digitalmars.D.bugs - [Issue 5519] New: Saner struct equality
- d-bugmail puremagic.com (98/98) Feb 02 2011 http://d.puremagic.com/issues/show_bug.cgi?id=5519
- d-bugmail puremagic.com (35/73) Feb 03 2011 http://d.puremagic.com/issues/show_bug.cgi?id=5519
- d-bugmail puremagic.com (9/24) Feb 03 2011 http://d.puremagic.com/issues/show_bug.cgi?id=5519
- d-bugmail puremagic.com (12/12) Feb 03 2011 http://d.puremagic.com/issues/show_bug.cgi?id=5519
- d-bugmail puremagic.com (33/33) Feb 03 2011 http://d.puremagic.com/issues/show_bug.cgi?id=5519
- d-bugmail puremagic.com (6/6) Feb 04 2011 http://d.puremagic.com/issues/show_bug.cgi?id=5519
- d-bugmail puremagic.com (12/12) Feb 04 2011 http://d.puremagic.com/issues/show_bug.cgi?id=5519
http://d.puremagic.com/issues/show_bug.cgi?id=5519
Summary: Saner struct equality
Product: D
Version: D2
Platform: All
OS/Version: All
Status: NEW
Severity: enhancement
Priority: P2
Component: DMD
AssignedTo: nobody puremagic.com
ReportedBy: bearophile_hugs eml.cc
Performing a comparison between two structs is a very common operation. Often
structs contain strings and other things. Currently (DMD 2.051) the struct
equality ignores the contents of strings contained inside structs, and this
behaviour is unacceptably bug-prone in a language like D that's otherwise
oriented toward code safety. So in the following four programs I'd like the
assertions to pass.
----------------
struct Foo { string s; }
void main() {
string s1 = "he";
string s2 = "llo";
string s3 = "hel";
string s4 = "lo";
auto f1 = Foo(s1 ~ s2);
auto f2 = Foo(s3 ~ s4);
assert((s1 ~ s2) == (s3 ~ s4));
assert(f1 == f2); // this asserts
}
----------------
struct Foo { int[] a; }
void main() {
auto a1 = [1, 2];
auto a2 = [3, 4, 5];
auto a3 = [1, 2, 3];
auto a4 = [4, 5];
auto f1 = Foo(a1 ~ a2);
auto f2 = Foo(a3 ~ a4);
assert((a1 ~ a2) == (a3 ~ a4));
assert(f1 == f2); // this asserts
}
----------------
class Bar {
int x;
this(int x_) { x = x_; }
bool opEquals(Object o) {
return x == (cast(Bar)o).x;
}
}
struct Foo { Bar a; }
void main() {
auto f1 = Foo(new Bar(1));
auto f2 = Foo(new Bar(1));
assert(f1 == f2); // this asserts
}
----------------
struct Foo { int[int] aa; }
void main() {
auto f1 = Foo([1:0, 2:0]);
auto f2 = Foo([1:0, 2:0]);
assert(f1.aa == f2.aa);
assert(f1 == f2); // this asserts
}
----------------
The title of this enhancement request is "Saner struct equality" instead of
"Sane struct equality" because (it seems) D struct equality isn't
meant/expected to be fully correct. This example shows a struct comparison
problem this enhancement request doesn't cover:
import core.stdc.string: memset;
struct Foo { long l; byte b; }
void main() {
Foo f1 = Foo(10, 20);
Foo f2;
memset(&f1, 'X', Foo.sizeof);
f2.l = 10;
f2.b = 20;
assert(f1 == f2); // this asserts
}
----------------
Surprisingly this works (DMD 2.051):
struct Bar {
int x;
const bool opEquals(ref const(Bar) o) {
return x == o.x || x == -o.x;
}
}
struct Foo { Bar a; }
void main() {
auto f1 = Foo(Bar(1));
auto f2 = Foo(Bar(-1));
assert(f1 == f2); // this doesn't assert
}
----------------
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Feb 02 2011
d-bugmail puremagic.com
http://d.puremagic.com/issues/show_bug.cgi?id=5519
Denis Derman <denis.spir gmail.com> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |denis.spir gmail.com
---
Performing a comparison between two structs is a very common operation. Often
structs contain strings and other things. Currently (DMD 2.051) the struct
equality ignores the contents of strings contained inside structs, [...]
----------------
struct Foo { string s; }
void main() {
string s1 = "he";
string s2 = "llo";
string s3 = "hel";
string s4 = "lo";
auto f1 = Foo(s1 ~ s2);
auto f2 = Foo(s3 ~ s4);
assert((s1 ~ s2) == (s3 ~ s4));
assert(f1 == f2); // this asserts
}
----------------
This issue is partially masked by the fact sring /literals/ are interned in D
(or is it an implementation optimisation of dmd?). Very annoying in interaction
with the present issue: since string interning makes comparison of structs
holding string sometimes behave as expected, other cases have an increased risk
of being bug-prone. Below another example showing this (all asserts pass). Note
that idup does not help & restore correct behaviour observed in case of
literals:
struct S {string s;}
unittest {
// literals
string s01 = "hello"; string s02 = "hello";
assert ( S(s01) == S(s02) );
// concat
string s1 = "he"; string s2 = "llo";
string s3 = "hel"; string s4 = "lo";
assert ( S(s1 ~ s2) != S(s3 ~ s4) );
// slice
string s = "hello";
assert ( S(s[1..$-1]) != S("ell") );
// idup'ed
assert ( S(s[1..$-1].idup) != S("ell") );
s5 = s[1..$-1].idup;
assert ( S(s5) != S("ell") );
}
Denis
----------------
Surprisingly this works (DMD 2.051):
struct Bar {
int x;
const bool opEquals(ref const(Bar) o) {
return x == o.x || x == -o.x;
}
}
struct Foo { Bar a; }
void main() {
auto f1 = Foo(Bar(1));
auto f2 = Foo(Bar(-1));
assert(f1 == f2); // this doesn't assert
}
----------------
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---
Surprisingly this works (DMD 2.051):
struct Bar {
int x;
const bool opEquals(ref const(Bar) o) {
return x == o.x || x == -o.x;
}
}
struct Foo { Bar a; }
void main() {
auto f1 = Foo(Bar(1));
auto f2 = Foo(Bar(-1));
assert(f1 == f2); // this doesn't assert
}
You probably meant in your comment: "this doesn't throw" didn't you? The
assertion passes, meaning the code behaves with expected semantics.
Denis
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Feb 03 2011
http://d.puremagic.com/issues/show_bug.cgi?id=5519 --- The core issue, I guess, is that '==' implicitely means comparison "by value equality". This sense is even more obvious in D which has a dedicated operator 'is' for comparison "by ref identity". Current behaviour comparing structs using by ref for their string & array members may not be wrong in itself, but it conflicts with expected semantics of '=='. And the case of (interned) string literals makes it inconsistent. Denis -- Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: -------
Feb 03 2011
http://d.puremagic.com/issues/show_bug.cgi?id=5519
---
One (hopefully last) more point:
A situation where one may constantly need to compare structs for equality (by
value!) is unittests:
unittest {
...
assert (result == S(a, b));
}
This is actually how I stepped on the present issue.
The general workaround is indeed to write custom opEquals methods which just
compare member per member. Thus, I ended up adding such opEquals to all
structs:
struct Lexeme {
string tag;
string slice;
Ordinal index; // for parser match error messages
string toString () {
return format(`Lexeme("%s","%s",%s)`, tag,slice,index);
}
// workaround for bug in default struct '=='
const bool opEquals (ref const(Lexeme) l) {
return (
this.tag == l.tag
&& this.slice == l.slice
&& this.index == l.index
);
}
}
Denis
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Feb 03 2011
http://d.puremagic.com/issues/show_bug.cgi?id=5519 This issue is essentially a dupe of bug 3789 -- Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: -------
Feb 04 2011
http://d.puremagic.com/issues/show_bug.cgi?id=5519
Simen Kjaeraas <simen.kjaras gmail.com> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|NEW |RESOLVED
CC| |simen.kjaras gmail.com
Resolution| |DUPLICATE
PST ---
*** This issue has been marked as a duplicate of issue 3789 ***
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Feb 04 2011