如何在重载运算符 = 中处理自赋值和异常时释放内存

Question

I am checking the Operator= overload in Scott Meyers

    Widget& Widget::operator=(const Widget& rhs)
    {
    Bitmap *pOrig = pb; // remember original pb
    pb = new Bitmap(*rhs.pb); // point pb to a copy of rhs’s bitmap
    delete pOrig; // delete the original pb
    return *this;
    }

Suppose the line pb = new Bitmap(*rhs.pb); // point pb to a copy of rhs's bitmap

gives some exception(may be due to lack of memory)

then i delete pOrig[It will free the memory holded by pOrig and will call Bitmap destructor] So after this,will pb not pointing to a deleted pointer?

Answer 1

No. If the new fails, the write to pb doesn't happend, because new throws instead of returning.

When you call a function, the function will return a value on a register.

So, you get something like:

call NewBitmap // From within here, you go directly to the catch clause if any
retval -> pb // <<-- this doesn't get executed
delete pOrig // <<-- this doesn't get executed either
*this -> retval // <<-- this either, you don't return, you rethrow as you don't have a catch

Think of this: you will only write to pb once you have a value. You'll have a value once NewBitmap returns a value. Will not return a value if it throws.

Anyway, this becomes sort of a rethorical question, because if new fails, it's not a good idea to continue the execution of the program.