Is there a C++ equivalent of a NullPointerException

Question

Both Java and C#, and probably many other languages too, have a predefined exception class that is thrown when a null parameter is used where it should not. Is there anything similar in C++? If not, is there another predefined exception I can use or should I define my own one?

Answer 1

Dereferencing a NULL pointer is undefined behaviour in C++ - which means the code can appear to work. An exception isn't guaranteed to be thrown. You can use the

std::invalid_argument

exception (provide a meaningful value to it - "p is NULL"), but you'll have to do the check yourself.

Answer 2

Usually, in C++ (or C for that matter), you never dereference a NULL pointer. Doing this has undefined behavior (likely a segfault on any implementation I know of, but anything could happen according to the standard). It's probably a bad thing in other languages as well, but I don't know those enough to assert that.

It's best to prevent the situation than to try to recover from it (which can't be done in C or C++ anyway).

The usual pattern to prevent some related programmer errors is to use assert() inside function bodies such as:

int foo(int* myint) {   // Ensure myint is not NULL   assert(myint);    // Do something with myint   (*myint)++;    return *myint; } 

Such assert() calls are completely ignored on release builds and thus have no cost in production. They just help the development. On debug builds, and if the condition is not met, the program aborts immediately with a very explicit error message. Running it through a debugger, you can easily check the call stack to investigate for the exact reason.