In C, you only have call-by-value (with some wierdness for arrays). To pass a reference, you explicitly take the variable’s address. This has the benefit that you can easily tell a function call which won’t modify a variable from one which probably will:
  int i;
  f(i); /* i is unchanged */
  g(&i); /* i is likely changed */
However, for composite types, you often want to avoid making a copy when you’re not modifying the variable, so you end up with a function h(const T*) which is called as h(&x) anyway.

C++ adds call-by-reference, so that a call which looks like f(i) no longer tells you that i isn’t modified. This is a shame. Some style rules will give you the benefit of being able to tell modifying from non-modifying functions, while still getting the copy avoidance without burdening the caller with that implementation detail:
  f(T x) /* Call by value: x is a local copy. Use this when you want to play with x's value in f() without affecting the caller. */
  f(const T x) /* Call by value: x is a local copy and const. Use this for easily copied types which f() will only be reading. */
  f(T& x) /* Call by reference: x is shared with the caller and modifyable. Avoid this. */
  f(const T& x) /* Call by reference: x is shared with the caller but not modifyable. Use this for composite types which you don't want to gratuitously copy. Because x is const to f(), the caller can pretty much treat it as if f() got a copy. (The gotcha is that f() has to be aware that it'll see modifications to x made through other references, unlike with a copy.) */
  f(T* x) /* Call by pointer: Use when you want to have f() modify *x for the caller. */
  f(const T* x) /* Call by pointer, but not modifyable through it. Avoid without good casue, as this is generally semantically a call-by-reference. */

With the f(T&) case gone, we once again can be confident that f(i) won’t change i but g(&i) is likely to. More likely than with the C rules even, because we prefer f(const T&) to f(const T*) in cases where we’re merely using pointers to avoid an expensive copy. Pretty nice. Now I just need wide adoption and lint enforcement.