在 C++ 中抛出 NullPointerException

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.
}

Answer 2

Another way of exception handling: calling a function using NULL pointer

#include <iostream>
#include <typeinfo>
using namespace std;

char str_NullPointer[25] = "NULL Pointer exception";
char str_Arithmetic[25] = "Arithmetic exception";

class A
{
public:
   int i = 20; 
public:
    int function()
    {
        printf("Function start\n");

         try
        {
            if (this)
            {
                printf("value of i = %d \n", i);  
            }
            else
            {
                throw str_NullPointer;          /* Exception thrown in Case 2 */
            }
        }
        catch(const int s)
        {
            printf("%d\n", s);
        }
        catch(const char* s)
        {
            printf("%s in %s\n", s, typeid(this).name());  /* Exception and Class Name */
        }

        printf("Function end\n\n\n");
    }
};

int main() {
    //code

printf("Case 1: With Pointer\n");
A *obj = new A();
obj->i = 20;
obj->function();

printf("Case 2: With NULL Pointer\n");
delete obj;
obj = NULL;
obj->function();

return 0;
}

Output:

Case 1: With Pointer
Function start
value of i = 20 
Function end


Case 2: With NULL Pointer
Function start
NULL Pointer exception in P4abcd
Function end