How can a returned object be assignable?
Question
In Effective C++, Item 3, Scott Meyers suggests overloading operator*
for a class named Rational
:
class Rational { ... };
const Rational operator*(const Rational& lhs, const Rational& rhs);
The reason for the return value being const
-qualified is explained in the following line: if it were not const
, programmers could write code such as:
(a * b) = c;
or, more probably:
if (a*b = c)
Fair enough. Now I’m confused as I thought that the return value of a function, here operator*, was a rvalue, therefore not assignable. I take it not being assignable because if I had:
int foo();
foo() += 3;
that would fail to compile with invalid lvalue in assignment
.
Why doesn’t that happen here? Can someone shed some light on this?
EDIT: I have seen many other threads on that very Item of Scott Meyers, but none tackled the rvalue problem I exposed here.
Solution
The point is that for class types, a = b
is just a shorthand to a.operator=(b)
, where operator=
is a member function. And member functions can be called on rvalues.
Note that in C++11 you can inhibit that by making operator=
lvalue-only:
class Rational
{
public:
Rational& operator=(Rational const& other) &;
// ...
};
The &
tells the compiler that this function may not be called on rvalues.