Accessing private members from inside class

[Bobruisk]

Hello all!

Let's say I have the following class:

class MyClass
{
private:
   int m_myInt;
public:
   int myInt() {return this->m_myInt;};
   int myFunc();
};

Which of the following is to prefer;

int MyClass::myFunc()
{
   return 2*this->m_myInt;
}

or

int MyClass::myFunc()
{
   return 2*this->myInt();
}

The second one seems better, but what do you guys thing? Are both OK? What's the best practice?

[MikeyBoy]

I'd prefer the latter, myself. The idea is that storing myInt as a data member is an implementation detail. If, at some point, you decide to change that - say, you were going to calculate it from some other data, or obtain it from another class - then you would only need to change your myInt() method. Everything else is calling myInt() to get the value, and doesn't need to know where that value comes from.

If you did it the former way, you'd also have to change myFunc(), along with any other method in MyClass that also uses that value.

[BlackSheep]

Honestly I would use the first because it just makes more sense intuitively to me. Why should I call a function if I have direct access? myInt() is good in a case where you need to be sure that a user of the class isn't editing the value m_myint, but in this case YOU'RE the author of the class and know all of its internals.

Although I've never coded in an environment with enforced guidelines (really just personal projects) so take that with a grain of salt.

[Bobruisk]

Thanks for your reply. That's a very good point.

What about this case, regarding the opposite, setting private members.

class MyClass
{
private:
   std::vector<int> m_intVec;
public:
   void addInt(int);
}

addInt should add an int to the vector. How would you implement this?

Let's say;

class MyClass
{
private:
   std::vector<int> m_intVec;
   std::vector<int> & intVec() {return this->m_intVec;};
public:
   void addInt(int n) {this->intVec().push_back(n);};
}

Now what if I want to access the vector from outside? I wouldn't want to return the reference to my internal variable without declaring it const. I would like to have this, kind of, but is it possible to overload by const?

class MyClass
{
private:
   std::vector<int> m_intVec;
   std::vector<int> & intVec() {return this->m_intVec;};  //Same...
public:
   void addInt(int n) {this->intVec().push_back(n);};
   std::vector<int> const & intVec() const {return this->intVec();}; //... function name
}

This won't work with 2 functions taking the same arguments (intVec()) differing only by const qualifier. How would you solve this? I'd rather not have one const_intVec() and one intVec() member, but do I have a choice?

[coder777]

MikeyBoy wrote:

or obtain it from another class - then you would only need to change your myInt() method

yes, at the first sight it looks intriguing. But then: Are you prepared for the new behavior? Like you call that function too often or something. I'd say better use your internal variables directly

Bobruisk wrote:

This won't work with 2 functions taking the same arguments (intVec()) differing only by const qualifier

No: due to this intVec() const the second function is different. You just cannot call a non const function from a const function. Again it's better to use the internal variable directly.

[Bobruisk]

@coder77
You are right

I ment something like this;

class MyClass
{
private:
   std::vector<int> m_intVec;
   std::vector<int> & intVec() {return this->m_intVec;};  //Same...
public:
   void addInt(int n) {this->intVec().push_back(n);};
   std::vector<int> const & intVec() const {return this->m_intVec;}; //... function name
} 

i.e. one public method that returns const, and one private method which returns a reference to a modifiable object.

[coder777]

both intVec() will work due to const on the right side of the function

[Bobruisk]

It will?

How does the compiler know which function I'm calling if i write

void MyClass::myMember()
{
   this->intVec();
}

[coder777]

How does the compiler know which function I'm calling if i write

the compiler doesn't care. The linker will resolve this with the first definition available.

[Peter87]

It will always use the non-const version if possible.

[ne555]

The const one will be called from constant objects.

@OP: you are too eager to show your organs.
I don't see any advantage in doing void addInt(int n) {this->intVec().push_back(n);};
Suppose that you change the container.

[Bobruisk]

@ne555

I don't quite follow. What would you do instead?

[Bobruisk]

I put the non-const access method as private so that I can use it internally in the class. Externally it give access only to the const method.

[ne555]

It may be fine if you are doing something like `string::c_str()'
¿what would be the meaning of `intVec()' if you are storing in a list?
Or you realize that a queue will be better, `push_back()' does not exist so `addInt()' is broken.

If you want to allow access to the elements, provide iterators.