Is there any way to access QByteArray member which is member of a union

I'm using union that have 2 members. One's type is QByteArray, other's *double. I want to access QByteArray members after changing value of the double array.

I've tried this using with char * (size = 8) and double union members. There was no problem.

 union smt{
        QByteArray qbyte;             
        double *data;
        ~smt(){}    // needs to know which member is active, only possible in union-like class
    };

     smt x={"sometng"};
     double *result =...;  
     memcpy(x.data,result,sizeof (double)); // copy result to x.data

In this code I want to access the values of the QByteArray after changing the data array. Basically I want to get the string values corresponding the double values. If they are sharing the same memory space, theoretically it's possible, right?

However, after changing the data array, I got runtime error, that says the QByteArray "not accessible"

Am I missing something or doing smtg wrong? Please help me

Thank you, in advance