Question

Is there an easy way to throw custom null pointer exception in c++?
My idea was to redefine this pointer, but it has 3 problems:

1. Not using this throws standard Acces Violation Exception
2. Pointer is checked every time it is used

3. Visual studio show this as InteliSense error (compilable) (do not know what other compilers do)

   #include <iostream>
   #define this (this != nullptr ? (*this) : throw "NullPointerException")
   
   class Obj
   {
   public:
       int x;
       void Add(const Obj& obj)
       {
           this.x += obj.x; // throws "NullPointerException"
                   //x = obj.x;  // throws Access Violation Exception
       }
   };
   
   
   void main()
   {
       Obj *o = new Obj();
       Obj *o2 = nullptr;
       try
       {
           (*o2).Add(*o);
       }
       catch (char *exception)
       {
           std::cout << exception;
       }
       getchar();
   }
   

Answer 1

Since this can never, ever be nullptr, compilers are free to treat this != nullptr the same as true. What you’re trying to do fundamentally doesn’t make sense. You can’t use exceptions to catch undefined behavior. The only way this can be nullptr is through undefined behavior.

Obj *o2 = nullptr;
try
{
    (*o2).Add(*o);
}

Dereferencing a nullptr is undefined behavior (8.3.2). This is trying to use an exception to catch undefined behavior. Fundamentally, you cannot do that in C++.

For one obvious reason this is undefined, consider this:

class Foo
{
   public:
   Foo { ; }
   virtual void func() = 0;
};

class Bar : public Foo
{
   public:
   Bar() { ; }
   virtual void func() { some_code() }
};

class Baz : public foo
{
    public:
    Baz() { ; }
    virtual void func() { some_other_code(); }
}

...

Foo * j = nullptr;
j->func(); // Uh oh, which func?