C++ Primer, Fifth Edition

6.2.1. Passing Arguments by Value

When we initialize a nonreference type variable, the value of the initializer is copied. Changes made to the variable have no effect on the initializer:

int n = 0;             // ordinary variable of type intint i = n;             // i is a copy of the value in ni = 42;                // value in i is changed; n is unchanged

Passing an argument by value works exactly the same way; nothing the function does to the parameter can affect the argument. For example, inside fact (§ 6.1, p. 202) the parameter val is decremented:

ret *= val--;  // decrements the value of val

Although fact changes the value of val, that ...