Question

I had a weird behavior of a piece code today under windows

std::vector<std::string> getMACs() {
  std::vector<std::string> macs;
  for(/*something*/) {
    char buffer[100];
    sprintf_s(buffer, size, "get the mac address here");
    std::string s = "";
    s.append(buffer);
    printf("mac=%s\n", s.c_str();  //print the mac address correctly
    macs.push_back(s);
  }
  return macs;
}

int main(int, char**) {
  std::vector<std::string> macs = getMACs();
  for (size_t i = 0; i < mac.size(); i++) {
    printf("mac=%s\n", macs[i]);   //prints garbage
  }
}

although the mac address inside the function has been printed correctly, in the main it prints garbage, the only explanation I have, that the macs vector is full of garbage strings, but how can this happen; a call to string.append(const char*), though passes by reference, the push_back() function should call the copy constructor of string, and so it should not point any more to a string reference that will turn into garbage after leaving the scope, right?

Answer 1

printf("mac=%s\n", macs[i]);   //prints garbage

Because macs[i] is of type std::string, and printf doesn’t know how to handle that. Try this:

printf("mac=%s\n", macs[i].c_str());

Or this:

std::cout << "mac=" << macs[i] << '\n';

Type safety, FTW