Four kinds of expression
Nikolay Ivchenkov
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n3010.pdf
proposes to separate expressions into 3 kinds: rref lvalue, non-rref
lvalue, and rvalue.
I can imagine 4 kinds of an expression:
1) ref lvalue (which is non-rref lvalue in N3010)
2) non-ref lvalue (new kind)
3) ref rvalue (which is rref lvalue in N3010)
4) non-ref rvalue (which is rvalue in N3010).
According to this concept, every expression is either an lvalue or an
rvalue, and every expression is either a ref expression or a non-ref
expression.
-- An rvalue expression designates an entity that shall be treated as
temporary (for example: an enumerator, an object returned by value or
by rvalue reference). Any other expression is an lvalue (for example:
a function, a local variable, an object returned by lvalue reference,
a bit-field).
-- A ref expression is a reference or an addressable expression that
can be converted to a reference type using reinterpret_cast (for
example: a function, a local variable, an object returned by lvalue
reference). Any other expression is a non-ref expression (for example:
enumerator, bit-field, an object returned by value).
Like some rvalues, bit-fields are not addressable. However, bit-fields
can be treated as temporary and as non-temporary:
struct X
{
X() : m(1) {}
int m : 10;
};
template <class T>
void f(T &);
template <class T>
void f(T const &) {}
int main()
{
X x;
f(X().m); // OK: X().m is an rvalue
f(x.m); // ill-formed: X().m is an lvalue
}
This example demonstrates collision between two concepts of lvalue:
designation of an addressable entity and designation of a possibly non-
temporary entity. It seems, the model with 4 kinds of expressions
looks more consistent. For the purposes of the overload resolution non-
ref lvalues could be treated like rvalues.
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