Re: Can you use a function reference passed in a template argument?

[Arthur Schwarz <home@slipbits.com>]

Thanks. You have clearly explained both my errors and my inability to do 
what I want in the way I want. Being wise, I will change my mind, and if 
I really feel distraught, will attempt to do it another way.

On 11/9/2023 2:00 PM, Jonathan Wakely wrote:
> On Thu, 9 Nov 2023 at 21:58, Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
>> On Thu, 9 Nov 2023 at 21:24, Arthur Schwarz <home@slipbits.com> wrote:
>>>
>>> On 11/9/2023 1:11 PM, Jonathan Wakely via Gcc-help wrote:
>>>> On Thu, 9 Nov 2023 at 20:46, Arthur Schwarz <home@slipbits.com> wrote:
>>>>> Is there any way to use a function passed as an argument to a template
>>>>> (example below)? Couldn't the existence of the referenced function be
>>>>> established durint instantiation (Stack<some class> obj)? I realize that
>>>>> just doing analysis of the template that the existence of a referenced
>>>>> function can't be determined, but during instantiation it can be validated.
>>>> I have no idea what that code is trying to do. You're trying to call a
>>>> member function on a _type_ T.
>>> As if I knew what I was trying to do.
>>>> And you said you want to call a function passed as an argument ... but
>>>> the template argument you pass is a type, not a function.
>>>>
>>> At the time of object instantiation the 'type' is a class and as a
>>> member of this
>>> class there is a function. It is resolvable that when a class is used
>>> that to satisfy
>>> the requirement that the template object is correct, the class must
>>> contain the
>>> indicated function. If a passed class does not contain the indicated
>>> function,
>>> then an error can be generated.
>> No, you have a category error. You call a non-static member function
>> on an object, not on a type.
>>
>> You could write T().toString() which would create a temporary object
>> of type T, and call the function on _that_. But you can't call a
>> non-static member function on a type. You need an object.

Wouldn't work. The values to be returned by toString() are the state 
variables for
the current object. New object. New state variables. (But it is 
interesting).

>>
>>> It looks like the determination of template instantiability is made when
>>> the
>>> template is 'compiled', and at this time it is not possible to determine
>>> that
>>> the referenced function, T.fun(), is available.
>> T.fun() is not even valid C++ syntax, it's just nonsense. So the
>> problem has nothing to do with when the template is instantiated or
>> compiled. You're just writing something that isn't C++.
>
> To see that this is not about template instantiation, consider:
>
> struct S {
>    string toString() { return "S"; }
> };
>
> int main()
> {
>    std::cout << S.toString();
> }
>
> This is the same as what you tried to write, it uses a type S where an
> object is required. It's not valid C++.