Hello!

I just started learning D and I come from a mostly C and C# background.
Hello World was working, so I figured that I should try something more complex: a template class and si I went with a simple stack.

Using dmd the code I wrote causes a segmentation fault and I am not sure why.

import std.stdio;
import std.container.slist;


class Stack(T)
{
    SList!(T) s;
    this() {}
    void push(T element)
    {
        s.insertFront(element);
    }
}

int main()
{
    Stack!(int) x;
    writeln("1");
    SList!(int) p;
    writeln("2");
    p.insertFront(42);
    writeln("3");
    x.push(42);
    writeln("4");
    return 0;
}


This program prints the following into the console:
% dmd test.d && ./test
1
2
3
zsh: segmentation fault  ./test


I do not understand why the normal SList works fine but the one inside the object doesn't. What am I overlooking?


Best regards,
Tim

Tim K. wrote:

>      Stack!(int) x;
x is not initialized.

`auto x= new Stack!(int);'
will do.

-manfred

On Saturday, June 06, 2015 09:44:36 Tim K. via Digitalmars-d-learn wrote:
> Hello!
>
> I just started learning D and I come from a mostly C and C#
> background.
> Hello World was working, so I figured that I should try something
> more complex: a template class and si I went with a simple stack.
>
> Using dmd the code I wrote causes a segmentation fault and I am not sure why.
>
> import std.stdio;
> import std.container.slist;
>
>
> class Stack(T)
> {
>      SList!(T) s;
>      this() {}
>      void push(T element)
>      {
>          s.insertFront(element);
>      }
> }
>
> int main()
> {
>      Stack!(int) x;
>      writeln("1");
>      SList!(int) p;
>      writeln("2");
>      p.insertFront(42);
>      writeln("3");
>      x.push(42);
>      writeln("4");
>      return 0;
> }
>
>
> This program prints the following into the console:
> % dmd test.d && ./test
> 1
> 2
> 3
> zsh: segmentation fault  ./test
>
>
> I do not understand why the normal SList works fine but the one inside the object doesn't. What am I overlooking?

>From the looks of it, you never initialized x, so it's null. So, the problem
has nothing to do with SList. It's the fact that you're trying to dereference a null reference. You need to call new to allocate a Stack!int, not just declare a reference to one.

void main()
{
    Stack!int x
    x.push(42);
}

would have exactly the same problem. You need something like

auto x = new Stack!int;

- Jonathan M Davis

On Saturday, 6 June 2015 at 10:10:15 UTC, Manfred Nowak wrote:
> x is not initialized.
>
> `auto x= new Stack!(int);'
> will do.

Thank you two.
But that leads me to another question: Why do I need to initialize x with a "new Stack" but I don't need to initialize p with a "new SList"?

Best regards,
Tim

On Saturday, 6 June 2015 at 10:16:12 UTC, Tim K. wrote:
> On Saturday, 6 June 2015 at 10:10:15 UTC, Manfred Nowak wrote:
>> x is not initialized.
>>
>> `auto x= new Stack!(int);'
>> will do.
>
> Thank you two.
> But that leads me to another question: Why do I need to initialize x with a "new Stack" but I don't need to initialize p with a "new SList"?

Because `SList` is a struct, which is a value type that is allocated where you declare it (here on the stack), while you used a class, which is a reference type and has to be allocated with `new`.

You can also turn your `Stack` type into a struct, then it works, too. (You just need to remove the constructor, which is empty anyway.)

On Saturday, 6 June 2015 at 10:20:34 UTC, Marc Schütz wrote:

> Because `SList` is a struct, which is a value type that is allocated where you declare it (here on the stack), while you used a class, which is a reference type and has to be allocated with `new`.
>
> You can also turn your `Stack` type into a struct, then it works, too. (You just need to remove the constructor, which is empty anyway.)

That makes sense. I overlooked that in SList's documentation and thought it was just a regular old class, too.

Best regards,
Tim

On Saturday, 6 June 2015 at 10:16:12 UTC, Tim K. wrote:
> On Saturday, 6 June 2015 at 10:10:15 UTC, Manfred Nowak wrote:
>> x is not initialized.
>>
>> `auto x= new Stack!(int);'
>> will do.
>
> Thank you two.
> But that leads me to another question: Why do I need to initialize x with a "new Stack" but I don't need to initialize p with a "new SList"?
>
> Best regards,
> Tim

Best way to construct a std.container is to use std.container.make.