Does sizeof(T) * CHAR_BIT guarantee bit size?

Question

There doesn't appear to be any library function for calculating the size of a type in bits.

Am I right to assume that this can be done in the following way?

#include <climits>

template <typename T>
size_t Size_In_Bits(){
    return sizeof(T) * CHAR_BIT;
}

Will this always give back the amount of bits that can be targeted on a type?

Answer 1

This is guaranteed to give you size (storage) in bits, but not the width (number of value bits). The latter could be less if the type has padding bits. For unsigned types there you can measure the number of value bits directly by converting -1 to the type (to get the max possible value in the type) and counting them. For signed types, std::numeric_limits<T>::max() can be used to get the max. Or, if you know the specific type already, you can use the xxx_MAX macros from limits.h or stdint.h.

Answer 2

sizeof(T) * CHAR_BIT returns the numbers of bits the type takes up in memory.

Yet the size of bits may be more than the bits the integer can be mathematically used - (consider padding bits).

---

Detail: integers have value bits, sign bit (signed integers) and possible padding bits. All these bits contribute to the storage size.

unsigned char will never have padding bits.