On Thu, 9 Nov 2023 at 21:24, Arthur Schwarz <home@xxxxxxxxxxxx> 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@xxxxxxxxxxxx> 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. > > 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++. > 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(); > >> }; > >> }; > >>