Question

What is the difference between the following parameter passing mechanisms in C++?

void foo(int &x) 
void foo(int *x)
void foo(int **x)
void foo(int *&x)

I’d like to know in which case the parameter is being passed by value or passed by reference.

Answer 1

void foo(int &x)

passes a reference to an integer. This is an input/output parameter and can be used like a regular integer in the function. Value gets passed back to the caller.

---

void food(int *x)

passes a pointer to an integer. This is an input/output parameter but it’s used like a pointer and has to be dereferenced (e.g. *x = 100;). You also need to check that it’s not null.

---

void foo(int **x)

passes a pointer to a pointer to an integer. This is an input/output parameter of type integer pointer. Use this if you want to change the value of an integer point (e.g. *x = &m_myInt;).

---

void foo(int *&x)

passes a reference to a pointer to an integer. Like above but no need to dereference the pointer variable (e.g. x = &m_myInt;).

---

Hope that makes sense. I would recommend using typedefs to simplify the use of pointers and reference symbols.