Pass by reference VS pass by pointer

Question

Whats the difference between two in C++?

Answer 1

A reference variable is an alias of an existing variable.

int a = 39;
int& b = a;
// both a and b refer to the same variable.

A pointer is a variable which stores the address of another variable,

int a = 39;
int* b = &a;

While both pass-by-reference and pass-by-pointer may be done due the same purpose, i.e. to change the value of the variable passed as a parameter to the function, within the function body, there are some subtle differences, such as the need to dereference a pointer in the function body.

void change1(int* a) {
*a = 39; // dereference the pointer before altering the value
}

void change2(int& a) {
a = 39; // alter the reference variable
}

int main() {
int a1 = 38, a2 = 38;

change1(&a1); // pass address of a1
change2(a2); // pass a2 directly
}

Answer 2

A practical recommendation I have read:

When you want to change the value you pass, use a pointer; when you just want to read the data, use a reference but make it const.

The point was to keep your code maintainable.

A call by value looks exactly like a call by reference:

f(x)

So by just calling the function you can't tell if it has the power to mess around with the original variable or not.

A call by pointer looks differently:

f(&x)

So here you explicitly and knowingly you grant the function the right to modify x.