struct String
{
	char[] data;
}

void main ()
{
	auto foo = String("foo");
	auto bar = String("foo".dup);

	assert(bar == foo);
}

Why isn't == used to compare the struct members in the code above? I mean, if I compare the structs with == it could also use == to compare the members. If I use "is" to compare the structs it could use "is" to compare them members.

Am Mon, 08 Feb 2010 12:19:12 +0100
schrieb Jacob Carlborg <doob@me.com>:

> struct String
> {
> 	char[] data;
> }
> 
> void main ()
> {
> 	auto foo = String("foo");
> 	auto bar = String("foo".dup);
> 
> 	assert(bar == foo);
> }
> 
> Why isn't == used to compare the struct members in the code above? I mean, if I compare the structs with == it could also use == to compare the members. If I use "is" to compare the structs it could use "is" to compare them members.

How should == be used for this struct?

struct Foo
{
	union
	{
		char[] dataA;
		long dataB;
	}
}

Foo a,b;

a.dataA = "abc";
b.dataA = "abc".dup;

Now a.dataA == b.dataA, but a.dataB != b.dataB.

> Why isn't == used to compare the struct members in the code above? I mean, if I compare the structs with == it could also use == to compare the members. If I use "is" to compare the structs it could use "is" to compare them members.

Structs are compared *bitwise*!
When you dup your pointer is different and thus the structs are different.

On 02/08/2010 01:48 PM, Trass3r wrote:
>> Why isn't == used to compare the struct members in the code above? I
>> mean, if I compare the structs with == it could also use == to compare
>> the members. If I use "is" to compare the structs it could use "is" to
>> compare them members.
>
> Structs are compared *bitwise*!
> When you dup your pointer is different and thus the structs are different.

I believe the question was *why* things are this way. I think it's weird.

> I believe the question was *why* things are this way. I think it's weird.
>

It's common behavior. In the end structs are just a way to map a memory range to some variable tuple.

If you really need == for all members you can always overload opEquals!!

On 2/8/10 14:58, Pelle Månsson wrote:
> On 02/08/2010 01:48 PM, Trass3r wrote:
>>> Why isn't == used to compare the struct members in the code above? I
>>> mean, if I compare the structs with == it could also use == to compare
>>> the members. If I use "is" to compare the structs it could use "is" to
>>> compare them members.
>>
>> Structs are compared *bitwise*!
>> When you dup your pointer is different and thus the structs are
>> different.
>
> I believe the question was *why* things are this way. I think it's weird.

Yes that was the question.

On 2/8/10 13:48, Trass3r wrote:
>> Why isn't == used to compare the struct members in the code above? I
>> mean, if I compare the structs with == it could also use == to compare
>> the members. If I use "is" to compare the structs it could use "is" to
>> compare them members.
>
> Structs are compared *bitwise*!
> When you dup your pointer is different and thus the structs are different.

Yes, I intentionally duped the string to make sure they're not using the same memory. I was thinking of something like a transitive ==.

> Yes, I intentionally duped the string to make sure they're not using the same memory. I was thinking of something like a transitive ==.

Overload opEquals.

Trass3r wrote:
>> Why isn't == used to compare the struct members in the code above? I mean, if I compare the structs with == it could also use == to compare the members. If I use "is" to compare the structs it could use "is" to compare them members.
> 
> Structs are compared *bitwise*!

Not in D2, any more. If a member has an opEquals, it's compared using ==.

Don wrote:
> Trass3r wrote:
>>> Why isn't == used to compare the struct members in the code above? I mean, if I compare the structs with == it could also use == to compare the members. If I use "is" to compare the structs it could use "is" to compare them members.
>>
>> Structs are compared *bitwise*!
> 
> Not in D2, any more. If a member has an opEquals, it's compared using ==.

Seems arrays inside structs still are not compared with ==.