Hello all!

I'm getting settled into D and I came into a problem. A code sample shows it best:

class SomeType
{
    string text;
    this(string input) {text = input;}
}


void main()
{
    SomeType foo = new SomeType("Hello");

    SomeType bar = foo;

    foo = new SomeType("World");

    writeln(bar.text); // Prints hello
    // I'd like it to print World
}

In the C++ world I could do this using pointers and changing the data underneath a given pointer, but I can't use pointers in D, so I'm not sure how I can get this behaviour?

I'd be open to other ways of achieving the same affect in D, using more D like methods.

On Thursday, 30 November 2017 at 00:40:51 UTC, David Colson wrote:
> Hello all!
>
> I'm getting settled into D and I came into a problem. A code sample shows it best:
>
> class SomeType
> {
>     string text;
>     this(string input) {text = input;}
> }
>
>
> void main()
> {
>     SomeType foo = new SomeType("Hello");
>
>     SomeType bar = foo;
>
>     foo = new SomeType("World");
>
>     writeln(bar.text); // Prints hello
>     // I'd like it to print World
> }
>
> In the C++ world I could do this using pointers and changing the data underneath a given pointer, but I can't use pointers in D, so I'm not sure how I can get this behaviour?
>
> I'd be open to other ways of achieving the same affect in D, using more D like methods.

I made an example demonstrating what I'd do in C++:

class SomeType
{
public:
    std::string text;
    SomeType(std::string input) {text = input;}
};


int main()
{
    SomeType foo = SomeType("Hello");

    SomeType* bar = &foo;

    foo = SomeType("World");

    std::cout << bar->text << "\n"; // Prints World
}

On Thursday, 30 November 2017 at 00:40:51 UTC, David Colson wrote:
> Hello all!
>
> I'm getting settled into D and I came into a problem. A code sample shows it best:
>
> class SomeType
> {
>     string text;
>     this(string input) {text = input;}
> }
>
>
> void main()
> {
>     SomeType foo = new SomeType("Hello");
>
>     SomeType bar = foo;
>
>     foo = new SomeType("World");
>
>     writeln(bar.text); // Prints hello
>     // I'd like it to print World
> }
>
> In the C++ world I could do this using pointers and changing the data underneath a given pointer, but I can't use pointers in D, so I'm not sure how I can get this behaviour?
>
> I'd be open to other ways of achieving the same affect in D, using more D like methods.


void main()
{
    SomeType foo = new SomeType("Hello");

    int * ptr;
    SomeType * bar;
    bar = &foo;

    foo = new SomeType("World");

    writeln(bar.text); // Prints World

}

On Thursday, 30 November 2017 at 00:52:25 UTC, codephantom wrote:
>...

sorry, don't know how the int * got in there ;-)

Anyway..who said you can't use pointers in D?

Just change:


//SomeType bar = foo;
SomeType * bar = &foo;

On 11/29/17 7:40 PM, David Colson wrote:
> Hello all!
> 
> I'm getting settled into D and I came into a problem. A code sample shows it best:
> 
> class SomeType
> {
>      string text;
>      this(string input) {text = input;}
> }
> 
> 
> void main()
> {
>      SomeType foo = new SomeType("Hello");
> 
>      SomeType bar = foo;
> 
>      foo = new SomeType("World");
> 
>      writeln(bar.text); // Prints hello
>      // I'd like it to print World
> }
> 
> In the C++ world I could do this using pointers and changing the data underneath a given pointer, but I can't use pointers in D, so I'm not sure how I can get this behaviour?
> 
> I'd be open to other ways of achieving the same affect in D, using more D like methods.

D does not support reassigning class data using assignment operator, only the class reference. You can change the fields individually if you need to.

e.g.:

foo.text = "World";

structs are value types in D and will behave similar to C++ classes/structs.

-Steve

On Thursday, 30 November 2017 at 00:40:51 UTC, David Colson wrote:
> Hello all!
>
> I'm getting settled into D and I came into a problem. A code sample shows it best:
>
> class SomeType
> {
>     string text;
>     this(string input) {text = input;}
> }
>
>
> void main()
> {
>     SomeType foo = new SomeType("Hello");
>
>     SomeType bar = foo;
>
>     foo = new SomeType("World");
>
>     writeln(bar.text); // Prints hello
>     // I'd like it to print World
> }
>
> In the C++ world I could do this using pointers and changing the data underneath a given pointer, but I can't use pointers in D, so I'm not sure how I can get this behaviour?
>
> I'd be open to other ways of achieving the same affect in D, using more D like methods.

You are dealing with a reference type. Reference types can be though of as a value type of an address. The new operator can be though of as giving the variable a new address. This means the foo and bar variables are not bound to the same value because their referencing different address. You need a struct. Which isn't a reference.

On Wednesday, November 29, 2017 21:12:58 Steven Schveighoffer via Digitalmars-d-learn wrote:
> On 11/29/17 7:40 PM, David Colson wrote:
> > Hello all!
> >
> > I'm getting settled into D and I came into a problem. A code sample shows it best:
> >
> > class SomeType
> > {
> >
> >      string text;
> >      this(string input) {text = input;}
> >
> > }
> >
> >
> > void main()
> > {
> >
> >      SomeType foo = new SomeType("Hello");
> >
> >      SomeType bar = foo;
> >
> >      foo = new SomeType("World");
> >
> >      writeln(bar.text); // Prints hello
> >      // I'd like it to print World
> >
> > }
> >
> > In the C++ world I could do this using pointers and changing the data underneath a given pointer, but I can't use pointers in D, so I'm not sure how I can get this behaviour?
> >
> > I'd be open to other ways of achieving the same affect in D, using more D like methods.
>
> D does not support reassigning class data using assignment operator, only the class reference. You can change the fields individually if you need to.
>
> e.g.:
>
> foo.text = "World";
>
> structs are value types in D and will behave similar to C++ classes/structs.

With classes, you could also assign the entire state of the object similar to what you'd get with structs and opAssign, but you'd have to write a member function to do it. There's no reason that you couldn't do the equivalent of opAssign. It's just that there's no built-in operator for it. So, whatever member function you wrote for it would be non-standard. Heck, technically, we don't even have a standard way to clone a class object like C# or Java do. Some folks write a clone function and others write a dup function; either way, there's nothing built-in or standard for it.

- Jonathan M Davis

On 11/29/17 10:22 PM, Jonathan M Davis wrote:
> With classes, you could also assign the entire state of the object similar
> to what you'd get with structs and opAssign, but you'd have to write a
> member function to do it. There's no reason that you couldn't do the
> equivalent of opAssign. It's just that there's no built-in operator for it.
> So, whatever member function you wrote for it would be non-standard.
It's mostly a bad idea. The whole point of disallowing assignment is to prevent the slicing problem. It's also why opAssign isn't overloadable for classes assigning to classes in the same hierarchy.

-Steve