Re: Insert object with const member in vector

From:

Paul Bibbings <paul.bibbings@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Sat, 17 Jul 2010 16:15:56 +0100

Message-ID:

<87iq4ennpv.fsf@gmail.com>

Joe Hesse <JoeHesse@gmail.com> writes:

On Sat, 17 Jul 2010 14:30:57 +0100, Paul Bibbings wrote:

Joe Hesse <JoeHesse@gmail.com> writes:

I added the following assignment operator and it still doesn't work.

  Data & operator == (const Data &d) {
    string *ps = const_cast<string *>(&data); *ps = d.data;
    return *this;
  }

You call this an assignment operator (op=), but what you have given is a
comparison operator (op==).

Regards

Paul Bibbings

Sorry to all. I made a mistake and should have written operator =, not
operator ==. Everything works.

But... Don't forget to take a close look at what Leigh is saying in his
reply. To give a (contrived) example:

   void increment(const int& i)
   {
      const_cast<int&>(i)++;
   }

   int main()
   {
      int i = 1;
      increment(i); // okay: i, non-const

      const int j = 1;
      increment(j); // undefined behaviour: j, const
   }

For the reference, see [expr.const.cast] ?5.2.11/7:

   "[Note: Depending on the type of the object, a write operation
   through the pointer, lvalue or pointer to data member resulting from
   a const_cast that casts away a const qualifier may produce undefined
   behavior (7.1.5.1). ]"

and [dcl.type.cv] ?7.1.5.1 for details.

Regards

Paul Bibbings