How to store a bit-array in C++?

The std::bitset will do, as long as your bit array is of fixed size.
As a side note there's also std::dynamic_bitset, but am not 100% sure it made it into the standard.

For vanilla C++, there's std::bitset.

Bitset is very similar to vector (also known as bit_vector): it contains a collection of bits, and provides constant-time access to each bit. There are two main differences between bitset and vector. First, the size of a bitset cannot be changed: bitset's template parameter N, which specifies the number of bits in the bitset, must be an integer constant. Second, bitset is not a Sequence; in fact, it is not an STL Container at all.

Matt Austern has a nice article on its use.

Also: If your byte array (bit array?) fits into an unsigned long, then you can assign it to a std::bitset directly:

unsigned long myByteArray = 0xABCD;
std::bitset<32> bitten( myByteArray );