Can not connect to an abstract unix socket in python
Your C++ doesn't do quite what you think it does. This line:
strncpy(addr.sun_path, UD_SOCKET_PATH, sizeof(addr.sun_path)-1);Copies a single null character '\0' into addr.sun_path. Note this line in the manpage for strncpy():
If the length of src is less than n, strncpy() writes additional null bytes to dest to ensure that a total of n bytes are written.
As a result your C++ actually connects to an abstract domain socket at "\0". Python does the right thing here and connects to an abstract domain socket at "\0hidden".