Why do the sizes of data types change as the Operating System changes?

Question

This question was asked to me in an interview, that size of char is 2 bytes in some OS, but in some operating system it is 4 bytes or different.

Why is that so?

Why is it different from other fundamental types, such as int?

Answer 1

That was probably a trick question. The sizeof(char) is always 1.

If the size differs, it's probably because of a non-conforming compiler, in which case the question should be about the compiler itself, not about the C or C++ language.

5.3.3 Sizeof [expr.sizeof]

1 The sizeof operator yields the number of bytes in the object representation of its operand. The operand is either an expression, which is not evaluated, or a parenthesized type-id. The sizeof operator shall not be applied to an expression that has function or incomplete type, or to an enumeration type before all its enumerators have been declared, or to the parenthesized name of such types, or to an lvalue that designates a bit-field. sizeof(char), sizeof(signed char) and sizeof(unsigned char) are 1. The result of sizeof applied to any other fundamental type (3.9.1) is implementation-defined. (emphasis mine)

The sizeof of other types than the ones pointed out are implementation-defined, and they vary for various reasons. An int has better range if it's represented in 64 bits instead of 32, but it's also more efficient as 32 bits on a 32-bit architecture.