Problem with dtor behavior

[SrMordred via Digitalmars-d-learn]

On Thursday, 27 July 2017 at 20:28:47 UTC, Moritz Maxeiner wrote:
> On Thursday, 27 July 2017 at 19:19:27 UTC, SrMordred wrote:
>> //D-CODE
>> struct MyStruct{
>>     int id;
>>     this(int id){
>>         writeln("ctor");
>>     }
>>     ~this(){
>>         writeln("dtor");
>>     }
>> }
>>
>> MyStruct* obj;
>> void push(T)(auto ref T value){
>>     obj[0] = value;
>> }
>>
>> void main()
>> {
>>     obj = cast(MyStruct*)malloc( MyStruct.sizeof );
>>     push(MyStruct(1));
>> }
>>
>> OUTPUT:
>> ctor
>> dtor
>> dtor
>>
>> I didnt expected to see two dtors in D (this destroy any 
>> attempt to free resources properly on the destructor).
>
> AFAICT it's because opAssign (`obj[0] = value` is an opAssign) 
> creates a temporary struct object (you can see it being 
> destroyed by printing the value of `cast(void*) &this` in the 
> destructor).
>
>> Can someone explain why is this happening and how to achieve 
>> the same behavior as c++?
>
> Use std.conv.emplace:
>
> ---
> import std.conv : emplace;
>
> void push(T)(auto ref T value){
> 	emplace(obj, value);
> }
> ---

It worked but isnt this odd?
like, if I change the push(MyStruct(1)) for obj[0] = MyStruct(1); 
(which is what I expected in case of compiler inlining for 
example) the behavior change:
OUTPUT:
ctor
dtor

I´m having the feeling that this "auto ref T" don´t have the same 
behavior that the "T&&" on c++.

I find this very strange because if i copy/paste/tweak code from 
c/c++ on D, and have some kind of malloc/free on the ctor/dtor 
the code will blow in my face without warning.