Creating new instance of class on stack with no empty no arguments constructor

Question

Possible Duplicates:
Calling the default constuctor
Why is it an error to use an empty set of brackets to call a constructor with no arguments?

I am wondering what it means to create an instance of a class using a no arguments constructor.

For example if I have a class "A" and try to create a variable like so:

A myVariable()

I tried to put together a little project to test it:

#include <iostream>

using namespace std;

class A
{
    public:
        A();
        A( int a, int b );
    public:
        int mA;
        int mB;
    private:
        virtual void init( int a , int b );
};

A::A()
{
    cout << "\tA()" << endl;
    this->init( 0, 0 );
} // end of function A::A

A::A( int a, int b = 0 )
{
    cout << "\tA( int a, int b )" << endl;
    this->init( a, b );
} // end of function A::A

void A::init( int a, int b )
{
    cout << "\tInit( int a, int b )" << endl;
    mA = a;
    mB = b;
} // end of function A::init

int main()
{
cout << "Start" << endl;
    cout << "A myA0()" << endl;
    A myA0();

    cout << "A myA1" << endl;
    A myA1;

    cout << "A myA2( 0 )" << endl;
    A myA2( 0 );

    cout << "A myA3( 0, 0 )" << endl;
    A myA3( 0, 0 );

    cout << "Stop" << endl;
    return 0;
}

The output looks like this:

Start
A myA0()
A myA1
    A()
    Init( int a, int b )
A myA2( 0 )
    A( int a, int b )
    Init( int a, int b )
A myA3( 0, 0 )
    A( int a, int b )
    Init( int a, int b )
Stop

So it doesn't seem to call any constructor. When I try to step through it in Visual Studio it just skips over it like it wasn't compiled and when I try to print out the variable i get an unknown extern symbol error.

Note: Of course when using the new operator doing "new A()" is required and will use the default constructor.

This question has been asked a million times. It's just a matter of finding the dupes. — Jon, Mar 23 '11 at 15:39
Have a look at this: http://stackoverflow.com/questions/180172/why-is-it-an-error-to-use-an-empty-set-of-brackets-to-call-a-constructor-with-no — razlebe, Mar 23 '11 at 15:41
Also this one: http://stackoverflow.com/questions/5300124/calling-the-default-constructor — Jon, Mar 23 '11 at 15:41
`A myA0();` defines a function called `myA0` with a return type of `A` - or at least that's what the compiler thinks - search for the "the most vexing parse". — Nim, Mar 23 '11 at 15:43
Thanks for all the comments. Sorry I tried to search for it first but didn't know what language to use in the search. — drewag, Mar 23 '11 at 15:53

score 4 · Accepted Answer · answered Mar 23 '11 at 15:42

The syntax:

A myA0();

Does not create a variable of type A by calling the default constructor, but rather declares a function taking no arguments and returning A by value. Lookup in the tags for most-vexing-parse or something similar. To create a local variable just do:

A myA0; // no ()!!!

A different issue that you have to be aware is that a virtual method called from the constructor, or destructor will not use dynamic dispatch. The call to init inside the A constructor is a call to A::init even if you are constructing an object of type B derived from A:

struct B : A {
   void init( int, int ) { std::cout << "B::init(int,int)" << std::endl; }
};
int main() {
   B b;      // will print "\tInit( int a, int b )" from the constructor of `A`
   b.init(); // will print "B::init(int,int)"
}

Creating new instance of class on stack with no empty no arguments constructor

1 Answers1