Re: copy ctor being private and compilation error

From:

Kai-Uwe Bux <jkherciueh@gmx.net>

Newsgroups:

comp.lang.c++

Date:

Tue, 03 Aug 2010 08:37:21 +0200

Message-ID:

<i38df2$kge$1@news.doubleSlash.org>

Ian Collins wrote:

On 08/ 3/10 04:12 PM, subramanian100in@yahoo.com, India wrote:

Consider the following program x.cpp:

<snip>

Just to be more explicit, I recall the declaration of Test:

  class Test
  {
  public:
        explicit Test(int arg = 10);
        ostream& writeTo(ostream& os) const;
  private:
        Test(const Test& rhs);
        int val;
  };

Note the private copy constructor.

int main()
{
         const Test& ref = Test();
         cout<< ref<< endl;

         return EXIT_SUCCESS;
}

When I compile this program as
g++ -std=c++98 -pedantic -Wall -Wextra x.cpp

I get the following compilation error:
x.cpp: In function `int main()':
x.cpp:12: error: `Test::Test(const Test&)' is private
x.cpp:33: error: within this context

Though the copy ctor of Test is private, the statement
         const Test& ref = Test();
involves only taking a 'const reference' to the temporary Test object
created by Test() on the RHS. Since only a reference is made, why does
this statement require the copy ctor ?

It appears to be a bug in the version (3.4.x?) of gcc you are using.

It's not a bug. The code is invalid in C++98 and C++03 by [8.5.3/5] which
requires the copy constructor to be visibile. The compiler is free (but not
required) to make a copy of the object before initializing the reference.
The bug is in newer g++ versions (starting with the 4.3 series or
thereabout).

The code will be legal in C++0x.

Best

Kai-Uwe Bux