Cast structs with certain common members

Lets say I have 2 structs:

typedef struct
{
    uint8_t useThis;            
    uint8_t u8Byte2;             
    uint8_t u8Byte3; 
    uint8_t u8Byte4;  
} tstr1

and

typedef struct
{
    uint8_t u8Byte1;            
    uint8_t u8Byte2;             
    uint8_t useThis;  
} tstr2

I will only need the useThis member inside a function, but in some cases I will need to cast one struct or the other:

void someFunction()
{
    someStuff();
    SOMETHING MyInstance;
    if(someVariable)
    {
        MyInstance = reinterpret_cast<tstr1*>(INFO_FROM_HARDWARE); //This line of course doesn't work
    }
    else
    {
        MyInstance = reinterpret_cast<tstr2*>(INFO_FROM_HARDWARE); //This line of course doesn't work
    }
    MyInstance->useThis; //Calling this memeber with no problem

    moreStuff();
}

• So I want to use useThis no matter what cast was done. How can this be done?

• I want to avoid someFunction() to be template (just to avoid this kind of things)

• Note tha questions like this have a kind of similar problem but the struct members have the same order

EDIT:

In RealLife these structs are way larger and have several "same named" members. Directly casting a uint8_t as reinterpret_cast<tstr1*>(INFO_FROM_HARDWARE)->useThis would be tedious and require several reinterpret_casts (althought it's a working solution for my question before this EDIT). This is why I insist on MyInstance being "complete".