struct A
{
     const (void *) p;
}

struct B
{
    A    a;
    this(void * _p)
    {
        a.p = _p;
    }
}

I cannot change the definition of A
how do I initialise b.a.p?

On Friday, 26 February 2016 at 02:32:44 UTC, Nicholas Wilson wrote:
> struct A
> {
>      const (void *) p;
> }
>
> struct B
> {
>     A    a;
>     this(void * _p)
>     {
>         a.p = _p;
>     }
> }
>
> I cannot change the definition of A
> how do I initialise b.a.p?

As you did:

    void main() {
        int i = 42;
        B b = B(&i);
        int* p = cast(int*)b.a.p;
        assert(*p == 42);
    }

On Friday, 26 February 2016 at 02:48:35 UTC, cym13 wrote:
> On Friday, 26 February 2016 at 02:32:44 UTC, Nicholas Wilson wrote:
>> struct A
>> {
>>      const (void *) p;
>> }
>>
>> struct B
>> {
>>     A    a;
>>     this(void * _p)
>>     {
>>         a.p = _p;
>>     }
>> }
>>
>> I cannot change the definition of A
>> how do I initialise b.a.p?
>
> As you did:
>
>     void main() {
>         int i = 42;
>         B b = B(&i);
>         int* p = cast(int*)b.a.p;
>         assert(*p == 42);
>     }

>         a.p = _p;
fails to compile
fixed by
a = typeof(a)(_p);

On Friday, 26 February 2016 at 02:32:44 UTC, Nicholas Wilson wrote:
> struct A
> {
>      const (void *) p;
> }
>
> struct B
> {
>     A    a;
>     this(void * _p)
>     {
>         a.p = _p;
>     }
> }
>
> I cannot change the definition of A
> how do I initialise b.a.p?

Use a constructor for A instead of trying to write to one specific member:

a = A(_p);

const variable must be initialized in constructors. Structs have automatically defined constructors that take all the member variables so even if it isn't explicitly written, you can still do this.

On Friday, 26 February 2016 at 03:18:02 UTC, Nicholas Wilson wrote:
> On Friday, 26 February 2016 at 02:48:35 UTC, cym13 wrote:
>> On Friday, 26 February 2016 at 02:32:44 UTC, Nicholas Wilson wrote:
>>> struct A
>>> {
>>>      const (void *) p;
>>> }
>>>
>>> struct B
>>> {
>>>     A    a;
>>>     this(void * _p)
>>>     {
>>>         a.p = _p;
>>>     }
>>> }
>>>
>>> I cannot change the definition of A
>>> how do I initialise b.a.p?
>>
>> As you did:
>>
>>     void main() {
>>         int i = 42;
>>         B b = B(&i);
>>         int* p = cast(int*)b.a.p;
>>         assert(*p == 42);
>>     }
>
>>         a.p = _p;
> fails to compile
> fixed by
> a = typeof(a)(_p);

What compiler and version are you using ? Because I literally just copied your example and added my main and got no error.