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digitalmars.D.learn - compare struct with string member is wield;

reply Cheng Wei <rivercheng gmail.com> writes:
struct S {
  string str;
};

S g_s;

unittest {
    S s1;
    S s2;
    assert(s1 == s2);         // Success
    assert(g_s == s1);        // Failed
}

Is this expected? If so, may I know the reason? Thanks.
Oct 11 2011
parent reply Jonathan M Davis <jmdavisProg gmx.com> writes:
On Wednesday, October 12, 2011 05:16:40 Cheng Wei wrote:

 struct S {
   string str;
 };
 
 S g_s;
 
 unittest {
     S s1;
     S s2;
     assert(s1 == s2);         // Success
     assert(g_s == s1);        // Failed
 }
 
 Is this expected? If so, may I know the reason? Thanks.


It succeeds on my box (Linux 64) with the latest from git. I assume that 
you're using 2.055? Maybe it's a bug that was fixed since the release.

- Jonathan M Davis
Oct 11 2011
parent reply Cheng Wei <rivercheng gmail.com> writes:
Sorry. The previous is not the one causes the problem.
Try this:

struct S {
    string str = "Hello";   // Adding an initial value here.
};

S g_s;

unittest {
    S s1;
    S s2;
    assert(s1 == s2); // Success
    assert(g_s == s1); // Fail
    auto s3 = g_s;
    assert(s3 == g_s);; // Even this will fail.
}

It seems if the string is initialized with a default value, then this
does not work.
Oct 11 2011
parent reply Jonathan M Davis <jmdavisProg gmx.com> writes:
On Wednesday, October 12, 2011 06:07:38 Cheng Wei wrote:

 Sorry. The previous is not the one causes the problem.
 Try this:
 
 struct S {
     string str = "Hello";   // Adding an initial value here.
 };
 
 S g_s;
 
 unittest {
     S s1;
     S s2;
     assert(s1 == s2); // Success
     assert(g_s == s1); // Fail
     auto s3 = g_s;
     assert(s3 == g_s);; // Even this will fail.
 }
 
 It seems if the string is initialized with a default value, then this
 does not work.


Yeah. It's failing for me too. It's obviously a bug of some kind. I'd have to 
going digging through d.puremagic.com/issues to see whether anything like it 
has been reported though.

By the way, the semicolon at the end of the definition of S is unnecessary in 
D.

- Jonathan M Davis
Oct 11 2011
next sibling parent Cheng Wei <rivercheng gmail.com> writes:
Thanks. Removing the ';' after struct and class is really helpful. The
";" keeps trapping me in C++ coding. :)
Oct 11 2011
prev sibling parent Dmitry Olshansky <dmitry.olsh gmail.com> writes:
On 12.10.2011 10:14, Jonathan M Davis wrote:

 On Wednesday, October 12, 2011 06:07:38 Cheng Wei wrote:

 Sorry. The previous is not the one causes the problem.
 Try this:

 struct S {
      string str = "Hello";   // Adding an initial value here.
 };

 S g_s;

 unittest {
      S s1;
      S s2;
      assert(s1 == s2); // Success
      assert(g_s == s1); // Fail
      auto s3 = g_s;
      assert(s3 == g_s);; // Even this will fail.
 }

 It seems if the string is initialized with a default value, then this
 does not work.


 Yeah. It's failing for me too. It's obviously a bug of some kind. I'd have to
 going digging through d.puremagic.com/issues to see whether anything like it
 has been reported though.

 By the way, the semicolon at the end of the definition of S is unnecessary in
 D.

 - Jonathan M Davis


It is strange. The reason could be: by default for structs it does 
bitlevel memcmp style comparison. Now in this case it will compare 
pointer-length pairs directly, if somehow global var got different 
string (address-wise) then it won't compare equal to S.init no matter what.

So two things:
- print out .str.ptr  for all of you vars to see if that's the case
- define sane opEquals*:
	bool opEquals(in S s)const{
		return str == t.str;
	}

* I haven't followed the timeline of the changes to opEquals that 
ultimately made things much simpler so you may have to rewrite it as 
bool opEquals(const ref S s)const in dmd 2.055.

-- 
Dmitry Olshansky
Oct 12 2011