calling overload constructor in constructor

[Christian2011]

Hi
Quick questions abour overload constructors. Lets say I have two overload constructors:

class MyFileExtractor{
    MyfileExtractor(string);
    MyfileExtractor(string, int);
    ~MyfileExtractor();

private:
   string name;
   int columns;
}

/*Class implementation*/
MyFileExtractor::MyFileExtractor(string fname,int c):columns(c)
{
   name=fname;
}

MyFileExtractor::MyFileExtractor(string fname)
{
   #define STD_NUMBER_OF_COLMS 1   //standard number of columns
   MyFileExtractor::MyFileExtractor(fname, STD_NUMBER_OF_COLMS);
}

MyFileExtractor::~MyFileExtractor()
{
   cout << "This file removed:" << name << endl;
}


int main()
{
   MyFileExtractor cobalt("co60.txt")
   MyFileExtractor zinck("zn65.dat",3)

   return 0;
}

When I run this code I notice that despite I instantiated 2 objects of type myFileExtractor but the destructor gets call 3 times. I am sure the problem comes from the way I created my second overloaded constructor which calls the first overloaded constructor. Why do I do that? Because the second overload constructor is the same as the first one except it has a default value for the columns parameter. I tried the following whcih i though it was going to solve my problem:

class MyFileExtractor{
    MyfileExtractor(string, int);
    ~MyfileExtractor();

private:
   string name;
   int columns;
}

/*Class implementation*/
MyFileExtractor::MyFileExtractor(string fname,int c=1):columns(c)
{
   name=fname;
}

MyFileExtractor::~MyFileExtractor()
{
   cout << "This file removed:" << name << endl;
}

(...)

However I couldn't instantiated objects in main in the following way:

MyFileExtractor cobalt("co60.txt")

I suppose my problem comes from calling the constructor inside another constructor. It seems c++ instantiates a second object inside the first object and that is the reason it calls thedestructor three times...

:~~~ My question here is: is it legal to call a constructor inside another constructor?

What would happen if I have 10 private members in my class and I have one constructor that initializes teh value of the ten memebers and then I have a second constructor that does the same thing except for one element which is initialized by an user parameter. Do i have to write the code for both construtors apart despite their body is the same except for the assignment of one parameter?

class MyFileExtractor{
    MyfileExtractor(void);
    MyfileExtractor(int);
    ~MyfileExtractor();

private:
   string name;
   int columns;
   (... 8 parameters more)
}
//class implementation
MyFileExtractor::MyFileExtractor(){
   //initialized all 10 parameters
   (...)
}

MyFileExtractor::MyFileExtractor(int a){
   //initialized 9 parameters
   (Lots of repeated lines which exist already in the default constructor)
   MyFileExtractor::a=a;
}

What is the most efficient way to do this?

Thanks,
C.

[Ancient Dragon]

Create another private function init() (or some other name) that can be called from all constructors

class MyFileExtractor{
public:
    MyFileExtractor(string str) 
    {
       const int STD_NUMBER_OF_COLMS = 1;   //standard number of columns
        init(str,STD_NUMBER_OF_COLMS);
    }
    MyFileExtractor(string str, int n)
    {
        init(str,n);
    }
    ~MyFileExtractor() {cout << "Destructor " << name << "\n"; }
    
private:
    void init(string str, int n)
    {
        name = str;
        columns = n;
    }
   string name;
   int columns;
}

The output is this:

Destructor zn65.dat
Destructor co60.txt
Press any key to continue . . .

[Christian2011]

Yes... I just thought a init() function would solve my issue. Thanks for your advise.

By the way... is it possible to conclude that you cannot call a contructor inside another constructor? Is this illegal? I don't recall this from my c++ classes and I would like to have this clear for future references,

Cristian

[ArkM]

The answer: yes, it's legal - but totally senseless. It's the same as:

int main()
{
    12345;
    return 0;
}

If you want, we call int "constructor" then immediatly discard created object. So internal constructor in actual fact creates local (in outer constructor body) object of the same type. Of course, after closing semicolon destructor was called.

Try this for the constructor with 1 arg (simple and clear code):

#define STD_NUMBER_OF_COLMS 1   //standard number of columns
MyFileExtractor::MyFileExtractor(string fname):name(fname),
columns(STD_NUMBER_OF_COLMS)
{}

If you need lots of constructors, move common codes in a private member function then call it in every constructor. It's a standard way to go.

[Ancient Dragon]

Calling one constructor from another cause the destructor to be called too many times. So don't do it.

[Christian2011]

Awesome guys, thxs!
Cristian