Re: "Not really an lvalue" warnings

• Subject: Re: "Not really an lvalue" warnings
• From: "Mark Wagner" <email@hidden>
• Date: Fri, 6 Apr 2007 15:27:53 -0700
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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.

--
Mark
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