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digitalmars.D.learn - Changing the class data underneath some reference

reply David Colson <david peripherallabs.com> writes:
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.
Nov 29 2017
next sibling parent David Colson <david peripherallabs.com> writes:
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
}
Nov 29 2017
prev sibling next sibling parent reply codephantom <me noyb.com> writes:
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

}
Nov 29 2017
parent codephantom <me noyb.com> writes:
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;
Nov 29 2017
prev sibling next sibling parent reply Steven Schveighoffer <schveiguy yahoo.com> writes:
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
Nov 29 2017
parent reply Jonathan M Davis <newsgroup.d jmdavisprog.com> writes:
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

function; either way, there's nothing built-in or standard for it.

- Jonathan M Davis
Nov 29 2017
parent Steven Schveighoffer <schveiguy yahoo.com> writes:
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
Nov 29 2017
prev sibling parent A Guy With a Question <aguywithanquestion gmail.com> writes:
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.
Nov 29 2017