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

[user42 via Digitalmars-d-learn]

On Saturday, 12 March 2016 at 15:32:39 UTC, Mike Parker wrote:
On Saturday, 12 March 2016 at 14:02:31 UTC, user42 wrote:
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 ?


Const member functions functions are not allowed to mutate any 
member state at all. This includes the state of any object 
instances that are members. Since the write method of the File 
type is not declared as const, then you can not call it on a 
member of type File from inside a const member function. 
Logging, by its very definition, mutates state.

Move the File instance outside of the class and it works.

import std.stdio;
private File f;
static this() { f = stdout; }

class X
{
  void p(string s) const
  {
     f.writeln!(string)(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));
}

Thanks for your reply.
Unfortunately it only solves the problem for this particular 
snippet.

The most interesting part of your reply is this line:
Since the write method of the File type is not declared as 
const,

At a quick glance I suppose it's because of the locking in 
LockingTextWriter.

I think I will probably pass this stuff to a C implementation, or 
override toString non-const, since adding const to it started 
this const avalanche in the first place.

Anyways, thanks for your input and have a nice weekend.