Pointer variable is declared null, but member function performs normally

Question

Below follows an example of the code I’m trying to execute.

//main.cpp

#include <iostream>

using namespace std;

class A{
public:
    A(){}
    ~A(){}
    void teste(){
        cout << "Teste" << endl;
    } 
};


int main(){
    A *pA = NULL;

    pA->teste();
    return 0;
}

//end main.cpp

Once I declare the pointer *pA as null, I expected the program to fail. However, it runs normally and displays the message "Test".

Why does the program normally run instead of throwing an error to type null Pointer Exception?

I’m using the Qt5 platform on Linux.

Answer 1

Why do you think you should make an exception?

The pointer is needed to refer to a data structure, i.e., to a state, to data in memory. To access class behavior, that is, its methods. the pointer does not need referencing to any memory position. By type the compiler knows where is the code of methods that are independent of the instance.

The code is in a static (fixed) area in the memory and the progressively compiler, with the linker, and then the executable load by the operating system will determine the code address. Methods are always available in the application, even if you have not created a valid instance.

Still it could generate an exception within of the method if it was trying to access some state of the class. But in this case it is not, then there is no reason to error.

What you’re telling me to do is to call out the method teste() of the kind A which happens to be referenced by the variable pA but as the class has no data, nothing needs to be stored in memory for its execution. If nothing needs to be stored, you do not need to have any memory address to refer to, then a NULL falls well. And as during the execution of the method you just make access to a stream sending a literal string and the end of line, does not access any of the instance (and neither could, since it does not exist), does not need to access anything in memory, does not need a valid address in the variable.

So what happened is logical. It would be strange for an exception to be thrown. Try creating a status field in the class and access it in the teste() or directly on main() and see what happens.

In C++ there is no need and it is not recommended to create classes without status, prefer to use simple functions and if you need to organize related functions in a single logical unit use namespace. C++ is not Java.

Do not forget to read the supplement in William Bernal’s comment below on undefined behavior that is highly undesirable (the behavior, not William’s comment :) ). It is very important.