Is memcopy extra data from std::string to std::array an undefined behavior?

Question

I don't mean to use this code in production environment, but I'm a little confuse about it.I thought it must be undefined,however,I ran this code without crash.Is this just conincidence?

#include <array>
#include <string>

auto main(int argc, char** argv) -> int {
  // for (int i = 0; i < 1000; ++i) {
  std::array<char, 1000> dst;
  std::string src = "hello world";
  memcpy(dst.data(), src.c_str(), dst.size()); // is this undefined behavior?
  // }
}

@StoryTeller-UnslanderMonica So I asked `Is this just conincidence?` — maidamai, May 25 '20 at 07:56
It is undefined behaviour, and undefined behaviour can mean a failure to crash. I'm not sure I'd describe that situation as a coincidence though. It just regular undefined behaviour. — john, May 25 '20 at 07:58
fyi Undefined Behaviour can cause Time Travel https://stackoverflow.com/questions/24527401/undefined-behavior-causing-time-travel — Richard Critten, May 25 '20 at 08:11
_Is it just coincidence?_ This question is generally unanswerable. It may be a coincidence, it may not. One would need to study the generated assembly, at least. At the assembly level, it's possible that nothing illegal happens. — Daniel Langr, May 25 '20 at 08:15

score 2 · Accepted Answer · answered May 25 '20 at 08:00

I thought it must be undefined

Yes, it is undefined behaviour. src.data points to an array of 12 chars (=12 bytes), but memcpy will attempt to read 1000 bytes from it, so it reads out of bounds.

however, I ran this code without crash

That's the point of undefined behaviour. Anything might happen. This includes running without any problems at all.

Is memcopy extra data from std::string to std::array an undefined behavior?

1 Answers1