Explicit instantiation of class template with deleted members

[Andrzej Krzemieński]

Hi All,
Is the following explicit class template instantiation well-formed?:

template <typename I>
class Ratio
{
public:
    Ratio (I, I  = 1) {};
    Ratio (double, double = 1.0) = delete;
};

template class Ratio<int>;

The second constructor is neither inherited, nor is it a template, so as per [temp.explicit]/8, it should be instantiated. Is such instantiation equivalent to being "used" in the sense provided by [dcl.fct.def.delete]/2? ("A program that refers to a deleted function implicitly or explicitly, other than to declare it, is ill-formed.")

GCC and Clang accept, it, but I cannot find a place in the Standard to support it.

Regards,
&rzej;

[Richard Smith]

On 11 Oct 2016 1:52 am, "Andrzej Krzemieński" <akrz...@gmail.com> wrote:

Hi All,
Is the following explicit class template instantiation well-formed?:

template <typename I>
class Ratio
{
public:
    Ratio (I, I  = 1) {};
    Ratio (double, double = 1.0) = delete;
};

template class Ratio<int>;

The second constructor is neither inherited, nor is it a template, so as per [temp.explicit]/8, it should be instantiated. Is such instantiation equivalent to being "used" in the sense provided by [dcl.fct.def.delete]/2? ("A program that refers to a deleted function implicitly or explicitly, other than to declare it, is ill-formed.")

The only reference to the function is the declaration generated by the instantiation. So the program arguably didn't refer to the function in any way other than to declare it.

GCC and Clang accept, it, but I cannot find a place in the Standard to support it.

Regards,
&rzej;

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[Andrzej Krzemienski]

2016-10-11 18:54 GMT+02:00 Richard Smith <ric...@metafoo.co.uk>:

On 11 Oct 2016 1:52 am, "Andrzej Krzemieński" <akrz...@gmail.com> wrote:

Hi All,
Is the following explicit class template instantiation well-formed?:

template <typename I>
class Ratio
{
public:
    Ratio (I, I  = 1) {};
    Ratio (double, double = 1.0) = delete;
};

template class Ratio<int>;

The second constructor is neither inherited, nor is it a template, so as per [temp.explicit]/8, it should be instantiated. Is such instantiation equivalent to being "used" in the sense provided by [dcl.fct.def.delete]/2? ("A program that refers to a deleted function implicitly or explicitly, other than to declare it, is ill-formed.")

The only reference to the function is the declaration generated by the instantiation. So the program arguably didn't refer to the function in any way other than to declare it.

Thanks for the response. Still, I feel a bit uncomfortable about this explanation. the explicit class template declaration did refer to a deleted function *indirectly*: I knew that the second deleted constructor is one of its non-inherited, non-template member functions, and I requested that its definition be instantiated. (Whatever it means for a deleted definition to be instantiated). I did not mention the function (class) name explicitly, but is it enough? The word "implicitly" in [dcl.fct.def.delete]/2 sounds very open to interpretation.

Regards,

&rzej;

[Robert Haberlach]

On 10/11/2016 07:35 PM, Andrzej Krzemienski wrote:
>
>
> 2016-10-11 18:54 GMT+02:00 Richard Smith <ric...@metafoo.co.uk <mailto:ric...@metafoo.co.uk>>:

>
> On 11 Oct 2016 1:52 am, "Andrzej Krzemieński" <akrz...@gmail.com <mailto:akrz...@gmail.com>> wrote:
>
> Hi All,
> Is the following explicit class template instantiation well-formed?:
>
> |
> template<typenameI>
> classRatio
> {
> public:
> Ratio(I,I =1){};
> Ratio(double,double=1.0)=delete;
> };
>

> templateclassRatio<int>;

> |
>
> The second constructor is neither inherited, nor is it a template, so as per [temp.explicit]/8, it should be instantiated. Is such
> instantiation equivalent to being "used" in the sense provided by [dcl.fct.def.delete]/2? ("A program that refers to a deleted function
> implicitly or explicitly, other than to declare it, is ill-formed.")
>
>
> The only reference to the function is the declaration generated by the instantiation. So the program arguably didn't refer to the function in any
> way other than to declare it.
>
>
> Thanks for the response. Still, I feel a bit uncomfortable about this explanation. the explicit class template declaration did refer to a deleted
> function *indirectly*: I knew that the second deleted constructor is one of its non-inherited, non-template member functions, and I requested that its
> definition be instantiated. (Whatever it means for a deleted definition to be instantiated). I did not mention the function (class) name explicitly,
> but is it enough? The word "implicitly" in [dcl.fct.def.delete]/2 sounds very open to interpretation.

Precisely: and is an interpretation sensible if it disallows defining templated member entities as deleted? No implementation thinks so, and neither
do we.

>
> Regards,
> &rzej;
>
>
>
>
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[Johannes Schaub]

Declarations are exempt from the "refering to a deleted function is illformed".

Anyway, this does not even require an explicit instantiation. Merely saying

   Ratio<int> r(5)

Refers to the deleted constructor in order to declare it as a member of "Ratio<int>".

I don't have the spec here, but I believe that you could argue that the constructor is not even deleted yet at that point and at the point that it is explicitly instantiated. Because the deletion is the function's definition, and when the explicit instantiation of the function definition (the = delete part) is performed, the function is not yet deleted. Only afterwards.

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[Matthew Woehlke]

On 2016-10-11 04:51, Andrzej Krzemieński wrote:
> Is the following explicit class template instantiation well-formed?:
>
> template <typename I>
> class Ratio
> {
> public:
> Ratio (I, I = 1) {};
> Ratio (double, double = 1.0) = delete;
> };
>
> template class Ratio<int>;
>
> The second constructor is neither inherited, nor is it a template, so as
> per [temp.explicit]/8, it should be instantiated. Is such instantiation
> equivalent to being "used" in the sense provided by [dcl.fct.def.delete]/2?
> ("A program that refers to a deleted function implicitly or explicitly,
> other than to declare it, is ill-formed.")

No.

I'm pretty sure "refers to" here means (at least for practical purposes)
"tries to execute or take the address of".

You aren't doing that. You're instantiating a class that has a deleted
function.

Consider this example instead:

class IntRatio
{
public:
Ratio(int, int = 1) {};
Ratio(double, double = 1.0) = delete;
};

There is nothing whatsoever wrong with this class. Why would you expect
the class being a template class to change things? The instantiation
`Ratio<int>` is not magically different from `IntRatio`; not anyway
w.r.t. the deleted function.

--
Matthew

[Andrzej Krzemienski]

:) Now, this is really convincing. Thank you.