C++ Creating an instance of a class?

Question

Possible Duplicate:
Default constructor with empty brackets
Instantiate class with or without parentheses?

Program:

class Foo
{
   public:
      Foo ( int bar = 1 )
      {
         cout << "bar=" << bar;
      }
};

int main() {

   cout << "0 - ";
   Foo foo_0 ( 0 ) ;
   cout << '\n';

   cout << "1 - ";
   Foo foo_1 ();
   cout << '\n';

   cout << "2 - ";
   Foo foo_4;
   cout << '\n';

   return 0;

}

Output:

0 - bar=0
1 - 
2 - bar=1

Question: why example #1 does not works, while examples #0 and #2 do?

score 7 · Accepted Answer · answered Sep 10 '12 at 08:51

7

Foo foo_1 ();

is a function declaration, no object is created. It's a function called foo_1 that takes no parameters and returns a Foo object.

The correct way to construct an object there is

Foo foo1;

This concept is called C++'s vexing parse. A short description is that anything that can be treated as a declaration, is.

answered Sep 10 '12 at 08:51

Luchian Grigore

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thanks! Comprehensive answer as usual! – Kolyunya Sep 10 '12 at 08:55
btw, exmaple #0 is correct too, right? – Kolyunya Sep 10 '12 at 08:56

score -2 · Answer 2 · answered Sep 10 '12 at 09:15

-2

foo_1(); in other words it's known as Null Intialize you must know difference between the object creation and value intialization.

answered Sep 10 '12 at 09:15

Chethan R

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2

C++ Creating an instance of a class?

2 Answers2