Tuesday, August 11, 2009

Modifying temporaries

Temporary objects are created and destroyed all the time in a C++ program. A simple example would be a function that returns by value. A temporary is as good as a const object because it makes little sense (usually) to change a temporary object, which is unnamed and has a very short time span. (Note: A temporary can be bound to a const reference in which case the scope of the temporary is the same as that of the reference.) However, as it turns out, in C++ you can change temporaries ... if they are of class type! You can call non-const member functions on a temporary. This is quite similar to binding a temporary to a non-const reference and changing it. Section 3.10.10 in C++ ISO/IEC 14882:1998 standard clearly mentions this exception. There are at least two practical use of such an exception. One is the "Named Parameter" idiom and the other one is the Move Constructor idiom. In case of the named parameter idiom, the member functions might prefer to return the object by non-const reference instead of a by value. Here is an example:

class X
{
public:
int a;
char b;
X() : a(0), b(0) {}
X setA(int i) { a = i; return *this; } // non-const function
X setB(char c) { b = c; return *this; } // non-const function
};

std::ostream & operator << (std::ostream & o, X const & x)
{
o << x.a << " " << x.b;
return o;
}

X createX() // returns X by value.
{
return X();
}

int main (void)
{
// The following code uses the named parameter idiom.
std::cout << createX().setA(10).setB('Z') << std::endl;
}