Question

The short version is: How do I learn the size (in bits) of an individual field of a c++ field?

To clarify, an example of the field I am talking about:

struct Test {     unsigned field1 : 4;  // takes up 4 bits     unsigned field2 : 8;  // 8 bits     unsigned field3 : 1;  // 1 bit     unsigned field4 : 3;  // 3 bits     unsigned field5 : 16; // 16 more to make it a 32 bit struct      int normal_member; // normal struct variable member, 4 bytes on my system };  Test t; t.field1 = 1; t.field2 = 5; // etc. 

To get the size of the entire Test object is easy, we just say

sizeof(Test); // returns 8, for 8 bytes total size 

We can get a normal struct member through

sizeof(((Test*)0)->normal_member); // returns 4 (on my system) 

I would like to know how to get the size of an individual field, say Test::field4. The above example for a normal struct member does not work. Any ideas? Or does someone know a reason why it cannot work? I am fairly convinced that sizeof will not be of help since it only returns size in bytes, but if anyone knows otherwise I’m all ears.

Thanks!

Answer 1

You can calculate the size at run time, fwiw, e.g.:

//instantiate Test t; //fill all bits in the field t.field1 = ~0; //extract to unsigned integer unsigned int i = t.field1; ... TODO use contents of i to calculate the bit-width of the field ...