. **Pass by reference** — alias. **Pass by pointer** — address.

**Pass by value:**
```cpp
void func(int x) {
    x = 100;  // Changes only local copy
}

int main() {
    int a = 10;
    func(a);
    std::cout << a;  // 10 (unchanged)
}
```

**Pass by reference:**
```cpp
void func(int& x) {
    x = 100;  // Changes original
}

int main() {
    int a = 10;
    func(a);
    std::cout << a;  // 100 (changed!)
}
```

**Pass by pointer:**
```cpp
void func(int* x) {
    *x = 100;  // Dereference and change
}

int main() {
    int a = 10;
    func(&a);  // Pass address
    std::cout << a;  // 100 (changed!)
}
```

**Key differences:**
- **Value** — copy, original unchanged.
- **Reference** — alias, original changed, cannot be null.
- **Pointer** — address, original changed, can be null.

**When to use:**
- **Value** — small types, no modification needed.
- **Reference** — modify parameter, avoid null.
- **Pointer** — optional parameter (null), arrays.

C++: What is the difference between pass by value, pass by reference, and pass by pointer?