Destructor order

Question

I have a C++ class hierarchy that looks something like this:

class A;

class B
{
public:
    B( A& a ) : a_( a )
    {
    };

private:
    A& a_;
};

class MyObject
{
public:
    MyObject() : b_( a_ )
    {
    };

private:
    A a_;
    B b_;
};

Occasionally, it will happen that in B's destructor I will get invalid access exceptions relating to its reference of A. It appears that A is destroyed before B.

Is there something inherently wrong with using class members to initialize other members? Is there no guarantee of the order of destruction?

Thanks, PaulH

Answer 1

EDIT: I missed the MyObject part of your question so my original answer will probably not be of much help. I guess your problem lies in the code you did not post, the stripped-down example should work fine.

---

Class B does not “own” the object passed by reference, therefore the objects a and b have different life cycles. If the object refered to by B::a_ is destroyed, B's destructor will access an invalid reference.

Some code to explain what I mean:

class A;

class B {
public:
    B(A a) : a_(a) {}  // a is copied to a_
    ~B() { /* Access a_ */ }
private:
    A a_;
};

class C {
public:
    C(A& a) : a_(a) {}  // a_ is a reference (implicit pointer)
                        // of an external object.
    ~C() { /* Access a_ */ }
private:
    A& a_;
};


int main(int argc, char** argv) {
    A* a = new A();

    B b(*a);
    C c(*a);

    delete a;
    // Now b has a valid copy of a, c has an invalid reference.
}