Question

I was debating with some colleagues about what happens when you throw an exception in a dynamically allocated class. I know that malloc gets called, and then the constructor of the class. The constructor never returns, so what happens to the malloc?

Consider the following example:

class B
{
public:
    B()
    {
        cout << "B::B()" << endl;
        throw "B::exception";
    }

    ~B()
    {
        cout << "B::~B()" << endl;          
    }
};

void main()
{
    B *o = 0;
    try
    {
        o = new B;
    }
    catch(const char *)
    {
        cout << "ouch!" << endl;
    }
}

What happens to the malloced memory o, does it leak? Does the CRT catch the exception of the constructor and deallocate the memory?

Cheers!
Rich

Answer 1

A call to

new B();

resolves in two things:

• allocating with an operator new() (either the global one or a class specific one, potentially a placement one with the syntax new (xxx) B())
• calling the constructor.

If the constructor throw, the corresponding operator delete is called. The case where the corresponding delete is a placement one is the only case where a placement delete operator is called without the syntax ::operator delete(). delete x; or delete[] x; don’t call the placement delete operators and there is no similar syntax to placement new to call them.

Note that while the destructor of B will not be called, already constructed subobjects (members or B and base classes of B) will be destructed before the call to operator delete. The constructor which isn’t called is the one for B.