Re: Overloaded [] operator + template

From:

"Cy Edmunds" <spamless.cedmunds@rochester.rr.com>

Newsgroups:

comp.lang.c++

Date:

Tue, 04 Jul 2006 16:49:51 GMT

Message-ID:

<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>'

I tried a number of code permutations but ended up with more errors (I am
a beginner :-),

Thanks,
Hans

#include <iostream>
using namespace std;

template<int len> class bool_vector;
template<int len> ostream &operator<< (ostream &os, const
bool_vector<len> &v);

template<int len> class bool_vector {
private:
   bool *v;
   int i;
public:
   int sz;
   bool_vector() {
       v=new bool[sz=len];
       for (i=0; i<sz; i++) v[i]=0;
   };
   ~bool_vector() {
       delete [] v;
   };

   friend ostream& operator << <>(ostream& os, const bool_vector<len>&);
// required?

   void write(const string& initstr) {
       for (i=0; i<len; i++) v[i]=initstr[i]-'0';
   }

   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