Re: When to use "call by reference"?
"Jacky" wrote:
I wonder when is the good time to use & with the actual
parameter?
I understand it does not make a "copy" to the callee but
passing the actual variable to the callee
but still unsure
Or in a mixed way
ostream& foo(int &a, char *string)
Or it shouldn't happen?
The rules of thumb are very simple, actually:
- If a parameter is changed within a function and you're
interested in the result, then pass by reference. Example:
void foo(int& n) { n = 42; }
int num = 0;
foo(num); // num is changed within foo
if(num == 42) { ... }
- If a parameter is big enough (i.e., its copy is expensive)
and you don't want it to be changed within a function, then
pass by const reference. Example:
std::vector<int> v;
v.reserve(100000);
void print_vector(
const std::vector<int>& vec)
{
// print content of vec
}
// v passed by reference, but cannot
// be changed within print_vector
print_vector(v);
- If a parameter is small and trivial enough (i.e., its copy
is cheap) and you don't care about its changes within a
function, then pass by value. Example:
void foo(int n) { n += 1; std::cout << n; }
int num = 42;
// copy of num is passed; foo can
// change the copy, but we don't care
foo(num);
That's all. Actually, you should pick up your favorite C++
textbook and reread a couple of first chapters. "By value vs
by reference" issue will be explained there in greater
details.
Alex