CODEWITHSUNDEEP
c++exceptionconstructor
St.Antario
St.Antario

Reputation: 27375

Understanding exceptions throwing by a constructor

I'm reading Scott Meyers' Effective C++ and he provided an example of a well-designed assignement operator. Here it is:

class Bitmap{ ... };
class Widget
{
    //...
private:
    Bitmap *pb;
};

Widget::operator=(const Widget& rhs)
{
    Bitmap *pOrig = pb;
    pb = new Bitmap(*rhs.pb);
    delete pOrig;
    return *this;
}

Now, the explanation he gives right after the example is about how the code is exception-safe.

Now, if new Bitmap throws an exception, pb (and the Widget it's inside of) remains unchaged)

I don't understand that. How can we ever talk about unchanging here if throwing execeptions by a constructor leads us to UB (because operator delete will no be called on the pointer return by the new operator which results in an exception) ?

Upvotes: 0

Views: 80

Answers (1)

MSalters
MSalters

Reputation: 179779

If a constructor throws, operator new will not keep the memory allocated. This prevents the memory leak.

Note that a memory leak wouldn't be Undefined Behavior.

Upvotes: 2

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