Why is it not possible to write to a file from a const member function ?

[user42 via Digitalmars-d-learn]

Hi

I have the following snippet to illustrate my problem/question:
class X
{
  import std.stdio: write, File, stdout;

  private File* f = &stdout;

  void p(string s) const
  {
     f.write(s);
  }
}

class Y
{
  private string s = "Y";

  override string toString() const
  {
     return s;
  }
}

void main()
{
  auto x = new X;
  auto y = new Y;

  import std.conv: to;
  x.p(to!string(y));
}

Why is this thing not compiling ?
Or, in other words, how is is possible to log something to a file 
from a const member function ?

Thanks in advance