C++: OOP: Friend Functions
Friend functions are functions defined outside a class (ie, not member functions), but which the
class declares to be friends so that they can use
the class's private members. This is commonly used in operator overloading.
Overloading input/output operators
Perhaps the most common use of friend functions is
overloading << and >> for I/O.
See Overloading << and >>.
Another use of friend functions is to permit operators to be
commutative. An operator is commutative if the result is the same
regardless of the order of its operands. Some typical examples are
addition and multiplication. Subtraction and division are not commutative.
x is an object of a class you've written
i is an integer,
x+i should have the same meaning as
The + in
x+i is ok if the + operator between an object and an
is defined as a member function in
However the + in the second case (
i+x) can only be written
as a friend function. That's because overloading (redefining) operators can
only be done for classes, not the primitive types, and the operator
definition that is used is based on the class of the left operand.
Declare friends before public and private
A class doesn't control the scope of friend functions so
friend function declarations are usually written at the
beginning of a .h file. Public and private don't apply to them.
Another possible use of friend functions is efficiency since
they can access the private members directly they can avoid
the cost of using getter functions. However, never do this
unless you really need to because it makes code harder to read
and introduces more possible sources of errors thru increased dependencies.
Another use is to make some
function calls more attractive. For example,
Time t1, t2;
. . .
if (t1.compareTo(t2) == 0) . . .
Might be more attractively written as
if (compareTo(t1, t2) == 0) . . .
Because compareTo needs to examine the private values in t1 and t2,
this second form is only possible if the class declares that
compareTo is a friend.