Wouldn't that variable have to be large enough to contain all the enums. At first glance this looks like:
fusion::vector<Enum_1,Enum_2,...Enum_n>:
The idea is that four enums which each only can contain four values, can together only represent 256 ( 4^4) values which can be stored in a char. This off cause assume that their associated values are defined from 0 to 3. It also works for numbers which are not powers of 2: enum A{ a0 = 0, a1 = 1, a2 = 2, a3 = 3, a4 = 4 }; enum B{ b0 = 0, b1 = 1, b2 = 2, b3 = 3, b4 = 4 }; CompressedEnums< unsigned char, CompressedEnum<unsigned char, 1, A>, CompressedEnum<unsigned char, 5, B> > ENUMS; sizeof(ENUMS) == sizeof(unsigned char); I do not know boost::fusion very well, but I do not think this use full for this.