Code not being generated for a constructor call

[adrian_h]

Hi,

Strange thing. I was trying to do some code and it wasn't working. Tried it in several different ways and still no luck. Finally, tried to make a very small case where it was failing, and I got this:

#include <iostream>
using namespace std;

class X
{
public:
	int x;
	X(int x) : x(x) { cout << "X(int x=" << x << ")" << endl; }
	X(X const & x) : x(x.x) { cout << "X(X const & x={" << x.x << "})" << endl; }
};
class Y : public X
{
public:
	Y(int x) : X(x) { cout << "Y(int x=" << x << ")" << endl; }
	Y(X const & x) : X(x) { cout << "Y(X const & x={" << x.x << "})" << endl; }
};


int main()
{
	cout << "1----" << endl;
	X x(3); // code generated
	cout << "2----" << endl;
	Y y(X(x)); // code not generated.  Expecting call to X(X const &) constructor then Y(X const &)
	cout << "3----" << endl;
	X x1(x); // code generated
	cout << "4----" << endl;
	Y y1(x1); // code generated
	cout << "5----" << endl;
	return 0;
}

Expecting to get output:

1----
X(int x=3)
2----
X(X const & x={3})
Y(X const & x={3})
3----
X(X const & x={3})
4----
X(X const & x={3})
Y(X const & x={3})
5----

but instead get:

1----
X(int x=3)
2----
3----
X(X const & x={3})
4----
X(X const & x={3})
Y(X const & x={3})
5----

Stepping through the code, it doesn't stop on Y y(X(x)); yet by explicitly making a variable of type X and passing it to constructor Y(X const &) it works. Only difference is that #2 uses a temporary var and #4 uses a standard var. Looking at the disassembly, it shows no code generated for that line.

This is very confusing . Any ideas? I'm thinking it could be the compiler puking. If someone could compile this on their system and verify the results, that would be grand.

Thanks.


A

Using g++ (Ubuntu 4.4.1-4ubuntu9) 4.4.1 with Eclipse Version: 3.5.1 Build id: M20090917-0800

[monarch_dodra]

Y y(X(x));

Be alert for C++'s most vexing parse.

--Scott Meyers

Are you sure that's a class object you are instantiating there...?

[jnmacd]

I think this is it right?...
http://www.parashift.com/c++-faq-lit....html#faq-10.2

[monarch_dodra]

I think this is it right?...
http://www.parashift.com/c++-faq-lit....html#faq-10.2

More or less:

This is a function declaration:

Y y(X x);

Now in C++, you are allowed to put parenthesis around parameter names, so it is equivalent to:

Y y(X(x));

The problem is that this can be both a function declaration, or an object instantiation. Unfortunately, the function declaration has "right of way"

To "fix" this, you can add an extra disambiguate parenthesis:

Y y((X(x)));

Now that "X(x)" is between parenthesis, there is no way for the above to be a function declaration, therefore it must be an object instantiation.

[adrian_h]

That's interesting. But isn't it illegal to declare a function in a function?


A

[Lindley]

That's interesting. But isn't it illegal to declare a function in a function?


A

Define, yes. Declare, no.

Not that anyone should do such a thing, but it's legal.

[adrian_h]

Oh yeah. Now I remember. Stupid C legacy.

Thanks all!


A

[adrian_h]

OT but how come my editor doesn't have the buttons for formatting?

Using Google Chrome 12.0.742.124