Question

Is the following code legal in C++. Accessing a friend class member public method? I know this sounds confusing, and the best way to show it is in code. I was wondering if the TestClassC::Method() is valid in the code below?

I’ve compiled (g++) and it works, however, I run into a situation, where it produces a segmentation fault on other machine/distros at TestClassC::Method(). Which is making me wonder if this->classA_Ptr->classB.Method(); is legal in C++.

#include <iostream>

using namespace std;

class TestClassB
{
   public:
      TestClassB(void){};
      ~TestClassB(void){};
      void Method(void){
         cout << "Hello Again" << endl;
      }
};


class TestClassC; //Forward Declaration
class TestClassA
{
   friend class TestClassC;

   public:
      TestClassA(void){};
      ~TestClassA(void){};

   private:
      TestClassB  classB;
};


class TestClassC
{
   public:
      TestClassC(TestClassA* a_Ptr){
         this->classA_Ptr = a_Ptr;
      }
      ~TestClassC(void){};

      void Method(void){
         //Is this Valid/Legal ???
         this->classA_Ptr->classB.Method();
      }

   private:
      TestClassA*  classA_Ptr;
};



int main()
{
    cout << "Hello world!" << endl;

    TestClassA testClassA;
    TestClassC classC(&testClassA);

    classC.Method();

    return 0;
}

Answer 1

The public/private/protected accessor modifiers are enforced at compile-time, not at runtime. The SEGFAULT is a runtime error, not a compile-time error. So, in the future, if you get a SEGFAULT, you can be sure that it is not related to the level of access. It sounds like your confusion is based on whether the access modifiers are applied directly, indirectly, transitively, etc. The access modifiers work in the most simple and straight-forward way: they apply only directly, to the functions or variables that have been declared in that class and control access through the given class to those items. Once you have access to such an item, further access is determined purely by that item’s own access modifiers.

A SEGFAULT is typically indicative of an illegal memory access. If you experience SEGFAULTS, look at where you dereference pointers (whenever you have *X or X->Y). Common causes of SEGFAULTS include dereferencing NULL, off-by-one array access, and using an object through a pointer where the object in question has already been deleted by going out of scope.