On May 19, 2012, at 9:34 PM, Jordan Rose wrote:
> On May 20, 2012, at 0:20, Richard Smith wrote:
>> On Sat, May 19, 2012 at 6:43 PM, Jordy Rose <[email protected]> wrote:
>>> I am not a standardista, but this doesn't seem right to me. The target
>>> constructor finishes before the delegating constructor runs, and presumably
>>> the target constructor has to be able to initialize the entire object by
>>> itself. I don't see anything in the standard that says 'x' is
>>> zero-initialized by the default constructor on its own, the default
>>> constructor as a target, or the delegating constructor.
>>>
>> [class.base.init]/7: The expression-list or braced-init-list in a
>> mem-initializer is used to initialize the designated subobject (or, in the
>> case of a delegating constructor, the complete class object) according to
>> the initialization rules of [dcl.init] for direct-initialization.
>> [dcl.init]/10: An object whose initializer is an empty set of parentheses,
>> i.e., (), shall be value-initialized.
>> [dcl.init]/7: To value-initialize an object of type T means: [...] if T is a
>> (possibly cv-qualified) non-union class type without a user-provided
>> constructor, then the object is zero-initialized [...]
>
> But X /does/ have a user-provided constructor -- X::X(int). Or it will after
> we finish defining it. Thus we should just call the implicit default
> constructor, which will not initialize 'x'.
Richard has quoted an old draft (to be fair, I think it's probably the official
standard). The most recent language (N3376) is more clear:
• To value-initialize an object of type T means:
• — if T is a (possibly cv-qualified) class type (Clause 9)
with either no default constructor (12.1) or a default constructor that is
user-provided or deleted, then the object is default-initialized;
• — if T is a (possibly cv-qualified) non-union class type
without a user-provided or deleted default constructor, then the object is
zero-initialized and, if T has a non-trivial default constructor,
default-initialized;
• — if T is an array type, then each element is
value-initialized;
• — otherwise, the object is zero-initialized.
> That's a technicality, but one that feels like C++ to me. Otherwise you have
> to pay for the zero-initialization whether you use it or not.
It's a delegating constructor call to an explicitly-defaulted default
constructor. If you don't want to pay for the zero-initialization, just omit
the delegating initialization, and your constructor will do all the
default-constructing that the defaulted default constructor would have.
Of course, the semantic difference between value-initialization and
default-initialization is really subtle, but that's just C++ for you.
John.
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