Why class size increases when int64_t changes to int32_t

Question

In my first example I have two bitfields using int64_t. When I compile and get the size of the class I get 8.

class Test
{
    int64_t first : 40;
         


        

Answer 1

Standard says:

§ 9.6 bit-fields

Allocation of bit-fields within a class object is implementation-defined. Alignment of bit-fields is implementation-defined. [Note: Bit-fields straddle allocation units on some machines and not on others. Bit-fields are assigned right-to-left on some machines, left-to-right on others. — end note]

c++11 paper

So layout depends on compiler implementation, compilation flags, target arch and so on. Just checked several compilers and output mostly is 8 8:

#include 
#include 

class Test32
{
    int64_t first : 40;
    int32_t second : 24;
};

class Test64
{
    int64_t first : 40;
    int64_t second : 24;
};

int main()
{
    std::cout << sizeof(Test32) << " " << sizeof(Test64);
}