• asdf (46/46) Mar 01 2016 import std.stdio : writeln;
• =?UTF-8?Q?Ali_=c3=87ehreli?= (5/30) Mar 01 2016 That's undefined behavior because f.bar is pointing at a dead object (d)...

asdf <k88a314 gmail.com> writes:

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?

=?UTF-8?Q?Ali_=c3=87ehreli?= <acehreli yahoo.com> writes:

On 03/01/2016 05:11 PM, asdf wrote:
 import std.stdio : writeln;

 struct foo
 {
      long* bar;

      this (long l)
      {
          long d = l;
          bar = &d;

Unfortunately, 'bar' is pointing at the temporary stack-based variable 'd'.

      }
 }

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

That's undefined behavior because f.bar is pointing at a dead object (d).

      //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

Yeah, undefined behavior sometimes produces "obvious" outputs. ;)

Ali