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digitalmars.D.learn - Comparison of struct with Nullable member

reply "TC" <chalucha gmail.com> writes:
Hello,
I came around a strange behavior and I'm not sure if it is a bug 
or feature.

import std.typecons : Nullable;
struct Foo
{
	string bar;
	Nullable!int baz;
}

auto a = Foo("bb");
auto b = Foo("bb");
assert(a == b);

This ends up with: Called `get' on null Nullable!int

But if I change bar to be an int for example, than it passes ok.
I would expect this to pass ok.

Tested on dmd 2.067.1 win64
Jul 17 2015
next sibling parent "Nicholas Wilson" <iamthewilsonator hotmail.com> writes:
On Friday, 17 July 2015 at 12:18:56 UTC, TC wrote:

 Hello,
 I came around a strange behavior and I'm not sure if it is a 
 bug or feature.

 import std.typecons : Nullable;
 struct Foo
 {
 	string bar;
 	Nullable!int baz;
 }

 auto a = Foo("bb");
 auto b = Foo("bb");
 assert(a == b);

 This ends up with: Called `get' on null Nullable!int

 But if I change bar to be an int for example, than it passes ok.
 I would expect this to pass ok.

 Tested on dmd 2.067.1 win64


what is happening is the complier generated default opEquals is 
accessing the payload for Nullable!T without checking for null 
(which as you've written `Foo("bb")` this constructs a and b as 
`Foo("bb",(Nullable!int).init)` i.e. in the null state)

why nullables don't automatically check for null when comparing 
for equality idk.

you can make this work by defining your own opEquals that 
explicitly checks for nulls.
Jul 17 2015
prev sibling next sibling parent reply "Meta" <jared771 gmail.com> writes:
On Friday, 17 July 2015 at 12:18:56 UTC, TC wrote:

 Hello,
 I came around a strange behavior and I'm not sure if it is a 
 bug or feature.

 import std.typecons : Nullable;
 struct Foo
 {
 	string bar;
 	Nullable!int baz;
 }

 auto a = Foo("bb");
 auto b = Foo("bb");
 assert(a == b);

 This ends up with: Called `get' on null Nullable!int

 But if I change bar to be an int for example, than it passes ok.
 I would expect this to pass ok.

 Tested on dmd 2.067.1 win64


https://issues.dlang.org/show_bug.cgi?id=14804

I'll probably be able to submit a PR for this sometime in the 
next few days.
Jul 17 2015
parent reply "TC" <chalucha gmail.com> writes:
On Friday, 17 July 2015 at 15:30:42 UTC, Meta wrote:

 https://issues.dlang.org/show_bug.cgi?id=14804

 I'll probably be able to submit a PR for this sometime in the 
 next few days.


Thanks.
What I don't get is why this one works ok?

import std.typecons : Nullable;
struct Foo
{
	int bar;
	Nullable!int baz;
}
auto a = Foo(1);
auto b = Foo(1);
assert(a == b);

Also is there some "nicer" way to init the struct with nullable 
member with default constructor?
Now I'm using Foo(1, Nullable!int(2)) but just Foo(1, 2) would be 
much nicer.
Same with calling functions with nullable params.
Jul 17 2015
parent "Meta" <jared771 gmail.com> writes:
On Friday, 17 July 2015 at 15:52:45 UTC, TC wrote:

 On Friday, 17 July 2015 at 15:30:42 UTC, Meta wrote:

 https://issues.dlang.org/show_bug.cgi?id=14804

 I'll probably be able to submit a PR for this sometime in the 
 next few days.


 Thanks.
 What I don't get is why this one works ok?

 import std.typecons : Nullable;
 struct Foo
 {
 	int bar;
 	Nullable!int baz;
 }
 auto a = Foo(1);
 auto b = Foo(1);
 assert(a == b);


This may be because if your struct only contains primitive 
values, the compiler will just do a memcmp to determine equality. 
Don't quote me on that though.


 Also is there some "nicer" way to init the struct with nullable 
 member with default constructor?
 Now I'm using Foo(1, Nullable!int(2)) but just Foo(1, 2) would 
 be much nicer.
 Same with calling functions with nullable params.


You can defined a custom constructor in Foo which will construct 
the Nullable.

struct Foo
{
     this(int n1, int n2)
     {
         bar = n1;
         baz = n2;
     }

     //...
}
Jul 17 2015
prev sibling parent "anonymous" <anonymous example.com> writes:
On Friday, 17 July 2015 at 12:18:56 UTC, TC wrote:

 Hello,
 I came around a strange behavior and I'm not sure if it is a 
 bug or feature.

 import std.typecons : Nullable;
 struct Foo
 {
 	string bar;
 	Nullable!int baz;
 }

 auto a = Foo("bb");
 auto b = Foo("bb");
 assert(a == b);

 This ends up with: Called `get' on null Nullable!int

 But if I change bar to be an int for example, than it passes ok.
 I would expect this to pass ok.

 Tested on dmd 2.067.1 win64


Bug. I reported a reduced version as 
<https://issues.dlang.org/show_bug.cgi?id=14806>.
Jul 17 2015