Re: "Not really an lvalue" warnings
Re: "Not really an lvalue" warnings
- Subject: Re: "Not really an lvalue" warnings
- From: Marshall Clow <email@hidden>
- Date: Fri, 6 Apr 2007 15:44:14 -0700
At 3:27 PM -0700 4/6/07, Mark Wagner wrote:
On 4/6/07, Marshall Clow <email@hidden> wrote:
At 2:48 PM -0700 4/6/07, Mark Wagner wrote:
The following code produces a warning from GCC about "target of
assignment not really an lvalue; this will be a hard error in the
future". What's going wrong, and how do I fix it?
char *typeOfQ = NULL;
static_cast<const char *>(typeOfQ) =
handle.FirstChildElement("fieldlabel").Element()->NextSiblingElement("fieldentry")->GetText();
GCC is telling you that there is a problem in your code (hence the
warning), and telling you that future versions of the compiler will
flag this as an error rather than a warning.
This expression:
static_cast<const char *>(typeOfQ)
is not a variable, it is a expression.
Think of the difference between 'x' and '3' - one can be assigned to,
the other cannot )
Googling for "rvalue" and "lvalue" will give more general information.
I know the difference between an lvalue and an rvalue. My
understanding is that "static_cast<const char *>(typeOfQ)" is
functionally and semantically equivalent to "(const char *)typeOfQ"
with the addition of compile-time checking to make sure the conversion
is defined.
In general, this is correct - there are some subtleties that are
different, IIRC.
Here's the appropriate bits of the standard:
Section 5.2.11 Const Cast
The result of the expression const_cast<T>(v) is of type T. If T is a
reference type, the result is an
lvalue; otherwise, the result is an rvalue and, the lvalue-to-rvalue
(4.1), array-to-pointer (4.2), and
function-to-pointer (4.3) standard conversions are performed on the
expression v.
- and -
Section 5.4 Explicit type conversion (cast notation)
1 The result of the expression (T) cast-expression is of type T. The
result is an lvalue if T is a reference
type, otherwise the result is an rvalue. [Note:if T is a non-class
type that is cv-qualified, the cv-qualifiers are ignored when
determining the type of the resulting rvalue; see 3.10. ]
Note that, as Clark Cox states, neither of them results in an lvalue,
which means they can't be assigned to.
--
-- Marshall
Marshall Clow Idio Software <mailto:email@hidden>
It is by caffeine alone I set my mind in motion.
It is by the beans of Java that thoughts acquire speed,
the hands acquire shaking, the shaking becomes a warning.
It is by caffeine alone I set my mind in motion.
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