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;
}

13 comments:

Wriiight said...

You'd be surprised how often you take advantage of this without thinking about it. We have some old Sun Solaris compile that insists temporaries are const, and we often find ourselves unable to compile our latest changes on that box for that very reason.

Wriiight said...

By the way, returning an auto_ptr from a function requires modifying the temporary return value, or else you wouldn't be able to use it on the right side of an equals.

Sumant said...

@Wriiight: In case of auto_ptr, move constructor/assignment idiom is used. This idiom partially depends on the fact that non-const member functions can be called on temporary objects. A non-const conversion function is called in the case of this idiom. Colvin-Gibbons were the first to come up with this subtle technique.

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Anabel said...

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internet marketing melbourne said...

>> Will VC10 support variable length array (VLA)?

> No; that is a C99 language feature.

Where do I put in a feature request for C99 support in VC10?

I really don't want to have to move to mingw (from VC9).

Chris said...

I think more useful would be using the command pattern:
class A
{
SetX(int x);
SetY(int y);
};
B b = CreateBFromA().SetX(1).SetY(2);

ie. Using class A to make a B object.

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