[SOLVED] C++ : Constructor in 'Private' section of class

thelink123 · 08-27-2010, 08:30 AM

Kindly take a look at the following code

Code:

 #include <iostream>

 using namespace std;

 class private_cons
 {

 private:

    int i;

    private_cons(int num)
    {
       i = num;
    }

 public:

    private_cons()
    {
       private_cons obj1(30);

    }

    int get_i()
    {

       return i;

    }

 };

 int main ()
 {
    private_cons obj;

    cout << "Value = " << obj.get_i();

    return 0;

 }

I expected the output to be

Quote:

Value = 30

Instead i got some big value like

Quote:

Value = 1629727663

Kindly let me know the reason for this.

Thanks
-Nitin

crabboy · 08-27-2010, 09:13 AM

The public constructor private_cons is creating his own object, obj1 which assigns the value of 30 to i to that object. Once the constructor exists, that object goes out of scope and your 30 is gone. The i for obj is never assigned.

Add this line to the end of your public constructor and maybe it will make sense to you.

Code:

i = obj1.get_i();

johnsfine · 08-27-2010, 09:58 AM

Quote:

Originally Posted by thelink123

Code:

    private_cons()
    {
       private_cons obj1(30);

    }

Do you have previous experience in some language, such as Java, that lets a constructor call another constructor for the same object?

C++ doesn't give you any way to do that. That fact is often inconvenient when writing C++ code.

As crabboy explained, what you actually did was have a constructor call a different constructor of the same class but for a different object.

One common, but imperfect, work around for not being able to call another constructor for the same object is to move all the actual construction to init methods:

Code:

#include <iostream>

 using namespace std;

 class private_cons
 {

 private:

    int i;

    private_cons(int num)
    {
       init(num);
    }

    void init(int num)
    {
       i = num;
    }
 public:

    private_cons()
    {
       init(30);

    }

    int get_i()
    {

       return i;

    }

 };

 int main ()
 {
    private_cons obj;

    cout << "Value = " << obj.get_i();

    return 0;

 }

Another method, that is often better, is to move the data members and true constructors of your class into a base class:

Code:

#include <iostream>

 using namespace std;

 class base_class_for_private_cons
 {
 protected:
    int i;
    base_class_for_private_cons(int num) :
      i(num)
    {}
 };
 class private_cons : private base_class_for_private_cons
 {

 private:

    private_cons(int num) :
      base_class_for_private_cons(num)
    {}

 public:

    private_cons() :
      base_class_for_private_cons(30)
    {}

    int get_i()
    {

       return i;

    }

 };

 int main ()
 {
    private_cons obj;

    cout << "Value = " << obj.get_i();

    return 0;

 }

Depending on details of what else you want to do, the part I put in red is probably not needed.

David1357 · 08-27-2010, 10:27 AM

Try

Code:

#include <iostream>

using namespace std;

class private_cons
{
private:
    int i;
    private_cons(int num) : i(num) { }

public:
    private_cons() { *this = private_cons(30); }

    int get_i() { return i; }
};

int main ()
{
    private_cons obj;

    cout << "Value = " << obj.get_i() << endl;

    return 0;
}

johnsfine · 08-27-2010, 12:05 PM

Quote:

Originally Posted by David1357

Try

Code:

    private_cons() { *this = private_cons(30); }

No. Do not do that!

You are violating basic concepts of the language. You are using the desired constructor on a different instance of the object (as the OP did) and that would be OK, but

You are using operator=() to copy to an incompletely constructed left hand side. That is wrong.

operator=() should be used only to replace contents of a fully constructed left hand side. It should not be used with a partially constructed left hand side.

We are looking at an example that is both trivial and POD. So you could do many things that are generally wrong C++ and they won't malfunction. But you should almost always code C++ as if you are working on a real project that will grow and evolve. Wrong code that happens to be safe because something else is unusually simple represents a land mine that is likely to blow up when unrelated reasonable looking changes are made.

Also, remember the OP didn't post the code He/She actually wants to use. We constantly tell people to cut their code down to a full compilable example that still contains the problem. That almost always involves keeping just the problem itself while discarding the reason it was coded that way. So when we make a suggestion for fixing the problem we should try not to assume the code was written that way for no good purpose.

If there is a good reason to have a private constructor and to want a public constructor to use the private constructor (rather than to simply initialize the members itself) then there is almost certainly also a reason that operator=() into a half constructed left hand side will malfunction.

David1357 · 08-27-2010, 12:13 PM

Quote:

Originally Posted by johnsfine

You are using operator=() to copy to an incompletely constructed left hand side.

How is it incompletely constructed? Please quote chapter and verse when you make such a statement.

If there were any other members that needed to be initialized specifically, they could be added to the private constructor. As long as the right hand side is completely constructed, and all you want is a copy of the right hand side, I fail to see how the code is "wrong".

paulsm4 · 08-27-2010, 12:49 PM

johnsfine is correct.

And, as he also pointed out, the bug is *especially* pernicious because most of the time (including in "toy" examples like this), it will *seem* to work.

IMHO .. PSM

David1357 · 08-27-2010, 01:12 PM

Quote:

Originally Posted by paulsm4

johnsfine is correct.

Please provide a reference.

thelink123 · 08-30-2010, 12:48 AM

Thank you all

... Very informative explanations by all LQ members who have contributed to this thread

-thelink123