What is the difference between Object b(); and Object b;?

Question

To be more explicit, I get a compile time error when I try accessing an instance variable when I create an object using (), but when I don't, the code compiles and runs as expected. Also, this problem only applies to the default constructor. I would like to understand why.

using namespace std;
#include <iostream>

class Student {

  public:

    int gpa;

    Student() { 
      gpa = 4;
    }

    Student( int x ) { 
      gpa = x; 
    }

};

int main() {

  Student zero;
  Student sally( 2 ); 
  Student jack();

  cout << zero.gpa << endl; //prints 4
  cout << sally.gpa << endl; // prints 2
  cout << jack.gpa << endl; //error: request for member 'gpa' in 'jack', which is of non-class type 'Student()'

}

This is not an issue of trying to get around the problem. I want to understand why I run into the error in the first place. — Kacy Raye, Jun 28 '13 at 20:46
@huseyintugrulbuyukisik Why should he? Automatic lifetime beats dynamic lifetime whenever it is applicable. — , Jun 28 '13 at 20:47
This has been answered here as well: http://stackoverflow.com/questions/180172/default-constructor-with-empty-brackets — Derek Peterson, Jun 28 '13 at 20:49
@delnan, you mean it gets auto deleted when out of scope? Just like a simple float var; ? — huseyin tugrul buyukisik, Jun 28 '13 at 21:10
Yes. Using `new` allocates the memory on the heap rather than on the stack, which means that it has to be explicitly deleted later on. — Derek Peterson, Jun 28 '13 at 21:36

sasha.sochka · Accepted Answer · 2013-07-21T15:37:47.930

8

The problem is that Student jack(); declares a function with Student as a return type. It doesn't declare an object of that class as you expect.

edited Jul 21 '13 at 15:37

answered Jun 28 '13 at 20:46

sasha.sochka

14,395
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ohhh! That's so subtle. I thought it had something to with that, but wasn't sure. Thanks. – Kacy Raye Jun 28 '13 at 20:48

score 6 · Answer 2 · answered Jun 28 '13 at 20:47

6

  Student jack();

declares a function that returns student and takes no arguments. Not an object!

See more in this gotw

answered Jun 28 '13 at 20:47

Balog Pal

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score 3 · Answer 3 · answered Jun 28 '13 at 20:48

3

"Object b();" declares a function b() returning an object of type Object, while "Object b;" defines a variable b of type Object.

No, it's not obvious, and it still comes back to bite me if I switch between C++, Java, and C#. :-)

answered Jun 28 '13 at 20:48

Steve

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4
10

score 2 · Answer 4 · answered Jun 28 '13 at 20:49

What is the difference between Object b(); and Object b;?

The difference exists because C++ interprets that as a function being declared, instead of an object being created.

Object b;

This is the object b of class Object being created by means of the default constructor.

Object b();

This is the function b(), being declared (it will be defined elsewhere) to return an object of class Object, and no parameters.

Hope this helps.

score 0 · Answer 5 · answered Jun 28 '13 at 20:49

0

I would try this

class Student {

public:

int gpa = 4;

Student() { };

answered Jun 28 '13 at 20:49

GManika

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What is the difference between Object b(); and Object b;?

5 Answers5

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