Question

i’ve got a function declaration in c++ and need to know how it’s working:

 template<class x>
    int fun(x, x(*)(x*) );

The first arg is an object of type x. And how to describe the second one?

Answer 1

That is the declaration of a template function that returns an integer and takes, as parameters, an x and a pointer to a function that returns an x and takes, as a parameter, an x*.

The part x(*)(x*) is the part that means “a pointer to a function that returns an x and takes, as a parameter, an x*“. The first x is the return type, the (*) indicates that it is a pointer to a function (if the parameter had a name, it would be written x(*argname)(x*)), and the third x* is just the argument.

Calling it would look like this:

int f(int* iptr) { return something; }

fun(4, f); // no need for the explicit template parameter because it can be deduced

Or more generally

template<typename x>
x functionname(x* xptr) { return something; }

It will not work in C because, as Daniel White said in a comment, C doesn’t have templates.