Hi all,
I'm gradually starting to make this transition from the world of C to C++, and now that I have a little progress, I have some things that are still a bit confusing to me. One of them is if I nest a class inside another class. If I'm inside the nested class, does "this" point to the nested class or the wrapper class? Likewise, what if I want to refer to one and the other separately. Can I get both this 's? If so, I suppose I could only do that from inside the nested class or else how would it know which instance of the nested class to get it from?...
Thanks,
Sean

If you have composition, then the this refers to whichever "object" the current function is a member of.

class A {
 private:
   int somePrvInt;
 public:
   A() : somePrvInt(0) {}
   int getPrvInt() {
     return this->somePrvInt;
     // this refers to the current A object
   }
   
};

class B {
 private:
   A somePrvA;
 public:
   B() {}
   A getPrvA() {
     return this->somePrvA;
     // this refers to the current B object
   }
   int getPrvAPrvInt() {
     return this->somePrvA.getPrvInt();
     // this still refers to the current B object
   }
};

If you have inheritance, in the base class, this refers to just the base class, in the derived class it refers to both simultaneously. This is what allows you direct access to the protected (but not private) members of A from inside B.

class A {
 protected:
   int someProtInt;
 public:
   A(int newInt = 0) : someProtInt(newInt) {}
   virtual int getSomeProtInt() {
     return this->someProtInt;
     // this refers to the A part of the current B:A object
   }
   
};

class B : public A {
 public:
   B(int newAInt = 0) : A(newAInt) {}

   int getSomeProtInt() {  //overrides A::getSomeProtInt()
     return this->someProtInt
     // this refers to both the current B : A object
   }
};

Edited 6 Years Ago by Fbody: n/a

@Fbody: you covered two good cases, but none are about the OP's question.

If you have a nested class, then the this pointer, within a call to a member of the nested class, refers to an instance of the nested class (not its wrapper class). However, the nested class has the advantage that it has potential access to all the methods and members of its wrapper. If you want access to both the nested class and the wrapper class, here's the usual trick: (and I put some const, as a wink to your previous posts ;) )

#include <iostream>

class Wrapper {
  private:
    int value;
 
    void bar() const {
      std::cout << "The answer is " << value << std::endl;
    };
 
  public:
    class Nested {
      private:
        Wrapper& Parent;
      public:
        Nested(Wrapper& aParent) : Parent(aParent) { };
        void foo() const {
          Parent.bar();
        };
    };
    Wrapper(int aValue) : value(aValue) { };
};

int main() {
  Wrapper A(42);
  Wrapper::Nested B(A);

  std::cout << "What is the answer to the ultimate question of life, the universe, and everything?" << std::endl;
  B.foo();
};

Ok, in that case, Parent is a member that gets a constructor call, so that makes sense, but how about in this case where the constructor is separate but seemingly related by nesting, but maybe not anyway:

#include <iostream>
class Wrapper 
{
public:
  int valueW;
  void bar() 
  {
    std::cout << "The answer is this->valueW = " << this->valueW <<", this->nest.valueN = "<<this->nest.valueN << ", this = "<<this<<std::endl;
    std::cout <<"this->nest.foo() = "<<std::endl;
    this->nest.foo();
  };
  class Nested 
  {
  public:
  int valueN;
    Nested(int v):valueN(v)
    {
    }
    void foo() 
    {
      std::cout <<"valueN = "<<valueN<<", this = "<<this<< std::endl;
      std::cout <<"however, I exist inside of a Wrapper but do I really not know which Wrapper::this, this is inside of on my own? "<<std::endl;
    };
  };
    Nested nest;
  Wrapper(int aValue) : valueW(aValue),nest(aValue-10)
  { 
    
  };
};
int main()
{
  Wrapper A(42);
  std::cout << "What is the answer to the ultimate question of life, the universe, and everything?" << std::endl;
  A.bar();
};

What is the answer to the ultimate question of life, the universe, and everything?
The answer is this->valueW = 42, this->nest.valueN = 32, this = 0x7fff5fbff680
this->nest.foo() =
valueN = 32, this = 0x7fff5fbff684
however, I exist inside of a Wrapper but do I really not know which Wrapper::this, this is inside of on my own?

Edited 6 Years Ago by smcguffee: n/a

I think you are getting confused. Think of the Nested class as a class of its own. The
only difference is its scope. You can only access it from the outer class. The nested
class has no association with the parent class, besides the scope resolution.

What you are doing is creating a instance of Nested class in the Wrapper class.
Therefore every different instance of wrapper will have different instance of Nested.

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