Question

I had a hard time debugging a crash on production. Just wanted to confirm with folks here about the semantics. We have a class like …

class Test {
public:
  Test()
  {
    // members initialized ...
    m_str = m_str;
  }
  ~Test() {}
private:
  // other members ...
  std::string m_str;
};

Someone changed the initialization to use ctor initialization-lists which is reasonably correct within our code semantics. The order of initialization and their initial value is correct among other things. So the class looks like …

class Test {
public:
  Test() 
    : /*other inits ,,, */ m_str(m_str)
  {
  }
  ~Test() {}
private:
  // other members ...
  std::string m_str;
};

But the code suddenly started crashing! I isolated the long list of inits to this piece of code m_str(m_str). I confirmed this via link text.

Does it have to crash? What does the standard say about this? (Is it undefined behavior?)

Answer 1

The first constructor is equivalent to

  Test()
  : m_str()
  {
    // members initialized ...
    m_str = m_str;
  }

that is, by the time you get to the assignment within the constructor, m_str has already been implicitly initialized to an empty string. So the assignment to self, although completely meaningless and superfluous, causes no problems (since std::string::operator=(), as any well written assignment operator should, checks for self assignment and does nothing in this case).

However, in the second constructor, you are trying to initialize m_str with itself in the initializer list – at which point it is not yet initialized. So the result is undefined behaviour.

Update: For primitive types, this is still undefined behaviour (resulting in a field with garbage value), but it does not crash (usually – see the comments below for exceptions) because primitive types by definition have no constructors, destructors and contain no pointers to other objects.

Same is true for any type that does not contain pointer members with ownership semantics. std::string is hereby demonstrated to be not one of these 🙂