Re: the meaning of lvalue in C++
On Mar 10, 12:26 am, "jam" <farid.mehr...@gmail.com> wrote:
On Mar 10, 12:25 am, "restor" <akrze...@interia.pl> wrote:
Does lvalue have any useful meaning in C++? The original (i.e.: as
used in C) was that it is whatever that can be assigned a value. But
in C++ the code:
std::complex<double> Complex() { return std::complex<double>(); }
Complex() = Complex();
Is valid. Also the code:
int Integer() { return 1; }
const int& cref = Integer();
int& ref = const_cast<int&>( cref ); // is it a cast from rvalue to
lvalue???
ref = 3;
Is valid, but it is unclear (at least for me) what is 'ref'. Is it an
lvalue pointing to a temporary?
We also have non-modifiable lvalues (consts) and modifiable rvalues
like here:
ReturnObj().ModifyMe();
So what is useful in having a concept of lvalue in C++?
What is the use of an lvalue by the way?
I should say that even const objects can be lvalue:
Even in C.
struct Lvalue{
Lvalue operator=(const Lvalue &)const{};//look at this!!
};
{
const Lvalue L1,L2;
L1=L2;//huh huh look at me
};
generally the only real Rvalue in C++ are literal numbers and
enumerations.
And most expressions. What's particular about class type
temporaries isn't that they aren't rvalues; it's that it's
possible, indirectly, to get their address (via the this
pointer) even though they are rvalues.
As for the "utility": I think it's largely a question of C
compatibility. Not just at the language level, but at a larger
"look and feel" level. One could easily do away with the
concept entirely, allowing things like "&(i + 3)" (with type
const int? and of course, the usual lifetime of a temporary),
but somehow, the language wouldn't feel the same.
--
James Kanze (GABI Software) email:james.kanze@gmail.com
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