Problem with insertBack

[John Nixon via Digitalmars-d-learn]

On Friday, 3 June 2016 at 15:03:45 UTC, ag0aep6g wrote:
> On 06/03/2016 04:34 PM, John Nixon wrote:
>> import std.stdio;
>> import std.container;
>> struct CS{
>>    char[] t;
>>    CS dup()const{
>>      CS cs;
>>      cs.t = this.t.dup;
>>      return cs;}
>> };
>
> Aside: No semicolon after struct declarations in D.
>
>> void main(){
>>    Array!CS cs_array = make!(Array!CS)();
>
> cs_array stores its data somewhere on the heap.
>
>>    CS cs;
>
> cs is on the stack.
>
>>    cs.t = "bb".dup;
>
> But cs.t's data is on the heap, like cs_array's. cs.t contains 
> a pointer to it.
>
>>    cs_array.insertBack(cs);
>
> This copies cs to cs_array's heap. The pointer in cs.t is being 
> copied, but the data that it points to is not being copied. So 
> cs.t and cs_array[0].t contain two distinct but equal pointers 
> now. Since they're equal, they refer to the same data.
>
>>    write("cs_array = 
>> ");foreach(i2;cs_array)write(i2);writeln("");
>>    cs.t[0] = ("a".dup)[0];//this changes cs_array!
>
> This is expected since cs.t and cs_array[0].t point to the same 
> location. A change to the data through one of them is visible 
> to the other.
>
>>    write("cs_array = 
>> ");foreach(i2;cs_array)write(i2);writeln("");
>>    cs.t = "c".dup;//but this does not
>
> Here you're not writing through the pointer in cs.t, but rather 
> you're assigning a whole new one. Since the pointers in cs.t 
> and cs_array[0].t are independent of each other, they now point 
> to different locations, with different data.
>
>>    write("cs_array = 
>> ");foreach(i2;cs_array)write(i2);writeln("");
>>    return;}

On Friday, 3 June 2016 at 15:03:45 UTC, ag0aep6g wrote:

Thanks very much to you and Steve for the detailed explanation. 
By the way I did try insertBack(cs.dup) but wasn't sure what I 
was doing. It seems that I just need to add all the .dup 's when 
needed to make my program work and find out a procedure to make 
sure I don't miss any.

John Nixon