• Perl

sub fact {
    my ($class, $str) = @_;
    return $class->new($str) if $class->$_isa('Parent');
}

• Ruby

def fact(klass, str)
  klass.new(str) if klass.is_a?(Parent)
end

• C++

template <typename T,
          std::enable_if_t<
            std::is_base_of_v<Parent, T>, bool> = true>
T fact(const std::string& str) {
  return T(str);
}

• D

auto fact(T, A...)(A a)
if (is(T==class) && is(T: Parent))
{
    return new T(a);
}

• Go

type ParentFactory func(string) Parent

var fact ParentFactory = func(str string) Parent {
	return Parent{
		name: str,
	}
}

• JS

class Parent {
    constructor(str) {}
    fact(new_class, str) {
        if (new_class.prototype instanceof Parent) {
            return new new_class(str)
        }
    }
}

class Child extends Parent {}

• Pascal

type Parent = class
  constructor create(const str: string);
end;

type ClassOfParent = class of Parent;

function fact(ClassType: ClassOfParent; const str: string): Parent;
begin
  result := ClassType.Create(str);
end;

• Python

def fact(a_class, str_):
    if issubclass(a_class, Parent):
        return a_class(str_)

• Rust

fn fact<Parent: std::str::FromStr>(str: String, _: Parent) -> Parent where <Parent as FromStr>::Err: Debug 
{
    return str.parse::<Parent>().unwrap();
}