default constructor == constructor without argument?

This is a discussion on default constructor == constructor without argument? within the C++ Programming forums, part of the General Programming Boards category; If an object is instantiated without brackets, is it the same as having empty brackets? Code: class A { ... ...

  1. #1
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    default constructor == constructor without argument?

    If an object is instantiated without brackets, is it the same as having empty brackets?

    Code:
    class A { ... };
    A a;
    Code:
    class A { ... };
    A a();
    Are they the same?

    The only way I can get different "behaviours" out of them is

    Code:
    class A {
    	A() {}
    };
    
    int main() {
    	A a();
    }
    vs
    Code:
    class A {
    	A() {}
    };
    
    int main() {
    	A a;
    }
    Note that the constructor is private. Both work if it's declared public.

    The first one compiles fine, but the second one gives
    cyberfish@cyberfish-desktop:/tmp$ g++ a.cpp
    a.cpp: In function ‘int main()’:
    a.cpp:2: error: ‘A::A()’ is private
    a.cpp:6: error: within this context
    Anyone care to enlighten me?

    Thanks

  2. #2
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    The first version doesn't declare an object of type A. It declares a function which returns an object of type A.

    Try this and see:
    Code:
    class A {
        A() {}
        public:
        void foo() {}
    };
    
    int main()
    {
        A a();
        a.foo();
    }

  3. #3
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    Uh...

    Nice call!

    Too much Java for me recently.

    Thanks for pointing it out.

  4. #4
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    What if it's allocated on the heap?

    Code:
    class A {
        public:
        A() {}
        void foo() {}
    };
    
    int main()
    {
        A *a = new A();
        a->foo();
    }
    Code:
    class A {
        public:
        A() {}
        void foo() {}
    };
    
    int main()
    {
        A *a = new A;
        a->foo();
    }
    They both compile. Are they the same?

  5. #5
    Algorithm Dissector iMalc's Avatar
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    For objects it is the same.
    For built-in types however, using () zero-initialises the variable.
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  6. #6
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    I see. Thanks.

  7. #7
    C++まいる!Cをこわせ! Elysia's Avatar
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    Using () would zero-initialize any POD-struct as well.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

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