[DMD 2.064.2]

Hello,

I've been strugling with a solution before bothering you guys here (again). I have my own complex code but i made a VERY simple test case to reproduce my problem, here's the code:

private import std.stdio;

class A
{
	private const ubyte* data;
	private ubyte[] abc;
	
	this()
	{
		abc = [1,2,3,4];
		data = cast(const ubyte*)abc.ptr;
	}
	
	public void print()
	{
		for(size_t i = 0; i < 4; i++)
		{
			writefln("%d", data[i]);
		}
	}
}

class B
{
	private const ubyte* data;
	private ubyte[] abc;
	
	this()
	{
		data = cast(const ubyte*)abc.ptr;
	}
	
	public void foo()
	{
		abc = [1,2,3,4];
	}
	
	public void print()
	{
		for(size_t i = 0; i < 4; i++)
		{
			writefln("%d", data[i]);
		}
	}
}

void main()
{
	A a = new A();
	a.print(); // OK.
	
	B b = new B();
	b.print(); // Crash.
}

The thing is that i can not use the data that const variable "data" in class B is referencing because in my original code i get "Invalid Memory Operation", in the test case i get "access violation" but it doesn't matter the name of the exception, the only thing that matters is that the exception raises for the same reason. I understand that i should not modify the const data outside a constructor but by stricts reason i can't avoid that.

Do you guys have any other aproach to get away with this? (modify a const reference data and then manipulate that data). THANK YOU very much in advance. D2 learner by the way.

Duhhh! i got the pink avatar by default. That sucks. Pink is just not my color.

Easy problem in class B: data is null!

On Monday, 9 December 2013 at 02:53:01 UTC, Heinz wrote:
> class B
> {
> 	private const ubyte* data;
> 	private ubyte[] abc;
> 	
> 	this()
> 	{
> 		data = cast(const ubyte*)abc.ptr;
> 	}


Since abc isn't initialized in this constructor, abc.ptr is null. So data is null too.


> 	public void print()
> 	{
> 		for(size_t i = 0; i < 4; i++)
> 		{
> 			writefln("%d", data[i]);
> 		}
> 	}
> }


And since data is null, data[i] will be a memory err/segfault/access violation/whatever it is called.

On 12/08/2013 07:24 PM, Adam D. Ruppe wrote:
> Easy problem in class B: data is null!
>
> On Monday, 9 December 2013 at 02:53:01 UTC, Heinz wrote:
>> class B
>> {
>>     private const ubyte* data;
>>     private ubyte[] abc;
>>
>>     this()
>>     {
>>         data = cast(const ubyte*)abc.ptr;
>>     }
>
>
> Since abc isn't initialized in this constructor, abc.ptr is null. So
> data is null too.
>
>
>>     public void print()
>>     {
>>         for(size_t i = 0; i < 4; i++)
>>         {
>>             writefln("%d", data[i]);
>>         }
>>     }
>> }
>
>
> And since data is null, data[i] will be a memory err/segfault/access
> violation/whatever it is called.

Apparently the OP intended to set it in foo(). If the data is actually mutable and there really is no way other than going against the type system, foo() must be called at least once and can be implemented like this:

    public void foo()
    {
        abc = [1,2,3,4];
        cast(ubyte*)data = abc.ptr;
    }

// ...

    B b = new B();
    b.foo();
    b.print(); // now OK

Ali

> Apparently the OP intended to set it in foo(). If the data is actually mutable and there really is no way other than going against the type system, foo() must be called at least once and can be implemented like this:
>
>     public void foo()
>     {
>         abc = [1,2,3,4];
>         cast(ubyte*)data = abc.ptr;
>     }
>
> // ...
>
>     B b = new B();
>     b.foo();
>     b.print(); // now OK
>
> Ali

Wow, i didn't know i could cast out constness from an lvalue. This is what i needed. I'll try it in my code as soon as i can. Thanks.

> Wow, i didn't know i could cast out constness from an lvalue. This is what i needed. I'll try it in my code as soon as i can. Thanks.

Yep, it compiles and works! By the way, i forgot to call b.foo() in my example. Thanks for your help guys.