Re: Is this an implicit use of the template?

[Alf P. Steinbach]

On 10/31/2015 11:59 AM, Stefan Ram wrote:
> The following program looks innocuous.
>
> #include <iostream>
> #include <ostream>
>
> int main() { constexpr int num{ 11 }; ::std::cout << num << '\n'; }
>
> Does the above constitute an »implicit use of the template
> »std::initializer_list«? I am thinking of 8.5.4p2:
>
> »The template std::initializer_list is not predefined; if the
> header <initializer_list> is not included prior to a use of
> std::initializer_list --- even an implicit use in which the
> type is not named (7.1.6.4) --- the program is ill-formed.«
>
> Or would only the following be such an implicit use?
>
> #include <iostream>
> #include <ostream>
>
> int main() { constexpr auto num{ 11 }; ::std::cout << num << '\n'; }
>

My gut feeling: None of these involve initializer_list, even though
they syntactically involve initializer-list (oh, the difference of a
hyphen: reminds me of Latin 1 versus Latin-1, not sure what that was).

They look like formally direct initializations.

And as far as I can recall the syntax

int x = {42};

(copy initialization, but with the braces so that it looks like an
initialization_list) has always been valid, from original C and onward.

So, my gut feeling is that you need something like

auto x = {42};

and then you'd be in initializer_list-land.


Cheers & hth.,

- Alf

[Victor Bazarov]

On 10/31/2015 6:59 AM, Stefan Ram wrote:
> The following program looks innocuous.
>
> #include <iostream>
> #include <ostream>
>
> int main() { constexpr int num{ 11 }; ::std::cout << num << '\n'; }
>
> Does the above constitute an »implicit use of the template
> »std::initializer_list«? I am thinking of 8.5.4p2:
>
> »The template std::initializer_list is not predefined; if the
> header <initializer_list> is not included prior to a use of
> std::initializer_list --- even an implicit use in which the
> type is not named (7.1.6.4) --- the program is ill-formed.«
>
> Or would only the following be such an implicit use?
>
> #include <iostream>
> #include <ostream>
>
> int main() { constexpr auto num{ 11 }; ::std::cout << num << '\n'; }

How do you know that the header <initializer_list> is NOT included by
some directive in <iostream>, <ostream> or any headers they include? If
it's included, it's not "implicit use", is it?

V
--
I do not respond to top-posted replies, please don't ask

[Juha Nieminen]

Stefan Ram <r...@zedat.fu-berlin.de> wrote:
> #include <iostream>
> #include <ostream>

Isn't that second line superfluous?

--- news://freenews.netfront.net/ - complaints: ne...@netfront.net ---

[Alf P. Steinbach]

On 11/3/2015 10:11 AM, Juha Nieminen wrote:
> Stefan Ram <r...@zedat.fu-berlin.de> wrote:
>> #include <iostream>
>> #include <ostream>
>
> Isn't that second line superfluous?

In practice, always. But in the formal it was needed in C++98 and C++03.
All the examples in the standard were wrong. Which was a bit
embarrassing. Happily James Kanze proposed a fix, and now it's OK.

[red floyd]

On 11/3/2015 1:11 AM, Juha Nieminen wrote:
> Stefan Ram <r...@zedat.fu-berlin.de> wrote:
>> #include <iostream>
>> #include <ostream>
>
> Isn't that second line superfluous?

Like Alf said, it was technically required in C++03. I don't know of
any implementation where it was needed, but technically, it was
required.

[Juha Nieminen]

You need both to use std::cout? I can't even figure out how you could
impose that requirement, even if you deliberately tried to do so.
Except perhaps have std::cout declared in one, and its operators
in the other.

[Martin Shobe]

On 11/3/2015 3:11 AM, Juha Nieminen wrote:
> Stefan Ram <r...@zedat.fu-berlin.de> wrote:
>> #include <iostream>
>> #include <ostream>
>
> Isn't that second line superfluous?

As far as I can tell, yes. (At least in n3797).

From 17.6.5.2p1

A C++ header may include other C++ headers. A C++ header shall
provide the declarations and definitions that appear in its synopsis. A
C++ header shown in its synopsis as including other C++ headers shall
provide the declarations and definitions that appear in the synopsis of
those other headers.

The synopsis for iostream includes ostream. (See 27.4.1).

Martin Shobe

[Victor Bazarov]

On 11/4/2015 4:10 AM, Juha Nieminen wrote:
> red floyd <no....@its.invalid> wrote:
>> On 11/3/2015 1:11 AM, Juha Nieminen wrote:
>>> Stefan Ram <r...@zedat.fu-berlin.de> wrote:
>>>> #include <iostream>
>>>> #include <ostream>
>>>
>>> Isn't that second line superfluous?
>>
>> Like Alf said, it was technically required in C++03. I don't know of
>> any implementation where it was needed, but technically, it was
>> required.
>
> You need both to use std::cout? I can't even figure out how you could
> impose that requirement, even if you deliberately tried to do so.
> Except perhaps have std::cout declared in one, and its operators
> in the other.

At least that's how the Standard used to define those. <iostream> had
the declarations of the objects, and the classes were in the other
header. Do you think Alf and red were making it up?