Rvalues in C++03

Question

How can you tell whether or not a given parameter is an rvalue in C++03? I'm writing some very generic code and am in need of taking a reference if possible, or constructing a new object otherwise. Can I overload to take by-value as well as by-reference and have the rvalue returns call the by-value function?

Or do I have a very sickening feeling that this is why rvalue references are in C++0x?

Edit:

is_rvalue = !(is_reference || is_pointer) ?

Answer 1

There apparently is a way to determine whether an expression is an rvalue or lvalue in C++03 (I say apparently because I'm not sure how well I understand the technique). Note that to make the technique usable, preprocessor macros are pretty much required. Eric Niebler has written a nice article about how it works and how it gets used in BOOST_FOREACH:

• Conditional Love: FOREACH Redux

Note that the article is pretty heavy reading (at least it is for me); as Neibler says in it:

There's no doubt that this is arcane stuff, but we are rewarded with a robust way to detect the rvalue-ness and lvalue-ness of any expression.

Using the rvalue detection described in the artcile might help you deal with at least some of the issues that C++0x's rvalue references solve.