When to return const refrence ?

a)When do we return by refrence?
Ans:. it avoids copying . But never return reference of local object .
     So that functions can be l-values
     We do this when we want to modify  lvalue  . we do this in operator overlaoding of " ="  to support a=b=c  chaining 

MyClass & operator=(const MyClass &rhs);

MyClass & operator+=(const MyClass &rhs) 

 
b) Returning just const
As const return can be assigned to nonconst variable and changed .
But if function is used in expression then temporary return as it is const will be prohibited .
struct foo
{
void bar() {}
void barfoo() const {}
};

foo foobar1() {return foo();}
const foo foobar2() {return foo();}

int main()
{
foobar1().barfoo(); //perfectly legal, calling const member function of non-const object
foobar1().bar(); //perfectly legal, calling non-const member function of non-const object
foobar2().barfoo(); // perfectly legal, calling const member function of const object
foobar2().bar(); // this willl not compile
}

In operator overloading we use it often to make sure

const MyClass MyClass::operator+(const MyClass &other) const

(a+b)=c // this kind of things will not get compile
We make sure (a+b).func()  is allowed only if func()is const function .
this is called const correectness

c)When to return const refrence
This make lot of sense .


http://www.cs.caltech.edu/courses/cs11/material/cpp/donnie/cpp-ops.html