Passing a string by reference

[Alexander Zhirov via Digitalmars-d-learn]

Do I understand correctly that in order for me to pass a string 
when creating an object, I must pass it by value? And if I have a 
variable containing a string, can I pass it by reference?

Should I always do constructor overloading for a type and a 
reference to it?
In the case of the variable `c`, a drop occurs. Why? An object is 
not being created on the stack?

```d
import std.stdio : writeln;

class A
{
    private string str = "base";

    this(ref string str)
    {
        writeln("type reference string");
        this.str = str;
    }

    this(string str)
    {
        writeln("type string");
        this.str = str;
    }

    this() {}

    void print()
    {
        writeln(str);
    }
}

void main()
{
    auto a = new A("Hello, World!"); // this type string
    a.print();
    string text = "New string";
    auto b = new A(text); // this type reference string
    b.print();
    A c;
    c.print();  // segmentation fault! Why not "base"?
}

```