Why does the author emphasize the fact that the operator [] cannot return 0?

Question

The code below was copied from Bruce Eckel Thinking in C ++ Volume 2 Chapter 16

//: C07:Wrapped.cpp // From Thinking in C++, 2nd Edition // Available at http://www.BruceEckel.com // (c) Bruce Eckel 2000 // Copyright notice in Copyright.txt // Safe, atomic pointers #include <fstream> #include <cstdlib> using namespace std; ofstream out("wrapped.out"); // Simplified. Yours may have other arguments. template<class T, int sz = 1> class PWrap { T* ptr; public: class RangeError {}; // Exception class PWrap() { ptr = new T[sz]; out << "PWrap constructor" << endl; } ~PWrap() { delete []ptr; out << "PWrap destructor" << endl; } T& operator[](int i) throw(RangeError) { if(i >= 0 && i < sz) return ptr[i]; throw RangeError(); } }; class Cat { public: Cat() { out << "Cat()" << endl; } ~Cat() { out << "~Cat()" << endl; } void g() {} }; class Dog { public: void* operator new[](size_t sz) { out << "allocating a Dog" << endl; throw int(47); } void operator delete[](void* p) { out << "deallocating a Dog" << endl; ::delete p; } }; class UseResources { PWrap<Cat, 3> Bonk; PWrap<Dog> Og; public: UseResources() : Bonk(), Og() { out << "UseResources()" << endl; } ~UseResources() { out << "~UseResources()" << endl; } void f() { Bonk[1].g(); } }; int main() { try { UseResources ur; } catch(int) { out << "inside handler" << endl; } catch(...) { out << "inside catch(...)" << endl; } } 

I have no problem with the code itself. But I had trouble understanding the following comment about RangeError class RangeError :

"The PWrap template shows a more typical use of exceptions than you have seen so far: A nested class called RangeError is created for use in operator[ ] if its argument is out of range. Since operator[ ] returns a link, it cannot return null. (There are no null references.) This is a true exceptional condition - you don’t know what to do in the current context, and you cannot return an incredible value. "

Answer 1

If the function was to return a pointer, not a link, then it could signal an error (i.e., an index outside the bounds), returning a NULL pointer. But you cannot have NULL links, so the only option available is an exception. ^*

As @Steve points out in the comments below, you do not want operator[] return a pointer, because that would mean you need to write something like:

 T x = *wrapper[5]; 

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_{* An alternative might be assert .}

Answer 2

He emphasizes this to explain why throwing an exception is the only option in this case.

If the operator returned a pointer, it could return a null pointer instead of throwing an exception in case of errors. But since it returns a link and there is no such thing as a null link, the only way to handle errors is to throw an exception.

Answer 3

The author explains this quite well, perhaps the null part consumes that it means that it cannot return any value that means there is no value as such (for example, a null pointer) and therefore throws it away.

Answer 4

The author said this: "Because the [] operator returns a link, it cannot return zero." Because the NULL link is very bad.