what does the following code do

Question

2.33/5 (3 votes)

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For the following C++ code :

C++

class X{
    public :
    X() {cout<<"constr" ;}
} ;

main () {
    X obj() ;
}

What happens during 'X obj()'. It does not invoke the default argument-less constructor. In fact this statement does not create an object at all because after this statement you can invoke any X member on obj. But neither does the statement give a compilation error.

Posted 29-Jan-14 1:06am

Member 10544034

Updated 29-Jan-14 4:15am

v2

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phil.o 29-Jan-14 10:24am

Please do not change the content of your original question after it has been answered; it can make the answer completely useless, if not false.

Richard MacCutchan 29-Jan-14 13:05pm

Compiling the above produces

visual studio\projects.c++\cpptest\cpptest\cpptest.cpp(71): warning C4930: 'X obj(void)': prototyped function not called (was a variable definition intended?)

.

3 solutions

Solution 2

You are defining a function prototype in main, that is why no object is being created:

C#

main () 
{
    X obj() ;
}

To correct this, use this syntax to create your object:

C#

main () 
{
    X obj;
}

This is one of the oddities that appears in C++ parsing. If your constructor had parameters, then the parenthesis would be appropriate to place around your construction call. Don't use parenthesis for a default constructed object.

Posted 29-Jan-14 12:38pm

Paul M Watt

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Member 10544034 · Accepted Answer · 2014-01-29T17:26:00

I fully agree that while creating a class instance via a default constructor, we do not use parenthesis, i.i., we create an instance of class X as follows :

C++

X  obj ;

But what I am confused about is this : what happens when we give the following

C++

X obj() ;

This statement does not give a compile-time error.
This statement does not call the default constructor of the class for obj either.
After this statement, a class member can not be accessed on obj - compiler says that request for member in ‘obj’, which is of non-class type ‘X()’.

So what is actually happening at this statement if the object is not being created.

pasztorpisti · Accepted Answer · 2014-01-29T03:51:00

Solution 1

This class has an automatically generated default constructor because you haven't provided any other constructors. What the automatically generated constructor does is the following: It just calls the default constructor of the base classes and the non-pod data members of the class. Since this class doesn't have any base classes or any non-pod data members the auto-generated default constructor does nothing. When compiled with optimization this constructor call is most probably optimized away.

Posted 29-Jan-14 3:51am

pasztorpisti

Comments

Pete O'Hanlon 29-Jan-14 9:53am

Well, that was the answer I was going to give. Great job. +5.

pasztorpisti 29-Jan-14 10:07am

Thank you!

Aescleal 31-Jan-14 14:12pm

Unless the questioner has edited the question (I didn't check) there is a default constructor. The problem is he/she/it's not creating an object in main, they're declaring a function.

Aescleal 31-Jan-14 14:13pm

Just seen the tit edited the question to add a default constructor. Why the muppet didn't ask a completely new question is beyond me...

Changed the 1 I gave you to a 5. Sorry about that...

pasztorpisti 31-Jan-14 15:05pm

No problem, with the "new question" the answer is indeed invalid. :-)