Can I create an instance of A without calling a constructor? (see below) Use case: for generic deserialiaztion, when the deserialization library encounters a class without default constructor for example (it knows what the fields should be set to, but doesn't know how to construct the object).

class A{
  int x=2;
  this(int x){
    this.x=x;
  }
}


This came up here:
https://github.com/msgpack/msgpack-d/issues/54#issuecomment-104136148
I provide some hacky solution for that in that thread but I suspect it's
not safe and something is missing.

On Thursday, 21 May 2015 at 09:06:59 UTC, Timothee Cour wrote:
> Can I create an instance of A without calling a constructor? (see below)
> Use case: for generic deserialiaztion, when the deserialization library
> encounters a class without default constructor for example (it knows what
> the fields should be set to, but doesn't know how to construct the object).
>
> class A{
>   int x=2;
>   this(int x){
>     this.x=x;
>   }
> }
>
>
> This came up here:
> https://github.com/msgpack/msgpack-d/issues/54#issuecomment-104136148
> I provide some hacky solution for that in that thread but I suspect it's
> not safe and something is missing.

For a start I'm pretty sure you want to be calling core.memory.GC.malloc not core.stdc.stdlib.malloc, otherwise you leak the memory.

On Thursday, 21 May 2015 at 09:06:59 UTC, Timothee Cour wrote:
> Can I create an instance of A without calling a constructor? (see below)
> Use case: for generic deserialiaztion, when the deserialization library
> encounters a class without default constructor for example (it knows what
> the fields should be set to, but doesn't know how to construct the object).
>
> class A{
>   int x=2;
>   this(int x){
>     this.x=x;
>   }
> }
>
>
> This came up here:
> https://github.com/msgpack/msgpack-d/issues/54#issuecomment-104136148
> I provide some hacky solution for that in that thread but I suspect it's
> not safe and something is missing.

based on this:

https://github.com/D-Programming-Language/druntime/blob/6698ee21d4eb00ec2e8c621993359d235618df75/src/rt/lifetime.d#L71

you can create an instance without calling the constructor like this:

---
CT construct(CT, A...)(A a)
if (is(CT == class))
{
    auto memory = malloc(typeid(CT).init.length);
    memory[0 .. typeid(CT).init.length] = typeid(CT).init[];
    return cast(CT) memory;
}
---

actually it only copies the fields with their initial values.

On 2015-05-21 11:06, Timothee Cour via Digitalmars-d-learn wrote:
> Can I create an instance of A without calling a constructor? (see below)
> Use case: for generic deserialiaztion, when the deserialization library
> encounters a class without default constructor for example (it knows
> what the fields should be set to, but doesn't know how to construct the
> object).
>
> class A{
>    int x=2;
>    this(int x){
>      this.x=x;
>    }
> }
>
>
> This came up here:
> https://github.com/msgpack/msgpack-d/issues/54#issuecomment-104136148
> I provide some hacky solution for that in that thread but I suspect it's
> not safe and something is missing.

Here's how I do it in my serialization library Orange [1]

[1] https://github.com/jacob-carlborg/orange/blob/master/orange/util/Reflection.d#L166

-- 
/Jacob Carlborg

On Thursday, 21 May 2015 at 19:06:35 UTC, Jacob Carlborg wrote:
> On 2015-05-21 11:06, Timothee Cour via Digitalmars-d-learn wrote:
>> Can I create an instance of A without calling a constructor? (see below)
>> Use case: for generic deserialiaztion, when the deserialization library
>> encounters a class without default constructor for example (it knows
>> what the fields should be set to, but doesn't know how to construct the
>> object).
>>
>> class A{
>>   int x=2;
>>   this(int x){
>>     this.x=x;
>>   }
>> }
>>
>>
>> This came up here:
>> https://github.com/msgpack/msgpack-d/issues/54#issuecomment-104136148
>> I provide some hacky solution for that in that thread but I suspect it's
>> not safe and something is missing.
>
> Here's how I do it in my serialization library Orange [1]
>
> [1] https://github.com/jacob-carlborg/orange/blob/master/orange/util/Reflection.d#L166

Thanks!