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digitalmars.D.learn - Diffrent instance of class share static array member?

reply Brian Hsu <brianhsu.hsu gmail.com> writes:
Content-Type: text/plain

Hello, everybody

When I tried to program an knight problem, I use a loop to test every case, but
only the answer of first case is correct. After print some variable, I suspect
it is caused by static array member.

So I wrote a simplest code to test, it looks like following (full code is in
the attachment), I found that every instance seems share the same array even
another instance changed the elements in array.

class TestArray {
    int [] x = [0,0,0,0,0];
    void doSomething () { // Increase every element in array by 1}
}

void main ()
{
    for (int i = 0; i < 5; i++) {
        TestArray t = new TestArray();
        t.doSomething();
        // Print array.
    }
}

I tried gdc/dmd at Linux/Windows all have same result, so I wondered is this a
design rationale for memory space and efficiency?

Currently I use add an constructor and in the constructor do x = x.dup in order
to create an whole new dynamic array which only belong to specific instance, is
my method correctly?

--
Brian Hsu
Sep 28 2007
next sibling parent Regan Heath <regan netmail.co.nz> writes:
Brian Hsu wrote:
 Hello, everybody
 
 When I tried to program an knight problem, I use a loop to test every
 case, but only the answer of first case is correct. After print some
 variable, I suspect it is caused by static array member.
 
 So I wrote a simplest code to test, it looks like following (full
 code is in the attachment), I found that every instance seems share
 the same array even another instance changed the elements in array.
 
 class TestArray { int [] x = [0,0,0,0,0]; void doSomething () { //
 Increase every element in array by 1} }
 
 void main () { for (int i = 0; i < 5; i++) { TestArray t = new
 TestArray(); t.doSomething(); // Print array. } }
 
 I tried gdc/dmd at Linux/Windows all have same result, so I wondered
 is this a design rationale for memory space and efficiency?

Not sure. It appears the array literal: [0,0,0,0,0,0,0,0,0,0] creates a single array of int's in memory and the assignment: int[] x = [0,0,0,0,0,0,0,0,0,0]; causes the array reference 'x' to refer to that signle instance in all class instances. Whereas what you actually want/expect is for the assignment to create a new array in memory and copy the contents of the literal.
 Currently I use add an constructor and in the constructor do x =
 x.dup in order to create an whole new dynamic array which only belong
 to specific instance, is my method correctly?

It works. Regan
Sep 28 2007
prev sibling parent "Jarrett Billingsley" <kb3ctd2 yahoo.com> writes:
"Brian Hsu" <brianhsu.hsu gmail.com> wrote in message 
news:fdj5bv$pab$1 digitalmars.com...
 Currently I use add an constructor and in the constructor do x = x.dup in 
 order to create an whole new dynamic array which only belong to specific 
 instance, is my method correctly?

Yes. Regan hit the nail on the head in explaining it, and yes, this is the right way to do it.
Sep 28 2007