www.digitalmars.com         C & C++   DMDScript  

digitalmars.D.learn - Bug with offsetof?

reply "Geancarlo" <geancarlo.rocha+d gmail.com> writes:
Hello, I'm using DMD32 D Compiler v2.060 for on Windows.


module main;

import std.stdio;

int main(string[] argv)
{
	writeln(TestStruct.x.offsetof);
	TestClass.test1();
	TestClass var = new TestClass();
	var.test2();
	return 0;
}

class TestClass
{
	static void test1()
	{
		writeln(TestStruct.x.offsetof);
	}

	void test2()
	{
		writeln(TestStruct.x.offsetof);//bug here
	}
}

struct TestStruct
{
	int x;
}

While test1 gives me no issues, test2 causes the following error:

Error: this for x needs to be type TestStruct not type 
main.TestClass		


Is this a known bug? How can I work around this issue in order to 
use offsetof from a class function that is not static?

Thanks
Nov 25 2012
next sibling parent "Geancarlo" <geancarlo.rocha+d gmail.com> writes:
This also works fine:

	void test3()
	{
		TestStruct dummy;
		writeln(dummy.x.offsetof);
	}
Nov 25 2012
prev sibling next sibling parent reply "jerro" <a a.com> writes:
This works for me if I add parentheses to the line where you get 
the error like this:

writeln(TestStruct().x.offsetof);//bug here

The error you were getting is not related to offsetof. The 
problem seems to be that if you write TestStruct.x inside a 
non-static method, the compiler thinks you are trying to get 
member TestStruct.x of the current instance. You obviously can't 
do that because the current instance is not a TestStruct. I've 
never used this feature, but it seems you can access members like 
this:

class Foo
{
     int x = 42;

     void test()
     {
         writeln(Foo.x); // prints 42
     }
}

Doing this seems pretty pointless, though. I assume the reason 
behind this is to allow you to access the members of a superclass 
that are named the same as current classes members, like this:

class Parent
{
     int x = 1;
}

class Child : Parent
{
     int x = 2;

     void test()
     {
         writeln(x);
         writeln(Parent.x);
     }
}

(new Child).test() prints:
2
1

When you add parentheses after TestStruct, you create an instance 
of TestStruct, and then you access its member x, so there is no 
ambiguity.
Nov 25 2012
parent Jacob Carlborg <doob me.com> writes:
On 2012-11-26 05:03, jerro wrote:

 This works for me if I add parentheses to the line where you get the
 error like this:

 writeln(TestStruct().x.offsetof);//bug here


This will create an instance of TestStruct.

-- 
/Jacob Carlborg
Nov 25 2012
prev sibling next sibling parent reply =?UTF-8?B?QWxpIMOHZWhyZWxp?= <acehreli yahoo.com> writes:
On 11/25/2012 07:23 PM, Geancarlo wrote:

 Hello, I'm using DMD32 D Compiler v2.060 for on Windows.


 module main;

 import std.stdio;

 int main(string[] argv)
 {
 writeln(TestStruct.x.offsetof);
 TestClass.test1();
 TestClass var = new TestClass();
 var.test2();
 return 0;
 }

 class TestClass
 {
 static void test1()
 {
 writeln(TestStruct.x.offsetof);
 }

 void test2()
 {
 writeln(TestStruct.x.offsetof);//bug here
 }
 }

 struct TestStruct
 {
 int x;
 }

 While test1 gives me no issues, test2 causes the following error:

 Error: this for x needs to be type TestStruct not type main.TestClass


 Is this a known bug? How can I work around this issue in order to use
 offsetof from a class function that is not static?

 Thanks


I don't know whether that is a bug but the class page at

   http://dlang.org/class.html

says ".offsetof can only be applied to expressions which produce the 
type of the field itself, not the class type".

Applying similar logic is a workaround for your case:

         writeln(TestStruct.init.x.offsetof);    // <-- works

As I said, I don't know whether it is a bug.

Ali
Nov 25 2012
parent reply "Geancarlo" <geancarlo.rocha+d gmail.com> writes:
Thanks jerro and Ali, I see your points. I thought offsetof was 
like C/C++'s sizeof... Guess while taking a crash course at a new 
language I will often bump into issues because I haven't read a 
specific doc.
Nov 25 2012
parent Jacob Carlborg <doob me.com> writes:
On 2012-11-26 05:49, Geancarlo wrote:

 Thanks jerro and Ali, I see your points. I thought offsetof was like
 C/C++'s sizeof... Guess while taking a crash course at a new language I
 will often bump into issues because I haven't read a specific doc.


You do have .sizeof in D as well.

-- 
/Jacob Carlborg
Nov 25 2012
prev sibling parent "Dan" <dbdavidson yahoo.com> writes:
On Monday, 26 November 2012 at 03:23:42 UTC, Geancarlo wrote:

 Hello, I'm using DMD32 D Compiler v2.060 for on Windows.



 		writeln(TestStruct.x.offsetof);//bug here
 


This works without creating your own instance:
           writeln(TestStruct.init.x.offsetof);

Thanks
Dan
Nov 26 2012