Re: Overloaded [] operator + template

From:

Kai-Uwe Bux <jkherciueh@gmx.net>

Newsgroups:

comp.lang.c++

Date:

Tue, 04 Jul 2006 23:26:27 +0200

Message-ID:

<e8emeq$gs2$1@murdoch.acc.Virginia.EDU>

Hans wrote:

"Cy Edmunds" <spamless.cedmunds@rochester.rr.com> wrote in message
news:P_wqg.12178$O35.3088@twister.nyroc.rr.com...

"Hans" <hans64@ht-lab.com> wrote in message
news:Iywqg.20130$1g.8617@newsfe1-win.ntli.net...

Hi All,

Can anybody explain how to fix the [] operator in the code below, Visual
C++ gives the following error message:

error C2678: binary '[' : no operator found which takes a left-hand
operand of type 'const bool_vector<len>'

[snip]

bool operator[] (const int& x){
return v[x];

Should be:
   bool operator[] (const int& x) const // note "const"
   {
        return v[x];
   }

[snip]

Your class has other problems too. It needs both a copy constructor and
an assignment operator. Otherwise if you copy this object the pointer
will be deleted twice causing the dreaded undefined behavior.

From a style point of view, I suggest getting out of the habit of adding
; at the end of function declarations and (more importantly) avoid public
data items.

Cy

Hi Cy/Kai-Uwe,

That fixed the problem although I don't fully understand why. My current
understanding is that const tells the compiler that the function doesn't
modify the state of the class (variables?), but if you don't change them
(like I do in the bool operator[] function) then why would you get a
compile error?

Have a look at your definition of operator<<:

  template<int len>
  ostream& operator << (ostream& os, const bool_vector<len>& v) {
    for (int i=0;i<v.sz;i++) os << v[i]; // **** Error C2678 ****
    return os;
  }

In the signature, you promise that the bool_vector reference that you pass
will be const. In the body of the function, you call a method on that
object that is not declared const, i.e., a method that potentially alters
the object. The compiler just takes your word for it, it does not check on
itself whether operator[] maybe secretly const.

Best

Kai-Uwe Bux