Will the following give a compilation error?
delete cout;
delete cin;
The answer is : No.
It is a flaw in the implementation of strea
It has apparently been fixed.
At least, in N3290 you have std::basic_ios::operator bool
instead of that void*
conversion, and this operator bool
is declared explicit
.
Note that C++98/C++03 did not support explicit
type conversion operators, but C++11 does.
An explicit
type conversion operator
N3290 §12.3.2/2;
is only considered as a user-defined conversion for direct-initialization (8.5)
And that might seem to be impractical for the condition in e.g. a while
or for
statement.
Happily,
N3290 §4/3;
An expressione
can be implicitly converted to a typeT
if and only if the declarationT t=e;
is well-formed, for some invented temporary variablet
(8.5). Certain language constructs require that an expression be converted to a Boolean value. An expressione
appearing in such a context is said to be contextually converted tobool
and is well-formed if and only if the declarationbool t(e);
is well-formed, for some invented temporary variablet
(8.5). The effect of either implicit conversion is the same as performing the declaration and initialization and then using the temporary variable as the result of the conversion.
where bool t(e);
is a direct-initialization.
E.g. you don’t have to explicit convert a stream object used as condition in a while
, because there is implicitly an explicit conversion (he he).
Unfortunately, searching N3290 I can’t find any list of the “certain language constructs” where this happens, but in comments to this answer JohannesD wrote:
Searched through the FDIS for “contextually”, and the whole list seems to be:
if
,while
,do
,for
,noexcept
, andstatic_assert
conditions; the first operand of?:
; both operands of&&
and||
; and the operand of!
.
Cheers & hth.,
If I can give my 2 cents, I think the standard library "flawed" a bit, with all the good intentions.
The operator void*()
had been introduced to allow code like
while(stream)
or if(!stream)
or while(stream && ...)
, without giving an implicit access to integer arithmetic (that operator bool
whould have given).
In fact, this disable integer arithmetic, but gives access to pointer features (like delete ...).
Now, in C++0x, an explicit oeprator bool()
had been introduced. It doesn't implicitly give access to whatever feature, since it requires an implicit conversion. But ... wait a bit: 'while(bool(stream))' or even while(static_cast<bool>(stream))
are so wordy...
Operator ! is explicit, and
'while(!!stream)' looks so effective that I even wonder why not accept this as a paradigm:
If I want something to be explicitly converted into bool, I just provide an operator!()
and give to !
the memaning of "is not valid" and of !!
as "is valid".
Much safer then an implicit conversion and not uselessly wordy: after all !
exist from ever!