Throwing a NullPointerException in C++

Question

Is it possible to let C++ throw a NPE when calling a method on a nullptr object, instead of going in undefined behaviour? I could create a handler for a SEGFAULT signal but this would be realy dangerous, because not every SEGFAULT is a NullPointerException. If i have to do it via just checking in an if clause, is there a efficiant way to do so? Maybe also on compiletime?

Answer 1

Yes you can but its not really a good idea (you should not be handling pointers anyway, in modern C++ pointers are held inside objects that manage their lifespan).

You can always define a class that holds the pointer. Then when you try and use operator->() it will throw if the held pointer is nullptr.

template<typename T>
class ThrowingUniquePtr
{
     T*   ptr;
     public:
        // STUFF to create and hold pointer.

        T* operator->()
        {
            if (ptr) {
                return ptr;
            }
            throw NullPointerException; // You have defined this somewhere else.
        }
};

class Run
{
    public:
        void run() {std::cout << "Running\n";}
};
int main()
{
    ThrowingUniquePtr<Run>    x(new Run);
    x->run();  // will call run.

    ThrowingUniquePtr<Run>    y(nullptr);
    y->run();  // will throw.
}