size of a datatype without using sizeof

Question

I have a data type, say X, and I want to know its size without declaring a variable or pointer of that type and of course without using sizeof operator.

Is this possible? I thought of using standard header files which contain size and range of data types but that doesn't work with user defined data type.

Answer 1

To my mind, this fits into the category of "how do I add two ints without using ++, += or + ?". It's a waste of time. You can try and avoid the monsters of undefined behaviour by doing something like this.

size_t size = (size_t)(1 + ((X*)0)); 

Note that I don't declare a variable of type or pointer to X.

Answer 2

Look, sizeof is the language facility for this. The only one, so it is the only portable way to achieve this.

For some special cases you could generate un-portable code that used some other heuristic to understand the size of particular objects[*] (probably by making them keep track of their own size), but you'd have to do all the bookkeeping yourself.

[*] Objects in a very general sense rather than the OOP sense.