passbyreference

Pass-by-reference is a parameter passing mechanism in which a function receives a reference to the actual data supplied by the caller, rather than a separate copy of that data. Because the parameter names refer to the same memory as the arguments, the function can modify the caller's variables. This is distinct from pass-by-value, where the function works on a copy and changes do not affect the caller.

Implementation varies by language. In C, functions receive values; to allow modification you pass pointers and dereference them. In C++, reference parameters use the & syntax in the function parameter list, so void increment(int& n) { ++n; }. In languages like PHP, pass-by-reference can be requested using a reference operator, so that the callee and caller share the same variable. In Java and Python, argument passing is by value of a reference; the function gets a copy of the reference to the object, so the mutable object can be changed, but reassigning the parameter does not affect the caller.

Advantages include avoiding copying large objects and enabling functions to modify inputs or to return multiple results efficiently. Disadvantages include potential side effects, difficulty in reasoning about program state, and risks of aliasing and data races in concurrent contexts. Some languages support explicit out or ref parameters to signal intent and control mutability.

Common guidance is to use pass-by-reference when large data structures must be mutated, or when a function must return more than a single value; otherwise, pass-by-value is often preferable to keep functions simpler and safer. When describing APIs, it is helpful to document whether parameters are passed by reference and whether modifications affect the caller.