Olivier Langlois's blog

For those who do not know what RAII is, you are probably using it without knowing it. One common use of this C++ idiom is to keep code manipulating mutexes exception safe. While I was writing a thread safe getter with a scoped lock like this:

A B::getA() const
{
  ScopedLock sc(m_lock);
  return m_a;
}

A doubt sparkled in my mind about whether or not the object copy performed by the return statement was protected by the lock. It must be since I have seen tons of functions using similar pattern but I could not explain why. I asked to another senior developer and he could not tell neither. By the way, this is what I love about the C++ language. You can have years of experience with it, there are millions of small details and any day, you may stumble on a mind boggling detail that force you to stop and think about it.

So if we come back to the original question, I guess that I could have checked what the standard document says about it but it is boring to do so. I prefer to do small scientific experiments to figure out myself what is the answer. So I wrote this small program:

#include <iostream>

class RAIIObj
{
  public:
  RAIIObj() { std::cout << "RAIIObj()\n"; }
  ~RAIIObj() { std::cout << "~RAIIObj()\n"; }
};

class A
{
  public:
  A() {}
  A( const A & ) { std::cout << "A( const A & )\n"; }
};

class B
{
  public:
  B() {}
  A getSyncAByVal() const;
  private:
  A m_a;
};

A B::getSyncAByVal() const
{
  RAIIObj scopedLock;
  return m_a;
}

int main( int argc, char *argv[] )
{
  B b;
  A retVal = b.getSyncAByVal();
  return 0;
}

and here is the output confirming that the getter code is correct:

RAIIObj()
A( const A & )
~RAIIObj()

Now that I know the answer, I have found another proof confirming that return statement object copies are performed before the local lock goes out of scope. If you were returning by value a local object, it absolutely must be copied before going out of scope.