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Re: [ #include <stack> ] What's the difference?

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On 7/6/21 3:50 PM, Andrey Tarasevich wrote:
> On 6/24/2021 2:23 PM, James Kuyper wrote:
>> On 6/23/21 1:17 PM, Andrey Tarasevich wrote:
>>> On 6/11/2021 10:55 AM, SIMON wrote:
>>>> Does anybody know the significant difference between:
>>>>
>>>>> myStack.push(30);
>>>>>
>>>>> myStack.emplace(30);
>>>>>
>>>> It does the same thing as far as I can see but surely there must be
>>>> something that I don't know about these two methods.
>>>
>>> Without the exact type of 'myStack' the question is meaningless and does
>>> not have an answer.
>>
>> What more do you need to know about the the type of myStack that you
>> don't already know?
>> The subject line says "#include <stack>", so you can reasonably presume
>> from its name that myStack is of type std::stack<T, Container> for some
>> values of T and Container. 
> 
> I need to know what `T` is. I need to know whether `myStack` holds 
> `int`s, or `myStack` holds something else, perhaps some non-trivial, 
> unrelated, possibly "heavy" type, which just happens to be implicitly 
> convertible from `int`.
> 
> For `T == int` the difference between the variants would be purely 
> conceptual, without any practical consequences.
> 
> For a "heavy" `T` the difference between these two variants can be 
> substantial. `push` would construct a temporary followed by two moves 
> from it into a new container entry, while `emplace` would construct a 
> new container entry in-place from the twice-forwarded `30`.

That doesn't make the question "meaningless", nor does it prevent the
question from having an answer. The question can be answered in full by
quoting what the standard says about std::stack<T, Container>::push()
and emplace(). The ways in which the behavior depends upon T and
Containter are completely determined by what the standard says about
those functions, directly or indirectly. The "significant difference"
between those two functions that the OP was talking about consists
entirely of the differences between what the standard says about them,
along with the T-dependent implications of those differences.

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[ #include <stack> ] What's the difference? SIMON <invalid@invalid.invalid> - 2021-06-11 17:55 +0000
  Re: [ #include <stack> ] What's the difference? SIMON <invalid@invalid.invalid> - 2021-06-11 18:08 +0000
    Re: [ #include <stack> ] What's the difference? scott@slp53.sl.home (Scott Lurndal) - 2021-06-11 18:21 +0000
  Re: [ #include <stack> ] What's the difference? Richard Damon <Richard@Damon-Family.org> - 2021-06-11 14:29 -0400
  Re: [ #include <stack> ] What's the difference? Kli-Kla-Klawitter <kliklaklawitter69@gmail.com> - 2021-06-11 20:35 +0200
  Re: [ #include <stack> ] What's the difference? Manfred <noname@add.invalid> - 2021-06-12 20:06 +0200
    Re: [ #include <stack> ] What's the difference? Kli-Kla-Klawitter <kliklaklawitter69@gmail.com> - 2021-06-12 20:12 +0200
    Re: [ #include <stack> ] What's the difference? MrSpook_mlauoyv4F_@o18rpnvgrx0dq.co.uk - 2021-06-13 08:34 +0000
      Re: [ #include <stack> ] What's the difference? Öö Tiib <ootiib@hot.ee> - 2021-06-13 04:31 -0700
  Re: [ #include <stack> ] What's the difference? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2021-06-23 10:17 -0700
    Re: [ #include <stack> ] What's the difference? James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-06-24 17:23 -0400
      Re: [ #include <stack> ] What's the difference? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2021-07-06 12:50 -0700
        Re: [ #include <stack> ] What's the difference? James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-07-06 21:17 -0400
          Re: [ #include <stack> ] What's the difference? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2021-07-06 21:59 -0700

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