How to make a generic function to take a class or struct by reference?

[JN via Digitalmars-d-learn]

I would like to have only one definition of getX if possible, 
because they both are doing the same thing. I can't remove the 
ref one, because without a ref it will pass the struct as a 
temporary and compiler won't like that.

```d
import std.stdio;
struct Foo
{
    int x;

    void doStuff()
    {
        *getX(this) = 5;
    }
}

class Bar
{
    int x;

    void doStuff()
    {
        *getX(this) = 5;
    }
}

int* getX(T)(ref T t)
{
    return &t.x;
}

int* getX(T)(T t)
{
    return &t.x;
}

void main()
{
    Foo foo;
    Bar bar = new Bar();

    foo.doStuff();
    bar.doStuff();

    assert(foo.x == 5);
    assert(bar.x == 5);
}
```