segmentation fault when moving std::vector [closed]

Question

The following program crashes with segmention fault:

#include <iostream>
#include <vector>

using namespace std;

struct data
{
  data() : a(random()), b(random()), v({random(), random(), random()}) {}
  data(data&& m) noexcept : a(m.a), b(m.b), v(std::move(m.v)) { }

  long int a;
  long int b;
  std::vector<long int> v;
};

data&& randomize()
{
  srandom(time(0));
  data d;
  d.a = random();
  return std::move(d);
}

int main( int argc, char** argv )
{
  data d = randomize();
  cout << d.a << " " << d.b << endl;
  return 0;
}

The code is compiled with g++ version 4.7.2 (Debian 4.7.2-5):

g++ -std=c++11 -g test.cpp

What am I doing wrong? The problem seems to be in std::vector move constructor, cause everything works fine without it. It looks like data object from randomize() is destroyed when function finishes, but shouldn't it rather be moved to the data object in main first?

Answer 1

This function:

data&& randomize()
{
    // ...
    data d
    // ...
    return std::move(d);
}

Returns a reference to a local object which is going to be destroyed when the call returns. Therefore, your program has Undefined Behavior. Therefore, the returned reference will be dangling by the time the move constructor of data is invoked here:

data d = randomize();

You should return a value of type data, and you shouldn't explicitly invoke std::move():

data randomize()
{
    // ...
    data d
    // ...
    return d;
}

This way, you will also give the compiler the opportunity to performed (Named) Return Value Optimization, possibly resulting in no call to the move constructor at all.