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?