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

[Arthur Schwarz <home@slipbits.com>]

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.

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. This validation only 
becomes
possible during template instantiation, that is, when 
template_name<class_name>
is instantiated. To me the question then becomes does the standard require
that checking is not deferred until instantiation.

>> As a nit, the repeated instances of "../header/" in the error message is
>> an annoyance.
> That looks like a problem with symlinks or your build system, not gcc's fault.
>
>> Diagnostic message and code given below.
>>
>> thanks
>> art
>>
>> ../header/../header/../header/../header/Stack.h: In member function
>> ‘std::string Stack<T>::toString()’:
>> ../header/../header/../header/../header/Stack.h:184:46: error: expected
>> primary-expression before ‘.’ token
>>     184 |       str << setw(3) << s.size() << ": " << T.toString();
>>         |                                              ^
>>
>> # include <iomanip>
>> # include <sstream>
>>
>> using namespace std;
>> template <class T>
>> class Stack {
>>      string toString() {
>>         stringstream str;
>>         str << setw(3) << s.size() << ": " << T.toString();
>>         return str.str();
>>      };
>> };
>>