Would love to override default ctor of struct

[Meta]

On Tuesday, 22 January 2019 at 01:18:24 UTC, aliak wrote:
> On Monday, 21 January 2019 at 20:50:45 UTC, Meta wrote:
>> On Monday, 21 January 2019 at 19:08:49 UTC, Alex wrote:
>>> Could you give an example, where a zero argument construction 
>>> has to be done, which cannot be accomplished by setting the 
>>> appropriate field with a default value?
>>
>> For the following reason, although the default argument 
>> constructor hack no longer works:
>>
>> import std.stdio;
>>
>> struct Test(bool useFieldDefaultVal)
>> {
>>     static if (useFieldDefaultVal)
>>     {
>>         int[] arr = [1, 2, 3];
>>     }
>>     else
>>     {
>>         int[] arr;
>>
>>         this(int dummy = 0)
>>         {
>>             arr = [1, 2, 3];
>>         }
>>     }
>>
>>     void doTest()
>>     {
>>     	writeln("Address of arr: ", arr.ptr);
>>     }
>> }
>>
>> void main()
>> {
>>     Test!true t1;
>>     Test!true t2;
>>
>>     // Deprecation: constructor 
>> `onlineapp.Test!false.Test.this`
>>     // all parameters have default arguments, but structs 
>> cannot
>>     // have default constructors.
>>     auto t3 = Test!false();
>>
>>     t1.doTest(); // Prints "Address of arr: 5622DAA0F010"
>>     t2.doTest(); // Prints "Address of arr: 5622DAA0F010"
>>     t3.doTest(); // Prints "Address of arr: null"
>> }
>
> Ok, so this confused me a bit, I seem to remember that when you 
> static inisialize a class in the declaration scope of a struct, 
> the same thing would happen as with arrays, i.e. they'd have 
> the same address. But this turned out different:
>
> import std.stdio;
>
> class C {}
>
> struct Test(bool useFieldDefaultVal)
> {
>     static if (useFieldDefaultVal)
>     {
>         C c = new C;
>     }
>     else
>     {
>         C c;
>
>         this(int dummy = 0)
>         {
>             c = new C;
>         }
>     }
>
>     void doTest()
>     {
>     	writeln("Address of c: ", &c);
>     }
> }
>
> void main()
> {
>     Test!true t1;
>     Test!true t2;
>
>     // Deprecation: constructor `onlineapp.Test!false.Test.this`
>     // all parameters have default arguments, but structs cannot
>     // have default constructors.
>     auto t3 = Test!false();
>
>     t1.doTest(); // Prints "Address of arr: 7FFE1322A7E8"
>     t2.doTest(); // Prints "Address of arr: 7FFE1322A7F0"
>     t3.doTest(); // Prints "Address of arr: 7FFE1322A7F8"
> }
>
> t1 and t2 have different addresses and t3 has t1's address?? 
> Huh?
>
> Vat Khappened Khere?

You're printing the address of the object reference, not the 
object itself (similar to printing the address of a pointer 
instead of the actual address it points to). Do `cast(void*)c` 
instead.