Why does an uint64_t needs more memory than 2 uint32_t's when used in a class? And how to prevent this?

As it was pointed out, this is due to padding.

To prevent this, you may use

#pragma pack(1)

class ... {

};
#pragma pack(pop)

It tells your compiler to align not to 8 bytes, but to one byte. The pop command switches it off (this is very important, since if you do that in the header and somebody includes your header, very weird errors may occur)