Question

I’m writing an own container class and have run into a problem I can’t get my head around. Here’s the bare-bone sample that shows the problem.

It consists of a container class and two test classes: one test class using a std:vector which compiles nicely and the second test class which tries to use my own container class in exact the same way but fails miserably to compile.

#include <vector>
#include <algorithm>
#include <iterator>

using namespace std;

template <typename T>
class MyContainer
{
public:
  
  class iterator
  {
  public:
    typedef iterator self_type;
    inline iterator() { }
  };

  class const_iterator
  {
  public:
    typedef const_iterator self_type;
    inline const_iterator() { }
  };
  
  iterator begin() {
    return iterator();
  }

  const_iterator begin() const {
    return const_iterator();
  }
};

// This one compiles ok, using std::vector
class TestClassVector
{
public:
  void test() {
    vector<int>::const_iterator I=myc.begin();
  }

private:
  vector<int> myc;
};

// this one fails to compile. Why?
class TestClassMyContainer
{
public:
  void test(){
    MyContainer<int>::const_iterator I=myc.begin();
  }

private:
  MyContainer<int> myc;
};


int main(int argc, char ** argv)
{
  return 0;
}

gcc tells me:

test2.C: In member function ‘void TestClassMyContainer::test()’:

test2.C:51: error: conversion from ‘MyContainer::iterator’ to non-scalar type ‘MyContainer::const_iterator’ requested

I’m not sure where and why the compiler wants to convert an iterator to a const_iterator for my own class but not for the STL vector class. What am I doing wrong?

Answer 1

When you call begin() the compiler by default creates a call to the non-const begin(). Since myc isn’t const, it has no way of knowing you mean to use the const begin() rather than the non-const begin().

The STL iterator contains a cast operator which allows an iterator to be silently converted to a const_iterator. If you want this to work you need to add one as well like so:

class iterator
{
public:
    typedef iterator self_type;
    inline iterator() { }

    operator const_iterator() { return const_iterator(); }
};

or allow const_iterator to be constructed from an iterator like so:

class const_iterator
{
public:
    typedef const_iterator self_type;

    const_iterator(iterator& ) {}
    inline const_iterator() { }
};