Question

I really can’t believe I couldn’t find a clear answer to this…

How do you free the memory allocated after a C++ class constructor throws an exception, in the case where it’s initialised using the new operator. E.g.:

class Blah
{
public:
  Blah()
  {
    throw "oops";
  }
};

void main()
{
  Blah* b = NULL;
  try
  {
    b = new Blah();
  }
  catch (...)
  {
    // What now?
  }
}

When I tried this out, b is NULL in the catch block (which makes sense).

When debugging, I noticed that the conrol enters the memory allocation routine BEFORE it hits the constructor.

This on the MSDN website seems to confirm this:

When new is used to allocate memory
for a C++ class object, the object’s
constructor is called after the memory
is allocated.

So, bearing in mind that the local variable b is never assigned (i.e. is NULL in the catch block) how do you delete the allocated memory?

It would also be nice to get a cross platform answer on this. i.e., what does the C++ spec say?

CLARIFICATION: I’m not talking about the case where the class has allocated memory itself in the c’tor and then throws. I appreciate that in those cases the d’tor won’t be called. I’m talking about the memory used to allocate THE object (Blah in my case).

Answer 1

You should refer to the similar questions here and here.
Basically if the constructor throws an exception you’re safe that the memory of the object itself is freed again. Although, if other memory has been claimed during the constructor, you’re on your own to have it freed before leaving the constructor with the exception.

For your question WHO deletes the memory the answer is the code behind the new-operator (which is generated by the compiler). If it recognizes an exception leaving the constructor it has to call all the destructors of the classes members (as those have already been constructed successfully prior calling the constructor code) and free their memory (could be done recursively together with destructor-calling, most probably by calling a proper delete on them) as well as free the memory allocated for this class itself. Then it has to rethrow the catched exception from the constructor to the caller of new.
Of course there may be more work which has to be done but I cannot pull out all the details from my head because they are up to each compiler’s implementation.