www.digitalmars.com         C & C++   DMDScript  

digitalmars.D.learn - Why is it not possible to write to a file from a const member function

reply user42 <user42 invalid.email> writes:
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
Mar 12 2016
parent reply Mike Parker <aldacron gmail.com> writes:
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));
}
Mar 12 2016
parent user42 <user42 invalid.email> writes:
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.
Mar 12 2016