Question

I have a testing struct definition as follows:

struct test{     int a, b, c;     bool d, e;     int f;     long g, h; }; 

And somewhere I use it this way:

test* t = new test;   // create the testing struct int* ptr = (int*) t; ptr[2] = 15;          // directly manipulate the third word cout << t->c;         // look if it really affected the third integer 

This works correctly on my Windows – it prints 15 as expected, but is it safe? Can I be really sure the variable is on the spot in memory I want it to be – expecially in case of such combined structs (for example f is on my compiler the fifth word, but it is a sixth variable)?

If not, is there any other way to manipulate struct members directly without actually having struct->member construct in the code?

Answer 1

It looks like you are asking two questions

Is it safe to treat &test as a 3 length int arrray?

It’s probably best to avoid this. This may be a defined action in the C++ standard but even if it is, it’s unlikely that everyone you work with will understand what you are doing here. I believe this is not supported if you read the standard because of the potential to pad structs but I am not sure.

Is there a better way to access a member without it’s name?

Yes. Try using the offsetof macro/operator. This will provide the memory offset of a particular member within a structure and will allow you to correctly position a point to that member.

size_t offset = offsetof(mystruct,c); int* pointerToC = (int*)((char*)&someTest + offset); 

Another way though would be to just take the address of c directly

int* pointerToC = &(someTest->c);