std.reflection prototype

[bitwise via Digitalmars-d]

On Wed, 15 Apr 2015 15:48:28 -0400, Jacob Carlborg <doob at me.com> wrote:

> On 2015-04-15 17:26, bitwise wrote:
>
>> Right now, this is the def:
>>
>> /**
>>   * Array of pairs giving the offset and type information for each
>>   * member in an aggregate.
>>   */
>> struct OffsetTypeInfo
>> {
>>      size_t   offset;    /// Offset of member from start of object
>>      TypeInfo ti;        /// TypeInfo for this member
>> }
>>
>> If "string name" esd added, and then offTi[] esd actually populated,
>> then I suppose you could do this:
>>
>> class Test {
>>      int a = 4;
>>      private int b = 5;
>>      void print(){ writeln(b); }
>> }
>>
>> void main()
>> {
>>      Test test = new Test;
>>      // offsetof would instead come from the TypeInfo/OffsetTypeInfo
>>      int* b = cast(int*)(cast(void*)test + Test.b.offsetof);
>>      *b = 1234;
>>      test.print();
>> }
>>
>> But AFAIK, this is NOT ok in C++ because of the way inheritance works..
>> is this safe in D?
>
> I'm not sure, I would assume so. Why is this not safe in C++?
>

One reason is that casting with multiple inheritance offsets the pointer,  
and I forget exactly virtual inheritance works, but I'm sure it breaks  
things too..

#include <iostream>
using namespace std;

class A {
     public: int a;
};

class B {
     public: int b;
};

class C {
     public: int c;
};

class D : public A, public B, public C {
     public: int d;
};

int main(int argc, const char * argv[])
{
     D *d = new D;

     cout << (intptr_t)d << endl;
     cout << (intptr_t)(C*)d << endl;
     cout << (intptr_t)(B*)d << endl;
     cout << (intptr_t)(A*)d << endl;
     return 0;
}

possible output:

1098416
1098424
1098420
1098416