Weird struct stuff

[asdf via Digitalmars-d-learn]

import std.stdio : writeln;

struct foo
{
    long* bar;
    this (long l)
    {
        long d = l;
        bar = &d;
    }
}

int main()
{
    foo f = foo(12345);
    writeln(*f.bar);
    //writefoo(f);
    writeln(*f.bar);

    return 0;
}

void writefoo(foo f)
{
    writeln(*f.bar);
}


If I compile this with dmd, I get the obvious output
12345
12345

however, if I uncomment the line writefoo(f);, I get
12345
140724376879320
140724376879320

(the second number changes each time, and appears to be a memory 
address)
Stranger still, if I change the constructor to
this (long l)
{
    writeln("wtf?");
    long d = l;
    bar = &d;
}

I get
wtf?
12345
12289
4422212

The second and third numbers don't change

Why does this happen? Were these functional programming guys 
right about impure functions having side effects after all?