Re: When to use "call by reference"?

"Alex Blekhman" <xfkt@oohay.moc>
Fri, 9 Feb 2007 11:18:36 +0200
"Jacky" wrote:

I wonder when is the good time to use & with the actual
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;

    void print_vector(
        const std::vector<int>& vec)
        // print content of vec

    // v passed by reference, but cannot
    // be changed within print_vector

- 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

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


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