C++ function template argument deduction guide

Question

I have a function and a simple class

template <class T, class U>
int foo(T t, U u) {
   return t + u;
}

template <class A, class B>
class C : public A {
  ...
};

C<something, something> c1;
foo(c1, c1);

In the call of foo with an instance of C, I would like the template argument to be derived to A, the base class of C. When creating a deduction guide like below, it fails to compile with error: expected initializer.

template <typename U, typename T, typename V>
int foo(C<U,T>, V) -> int foo<U, V>(U, V);

I failed to find any advise for what would be the syntactically correct way of providing such a deduction guide. Thanks for any advise.

Functions can't have deduction guides. Sounds like you want a template template or a specialization. — NathanOliver, Jan 24 '20 at 20:24
Does `foo(c1, c1)` not do what you want? Note that `foo` is taking it's argument by value so you'd be copying a `C` into an `A` and thus presumably losing some of what make the subclass useful. Perhaps you want to change it to `int foo(T& t, U& u)` instead? — Oliver Dain, Jan 24 '20 at 20:33
Even if you could do this, you shouldn't. It would be extremely confusing to a reader of the code if `foo` had a special case where calling it with `C` objects would slice them into `A`'s. — Brian Bi, Jan 24 '20 at 20:37

C++ function template argument deduction guide

0 Answers0