变异和C ++ lambda

Question

Trying to do functional programming in C++ and getting bit by some kind of mutation rule.

I have this code:

  bpf_u_int32 netp, maskp;
  struct in_addr addr;
  // ...code
  auto set_address_string = [&](auto &do_assign) {
    if ((do_assign = inet_ntoa(addr)) == NULL)
      cerr << "error - inet_ntoa() failed" << endl;
    else
      cout << "Some IP: " << do_assign << endl;
  };

  char *net, *mask;
  addr.s_addr = netp;
  set_address_string(net);
  addr.s_addr = maskp;
  set_address_string(mask);
  cout << string(net) << endl;

Which is printing the value of mask when I expected the contents of net to be printed, say that net is supposed to be "10.0.0.0" and mask is supposed to be "255.255.255.0".

I assume this has to do with the & I used for the variable do_assign?

EDIT:

Actual output is:

Some IP: 10.0.0.0
Some IP: 255.255.255.0
255.255.255.0

but expected was:

Some IP: 10.0.0.0
Some IP: 255.255.255.0
10.0.0.0

Answer 1

From the manual:

The inet_ntoa() function converts the Internet host address in, given in network byte order, to a string in IPv4 dotted-decimal notation. The string is returned in a statically allocated buffer, which subsequent calls will overwrite.

Both net and mask will point to the same buffer, and, of course, the buffer will contain the result of the last call.