Query regarding pointer member variable

Question

I am learning c++ from 'Teach yourself c++ in 21 days' by Liberty and Jones. Compiler I am using is gcc 6.3.0. I am on pointer topic. First I am writing one code similar to the one book gives which works fine. Here it is:

#include <iostream>
 using namespace std;
 class Cat 
{
 public:
   Cat();
   int Getage() {return *itsage;}
 private:
  int *itsage;
};

 Cat::Cat()
{
 itsage=new int;
 *itsage=2;
}
 int main()
{
 Cat *Mani=new Cat;
 cout << "Mani is a cat whose age is: " << Mani->Getage();
 cout << endl;
 return 0;
}

Output: Mani is a cat whose age is: 2

Now instead of using constructor, I thought of using accessor function to set age. I wrote the following code which gave me output as above but I have doubt why this code work. Here is the code I wrote.

#include <iostream>
 using namespace std;
 class Cat 
{
 public:
   void Setage(int age) {*itsage=age;}
   int Getage() {return *itsage;}
 private:
  int *itsage;
};

 
 int main()
{
 Cat *Mani=new Cat;
 Mani->Setage(2);
 cout << "Mani is a cat whose age is: " << Mani->Getage();
 cout << endl;
 return 0;
}

Doubt: What I studied so far tells me that whenever pointer declared, it should get initialized, i.e. we should assign some memory address to it. In the first code, we are initializing our pointer member variable in the constructor by this command itsage=new int; which looks fine. But in the second code, I nowhere assigned any memory address to pointer itsage. But still code worked fine. I am little confused here.

I have one more doubt(I am asking here itself as it is related to the above code. If it's bad, I will ask it in different question.) I thought of making pointer itsage in the above code public. But I don't know how can I access it. I tried the following but didn't work.

#include <iostream>
 using namespace std;
 class Cat
{
 public:
  int *itsage;
}; 
 
 int main()
{
 Cat *Mani=new Cat;
 Mani->*itsage=2;
 cout << "Frisky's age is " << Mani->*itsage;
 return 0; 
}

So how to access public pointer member variable of object created on heap?

Thanks.

score 2 · Answer 1 · answered Aug 08 '20 at 08:11

About your first question. I was confused at first, this is a neat answer, which shows that the compiler did it for you.
About your second question, it's nothing but an improper way of accessing the pointer. You should realize that when trying to access the content of a pointer, you must get its address then go to its content. So you need to:

Get its address: Mani -> itsage
Access its content: *(Mani -> itsage)

score 1 · Answer 2 · answered Aug 08 '20 at 07:52

But in the second code, I nowhere assigned any memory address to pointer itsage. But still code worked fine. I am little confused here.

That's a fair doubt. In fact, your 2nd code should never work, as no memory had ever been allocated to itsage. If it still somehow "worked" for you then it is still a typical undefined behaviour.

You should still allocate the memory before usage as usual, and free it in the constructor. As your class doesn't even have a destructor, perhaps you should choose unique_ptr instead:

class Cat
{
public:
    void Setage(int age) { itsage = make_unique<int>(age); }
    int Getage() { return *itsage; }

private:
    unique_ptr<int> itsage;
};

score 1 · Accepted Answer · answered Aug 08 '20 at 15:13

In this function:

void Setage(int age) { *itsage=age; }

you are dereferencing itsage, but it's not pointing to valid memory. This invokes undefined behavior. You need to do:

void Setage(int age) {
  itsage = new int;
  *itsage=age;
}

For the second question, the correct syntax to access a member variable of pointer type is:

*(Mani->itsage) = 2;

Note that this is undefined behavior too, for the same reason as above. The fix is also the same; you need to do:

Mani->itsage = new int;
*(Mani->itsage) = 2;

Query regarding pointer member variable

3 Answers3