Question

Does C++ say anything on bit-ordering? I’m especially working on protocol packet layouts, and I’m doubting whether there is a portable way to specify that a certain number be written into bits 5,6,7, where bit 5 is the ‘most significant’.

My questions:

• is 0x01 always represented as a byte with bit 7 set?
• is bitset<8>().set(7).to_ulong() always equal to 1?

Answer 1

From 20.5/3 (ISO/IEC 14882:2011)

When converting between an object
of class bitset and a value of some integral type, bit position pos corresponds to the bit value 1 << pos.

That is, bitset<8>().set(7).to_ulong() is guaranteed to be (1 << 7) == 128.