Calling a Overloaded constructor winthin a constructor

[shanakard]

I m a beginner programmer and new to this site so i m not quite sure whether this is the correct place to ask this question.

I saw the 2 articles published here about this question but can't seem to understand it.

i have a class like this withe some overloaded constructors


class Abcd{
public: //sorry for forgetting this earlier

Abcd();

Abcd(int i);

}

Abcd::Abcd(int i){

............... ............... .......

..Big code here calculating

something using int i.......

............... ............... ............... ...

}

// Why won't This work?
Abcd::Abcd(){

Abcd::Abcd(10);

}



i m quite sure c++ supports such things like in java. and if it don't can someone plesae tell me a alternate way.

-----------------------------------------------------------------------------------------
ThankX everyone for the fast replies.
What I actually wanted to do was to cerate a class 'Encode' with two constructors

Encode(QCString * value){ .....big code in herer......}

Encode(const char* value){ Encode(new QCString(value) ); }

[weaknessforcats]

This call:

Abcd::Abcd(10);
 

Calls the Abcd::Abcd(int) constructor. But no object is is specified. So, the compiler creates a temporary object as local to the Abcd::Abcd() function and initializes it with the 10. Unfortunately, this is NOT the object being initialized by the Abcd::Abcd() constructor. This temporary object will be destroyed at the end of the Abcd::Abcd() function.

If you want 10 as the default value, use the initializer list:

Abcd::Abcd() : datavariablename(10)
{
}
 

[DhananjayNangare]

I m a beginner programmer and new to this site so i m not quite sure whether this is the correct place to ask this question.

I saw the 2 articles published here about this question but can't seem to understand it.

i have a class like this withe some overloaded constructors


class Abcd{

Abcd();

Abcd(int i);

}

Abcd::Abcd(int i){

............... ............... .......

..Big code here calculating

something using int i.......

............... ............... ............... ...

}

// Why won't This work?
Abcd::Abcd(){

Abcd::Abcd(10);

}



i m quite sure c++ supports such things like in java. and if it don't can someone plesae tell me a alternate way.

Hi,
I tried it this way on vc++ 6.0
Works fine
#include <iostream.h>

class myclass
{

public:

myclass()
{
cout << "\n Defualt";
myclass::myclas s(10);
}
myclass(int i)
{
cout <<"\n Overloaded :" <<i;
}
};
void main()
{
myclass m;
}

Output:

Defualt
Overloaded :10Press any key to continue

[dcollins]

What you've told the compiler is that there is a class named
Abcd that has two private constructors, one is a default
constructor (it takes no arguments) and one that takes an
int. Member functions and data are private unless
explicitly declared otherwise as "public" or "protected" . In
this case you want to declare at least one public
constructor so you can instantiate the object.

There isn't a need to call one constructor from the
other. Both functions create an object of type Abcd. The
correct constructor is selected by the compiler depending on
the arguments that are supplied when you create it.

The change to your class declaration and the syntax for
instantiating the object is shown below.

class Abcd {
public:
    Abcd( ) { };
    Acbd( int i ) { /* some code */ };
}; // <- Note the semi-colon
 
 
// Instantiate an Abcd object
 
   Abcd abcd(10);

[shanakard]

Hi,
I tried it this way on vc++ 6.0
Works fine
#include <iostream.h>

class myclass
{

public:

myclass()
{
cout << "\n Defualt";
myclass::myclas s(10);
}
myclass(int i)
{
cout <<"\n Overloaded :" <<i;
}
};
void main()
{
myclass m;
}

Output:

Defualt
Overloaded :10Press any key to continue

that must be why i remember dowing this earlier. but it dosen't work in linux (g++)

[shanakard]

What you've told the compiler is that there is a class named
Abcd that has two private constructors, one is a default
constructor (it takes no arguments) and one that takes an
int. Member functions and data are private unless
explicitly declared otherwise as "public" or "protected" . In
this case you want to declare at least one public
constructor so you can instantiate the object.

There isn't a need to call one constructor from the
other. Both functions create an object of type Abcd. The
correct constructor is selected by the compiler depending on
the arguments that are supplied when you create it.

The change to your class declaration and the syntax for
instantiating the object is shown below.

class Abcd {
public:
    Abcd( ) { };
    Acbd( int i ) { /* some code */ };
}; // <- Note the semi-colon
 
 
// Instantiate an Abcd object
 
   Abcd abcd(10);

U are write both the constructors are public, my mistake

ThankX for the fast reply.
What I actually wanted to do was to cerate a class 'Encode' with two constructors

Encode(QCString * value){ .....big code in herer......}

Encode(const char* value){ Encode(new QCString(value) ); }

some thing like that

[dcollins]

What I actually wanted to do was to cerate a class 'Encode' with two constructors

Encode(QCString * value){ .....big code in herer......}

Encode(const char* value){ Encode(new QCString(value) ); }

some thing like that

I can't answer this case specifically because I don't know what a
QCString is. In general a QCString* and a const char* point to
different, incompatible types, so you wouldn't be able to use the same
code for both. Only if QCString has public member functions or
operators that return a pointer to its "raw data", can you use the
same code. This pointer would also need to be either a const char* or
something that can be safely cast to one. If a QCString is designed to
be used then obtaining this pointer should be easy.

I'll define QCString as a std::string in the following example. You
get a const char* from a std::string from its data( ) member
function.

#include <string>
#include <iostream.h>
 
    // make a guess at what a QCString is
    typedef std::string QCString;
 
    class Encode
    {
    public:
        Encode( const QCString* value )
        {
            Encode( value->data( ) );
        };
        Encode( const char* value )
        {
            cout << "Some complex function of " << value << endl;
        };
    };
 
int main( int argc, char* argv[ ] )
{
    // Allocate a QCString on the heap
    QCString* pQCString = new QCString( "A QCString." );
 
    // create a local Encode object, initialized from a constant QCString pointer
    Encode encode_one( pQCString );
 
    // Deallocate the QCString to prevent memory leaks
    delete pQCString;
 
    // create a local Encode object, initialized from a const char array
    Encode encode_two( "A const char array." );
 
     return 0;
}

[shanakard]

"weaknessforcat s" is right
calling a constructor from within a constructor only creates a temporary object

ive tried the following program and it can be confirmed

#include <string>
#include <iostream.h>
 
// make a guess at what a QCString is
typedef std::string QCString;
 
class Encode
{
public:
    int v;
 
    Encode( const QCString* value )
    {    v=5;
        cout << this << " Some complex function of " << *value << endl;
    }
    Encode( const char* value )
    {    v=3;
        Encode( new QCString(value) );
        cout<<"ok "<<endl;
    }
    ~Encode()
    {
        cout<<"destroying"<<endl;
    }
};
 
int main( int argc, char* argv[ ] )
{
    // Allocate a QCString on the heap
    QCString* pQCString = new QCString( "A QCString." );
 
    // create a local Encode object, initialized from a constant QCString pointer
    Encode* i =new Encode( pQCString );
 
    cout<< i->v<<endl;
    // Deallocate the QCString to prevent memory leaks
    delete pQCString;
    //delete i;
 
    // create a local Encode object, initialized from a const char array
    i=new Encode( "A const char array." );
 
    cout<< i <<" new addr"<<endl;
    cout<< i->v<<endl;
 
    return 0;
}
 
 

[shanakard]

anyway BIG thankX for everyone who repiled :-)