| |
 | Posted by Nicholas Wilson
in reply to Voitech |  Permalink Reply |
|
Nicholas Wilson 
Posted in reply to Voitech
| On Wednesday, 6 January 2016 at 11:39:44 UTC, Voitech wrote:
> Hello, i am new to D language and trying to learn it by coding.
> I compile my programs on Xubuntu 14.04 with DMD64 D Compiler v2.069.2.
> So i have a struct/union which contains two fields representing real and string values:
>
> public union Element
> {
> private real _value;
> private string _rawValue;
>
> @property{
> real value(){
> return _value;
> }
> void value(real value){
> _value=value;
> }
>
> }
> @property {
> ref string rawValue(){
> return _rawValue;
> }
> void rawValue(ref string value){
> _rawValue=value;
> }
>
> }
>
>
> public Element opBinary(string op)(Element other){
> Element newElement;
> real value=mixin("this.value "~op~"other.value");
> newElement.value=value;
> return newElement;
> }
>
> public bool opEquals(Element other){
> if(this.rawValue == null && other.rawValue == null){
> return this.opEquals(other.rawValue);
> }
> return this.opEquals(other.value);
> }
> public bool opEquals(real value){
> return this.value==value;
> }
> public bool opEquals(string rawValue){
> return this.rawValue==rawValue;
> }
>
> unittest{
> Element e1 = {4},e2 ={5};
> writeln("e1 is at address: ",&e1);
> writeln("e2 is at address: ",&e2);
> writeln("e1.value is at address: ",&e1._value);
> writeln("e2.value is at address: ",&e2._value);
> assert(e1+e2==9);
> }
>
> unittest{
> Element s1={_rawValue:"+"},s2,s3,s4;
> writeln("s1 is at address: ",&s1);
> writeln("s2 is at address: ",&s2);
> writeln("s3 is at address: ",&s3);
> writeln("s1.value is at address: ",&s1._value);
> writeln("s2.value is at address: ",&s2._value);
> writeln("s3.value is at address: ",&s3._value);
> writeln("s4.value is at address: ",&s4._value);
> writeln("s1.rawValue ",s1._rawValue);
> writeln("s1.value: ",s1._value);
> writeln("s2.value: ",s2._value);
> writeln("s3.value: ",s3._value);
> writeln("s4.value: ",s4._value );
> assert(s1!=s2);
> }
> }
>
> The unit test results are:
>
> e1 is at address: 7FFCEFBA7510
> e2 is at address: 7FFCEFBA7520
> e1.value is at address: 7FFCEFBA7510
> e2.value is at address: 7FFCEFBA7520
> s1 is at address: 7FFCEFBA7500
> s2 is at address: 7FFCEFBA7510
> s3 is at address: 7FFCEFBA7520
> s1.value is at address: 7FFCEFBA7500
> s2.value is at address: 7FFCEFBA7510
> s3.value is at address: 7FFCEFBA7520
> s4.value is at address: 7FFCEFBA7530
> s1.rawValue +
> s1.value: 0.125
> s2.value: 4
> s3.value: 5
> s4.value: 9
>
> Can anyone tell me why s2,s3,s4 have initialized _value field? Shouldn't it be collected by GC when first unit test (with e1,e2) finishes ? If not how to handle this behavior, to use union without preinitialized fields.
Probably because you are accessing uninitialised memory. the values 4,5,9 appear in the first unittest and are left on the stack. Unions ,unlike structs, do not initialise their fields because it does not make sense to do so because the memory aliases. This can be observed by looking at the size of element (16 bytes) which is less than the size of an array (on 64 bit systems) , 2*8 bytes plus size of a real (8 or 10 bytes (x86/64)).
other things unrelated to your question. Things are public by default so the 'public's are redundant. strings are arrays so (most of the time) don't need ref, as the elements are accessed through a pointer.
See also Algebraic for cases (most) when you want to know the type of the value of the union.
Comparisons to null should use 'is' expressions
Having _value and _rawvalue as private and then providing getters and setters is redundant. the same behaviour can be achieved by making the public. getters and setters are for either heaving only a getter or calling a function when a field is changed (e.g. updating a cache)
Nic
|
