creation of objects?

[Rahul]

Hi Everyone,

class Base
{
public : Base(int i)
{
printf("constru ctor\n");
}
void disp()
{
printf("display ed...\n");
}
virtual ~Base()
{
}
};

int main()
{
Base obj();
// obj.disp();
return(0);
}

What does the first line in main() function mean?
The second commented line causes a compilation error saying "left of
'.disp' must have class/struct/union type"

Thanks in advance!!!

[Daniel T.]

Rahul <sa*****@yahoo. co.inwrote:

class Base
{
public : Base(int i)
{
printf("constru ctor\n");
}
void disp()
{
printf("display ed...\n");
}
virtual ~Base()
{
}
};

int main()
{
Base obj();
// obj.disp();
return(0);
}

What does the first line in main() function mean?

It defines a function called "obj" that returns an object of type Base.
Much like "int main()" defines a function called "main" that returns an
int.

The second commented line causes a compilation error saying "left of
'.disp' must have class/struct/union type"

Try this instead:

int main()
{
Base obj;
obj.disp();
// return not necessary in main
}

[Salt_Peter]

On Jan 7, 10:16 pm, Rahul <sam_...@yahoo. co.inwrote:

Hi Everyone,

class Base
{
public : Base(int i)
{
printf("constru ctor\n");
}
void disp()
{
printf("display ed...\n");
}
virtual ~Base()
{
}
};

int main()
{
Base obj();
// obj.disp();
return(0);

}

What does the first line in main() function mean?
The second commented line causes a compilation error saying "left of
'.disp' must have class/struct/union type"

Thanks in advance!!!

The error is referring to left of disp(). So that error occurs in the
previous line.
Its telling you 'obj' is not an instance of a known type or union.
Look at your constructor for type Base....
Is it not declared as Base(int) ?

instead of
Base obj();
try
Base obj(8); // now that looks like Base(int)

See if you can follow this example:

#include <iostream>

class Base
{
int m_n;
public:
Base() : m_n(0)
{
std::cout << "default ctor\n";
}
Base(int i) : m_n(i)
{
std::cout << "Base(int)\ n";
}
void disp() const
{
std::cout << "Base::disp()\t ";
std::cout << "m_n = " << m_n;
std::cout << std::endl;
}
~Base()
{
std::cout << "~Base()\n" ;
}
};

void obj()
{
std::cout << "obj()\n";
}

int main()
{
Base base;
base.disp();

Base another_base(8) ;
another_base.di sp();

obj();
}

/*
default ctor
Base::disp() m_n = 0
Base(int)
Base::disp() m_n = 8
obj()
~Base()
~Base()
*/

[Rahul]

On Jan 8, 8:45 am, "Daniel T." <danie...@earth link.netwrote:

Rahul <sam_...@yahoo. co.inwrote:

class Base
{
public : Base(int i)
{
printf("constru ctor\n");
}
void disp()
{
printf("display ed...\n");
}
virtual ~Base()
{
}
};

int main()
{
Base obj();
// obj.disp();
return(0);
}

What does the first line in main() function mean?

It defines a function called "obj" that returns an object of type Base.
Much like "int main()" defines a function called "main" that returns an
int.

The second commented line causes a compilation error saying "left of
'.disp' must have class/struct/union type"

Try this instead:

int main()
{
Base obj;
obj.disp();
// return not necessary in main

}

Yeah, its a function prorotype visible only in main function, however
i tried the following code,

class Base
{
public: Base(int i = 0)
{
cout<<"Construc tor"<<endl;
}
};

int main()
{
Base obj();
Base obj1(10); // Invokes constructor
Base obj2; // Invokes constructor
return ( 0 ) ;
}
i expected the constructor to be called in first case, as default
arguement is provided for the constructor...

[Salt_Peter]

On Jan 8, 1:41 am, Rahul <sam_...@yahoo. co.inwrote:

On Jan 8, 8:45 am, "Daniel T." <danie...@earth link.netwrote:

Rahul <sam_...@yahoo. co.inwrote:

class Base
{
public : Base(int i)
{
printf("constru ctor\n");
}
void disp()
{
printf("display ed...\n");
}
virtual ~Base()
{
}
};

int main()
{
Base obj();
// obj.disp();
return(0);
}

What does the first line in main() function mean?

It defines a function called "obj" that returns an object of type Base.
Much like "int main()" defines a function called "main" that returns an
int.

The second commented line causes a compilation error saying "left of
'.disp' must have class/struct/union type"

Try this instead:

int main()
{
Base obj;
obj.disp();
// return not necessary in main

}

Yeah, its a function prorotype visible only in main function, however
i tried the following code,

class Base
{
public: Base(int i = 0)
{
cout<<"Construc tor"<<endl;
}

};

int main()
{
Base obj();
Base obj1(10); // Invokes constructor
Base obj2; // Invokes constructor
return ( 0 ) ;

}

i expected the constructor to be called in first case, as default
arguement is provided for the constructor...

This invokes default construction:
Base obj;
this does not:
Base obj(); // is a function call, not a ctor

A function is looked for that isn't a constructor and matches the
provided signature.
example:
Base obj() { return Base(); } // returns a temporary by value
Base obj(); // does nothing [bad code example !!]

Now, you can use Base() in the case of a temporary, since a function
will enforce the type of the parameter.
ie:

void foo(const Base& b) { /*do stuff to a temp*/ }
int main()
{
foo(Base()); // Base() generates a temporary, this is ok
}

There is a distinction to be made here, since the function will only
accept a Base.
The program has no choice but to consider Base constructors alone. So
you can think of Base() as a direct call to the default constructor
(which generates the resulting temporary). And the code is safe
because the reference is a reference_to_co nst (the lifetime of the
temporary is guarenteed to last until function ends).

Recap:

the following statement creates a default object:
Base obj;
and this statement tries to run a function
Base obj();
and finally, the following creates a temporary
Base();

It will be crystal-clear soon enough. Its a subject that rears its
head daily in this newsgroup.

[James Kanze]

On Jan 8, 7:41 am, Rahul <sam_...@yahoo. co.inwrote:

On Jan 8, 8:45 am, "Daniel T." <danie...@earth link.netwrote:

Rahul <sam_...@yahoo. co.inwrote:

class Base
{
public : Base(int i)
{
printf("constru ctor\n");
}
void disp()
{
printf("display ed...\n");
}
virtual ~Base()
{
}
};

int main()
{
Base obj();
// obj.disp();
return(0);
}

What does the first line in main() function mean?

It defines a function called "obj" that returns an object of
type Base. Much like "int main()" defines a function called
"main" that returns an int.

Not "defines", declares.

The second commented line causes a compilation error
saying "left of '.disp' must have class/struct/union type"

Try this instead:

int main()
{
Base obj;
obj.disp();
// return not necessary in main
}

Yeah, its a function prorotype visible only in main function,
however i tried the following code,

class Base
{
public: Base(int i = 0)
{
cout<<"Construc tor"<<endl;
}
};

int main()
{
Base obj();
Base obj1(10); // Invokes constructor
Base obj2; // Invokes constructor
return ( 0 ) ;
}

i expected the constructor to be called in first case, as
default arguement is provided for the constructor...

The constructor is called when an object is constructed. The
first line in main, above, is not a definition of an object, but
a declaration of an external function. No object, no
constructor gets called.

This is sometimes called "C++'s most embarassing parse", and it
occasionally catches out even the experts. Regretfully, for
historical reasons, there's not much that can be done about it.
C++ is based on C, and as someone (I think it was Bjarne
Stroustrup) said: "C's declaration syntax can be considered an
experiment which failed." If the language were being designed
from scratch, there would probably be a keyword which would make
the distinction, e.g.:

int
main()
{
var Base obj() ; // Defines a variable...
fnc Base obj() ; // Declares a function...
//...
}

It would make reading the code easier for both compiler and
human readers.

--
James Kanze (GABI Software) email:ja******* **@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, [phone]

[Daniel T.]

In article
<e2************ *************** *******@i7g2000 prf.googlegroup s.com>,
James Kanze <ja*********@gm ail.comwrote:

On Jan 8, 7:41 am, Rahul <sam_...@yahoo. co.inwrote:

On Jan 8, 8:45 am, "Daniel T." <danie...@earth link.netwrote:

Rahul <sam_...@yahoo. co.inwrote:
class Base
{
public : Base(int i)
{
printf("constru ctor\n");
}
void disp()
{
printf("display ed...\n");
}
virtual ~Base()
{
}
};

int main()
{
Base obj();
// obj.disp();
return(0);
}

What does the first line in main() function mean?

It defines a function called "obj" that returns an object of
type Base. Much like "int main()" defines a function called
"main" that returns an int.

Not "defines", declares.

The second commented line causes a compilation error
saying "left of '.disp' must have class/struct/union type"

Try this instead:

int main()
{
Base obj;
obj.disp();
// return not necessary in main
}

And while we are at it, Salt Peter caught the fact that the OP doesn't
have a default constructor so the above won't work either.

[Andre Kostur]

Rahul <sa*****@yahoo. co.inwrote in news:73a87338-c616-4406-84d7-
35**********@l6 g2000prm.google groups.com:

Hi Everyone,

class Base
{
public : Base(int i)
{
printf("constru ctor\n");
}
void disp()
{
printf("display ed...\n");
}
virtual ~Base()
{
}
};

int main()
{
Base obj();
// obj.disp();
return(0);
}

What does the first line in main() function mean?

You are declaring the existence of a function named 'obj' which takes no
parameters and returns an instance of a Base object.

The second commented line causes a compilation error saying "left of
'.disp' must have class/struct/union type"

Yep. obj isn't a class/struct/union. It's a function (which has no body
yet).

The extra parentheses mean something. Without the parens, the first line
becomes the definition of an object named 'obj' of type Base.

[Howard]

"Salt_Peter " <pj*****@yahoo. comwrote in message
news:5695f89f-5d29-4235-a789-

and this statement tries to run a function
Base obj();

No, it declares a function named obj which takes no parameters and returns
an object of type Base. That's different from actually calling such a
function. (And I suspect it will lead to a linker error, unless you provide
a function body for it somewhere.)

-Howard

[Rahul]

On Jan 9, 9:07 am, "Howard" <[e-mail] rote:

"Salt_Peter " <pj_h...@yahoo. comwrote in message

news:5695f89f-5d29-4235-a789-

and this statement tries to run a function
Base obj();

No, it declares a function named obj which takes no parameters and returns
an object of type Base. That's different from actually calling such a
function. (And I suspect it will lead to a linker error, unless you provide
a function body for it somewhere.)

-Howard

A prototype specification alone doesn't cause any linker error, may be
when the actual call is done and the library is not found...