Question

I have a working iterator for MFC CObList – BaseMFCIter. It works for iterating in loop but i still didn’t managed to make ListIter to work properly with STL algorithm find_if.
Code

#include < iterator >
#include "afxwin.h"
#include "afxtempl.h"
#include <iostream>
#include <algorithm>
#include <cstdlib> 
class myCObject : public CObject
{
public:
    myCObject( std::string val )
    {
        x = val;
    }
    std::string x;
};
template < typename Item, class Cont, class Key = POSITION >
class BaseMFCIter : public std::iterator < std::input_iterator_tag, Item >
{
public:
    // Define types for the 2 member functions to be used:
    typedef Key  (Cont::*GetFirstFunctionPtr) ()     const;
    typedef Item    (Cont::*GetNextFunctionPtr)  (Key&) const;


    // Default constructor, makes a null iterator, equal to BaseMFCIter::end()
    BaseMFCIter() : m_pCont(0), m_Pos(0), m_GetFirstFunc(0), m_GetNextFunc(0), m_End(true) {}

    // Constructor taking pointer to container and the iteration functions
    BaseMFCIter(Cont* pCont, GetFirstFunctionPtr pFF, GetNextFunctionPtr pNF) 
        : m_pCont(pCont), m_Pos(0), m_GetFirstFunc(pFF), m_GetNextFunc(pNF)
    { init(); }

    // Copy constructor, initialises iterator to first element
    BaseMFCIter(const BaseMFCIter& vi) : m_pCont(vi.m_pCont), m_Pos(0),
        m_GetFirstFunc(vi.m_GetFirstFunc), m_GetNextFunc(vi.m_GetNextFunc)
    { init(); }

    // Assignment operator, initialises iterator to first element
    BaseMFCIter& operator=(const BaseMFCIter& vi)
    {
        m_pCont     = vi.m_pCont; 
        m_GetFirstFunc  = vi.m_GetFirstFunc;
        m_GetNextFunc   = vi.m_GetNextFunc;
        init();  
        return *this; 
    }

    bool operator == (const BaseMFCIter& rhs) const
    { return (m_Pos == rhs.m_Pos && m_End == rhs.m_End); }

    bool operator != (const BaseMFCIter& rhs) const 
    { return !operator==(rhs); }

    BaseMFCIter&    operator ++ ()    { advance(); return *this; }
    BaseMFCIter&    operator ++ (int) { BaseMFCIter ret(*this); advance(); return ret; }
    Item            operator *  ()    { return m_Item; }
    Item            operator -> ()    { return m_Item; }

    static BaseMFCIter end   ()    { return BaseMFCIter(); }    // end() returns default null iterator

private:
    Item     m_Item;      // Current item from container
    Cont* m_pCont;     // Pointer to container
    Key  m_Pos;       // Key to item in container
    bool     m_End;       // Flag to indicate end of container reached

    // Pointers to container iteration functions
    GetFirstFunctionPtr m_GetFirstFunc;
    GetNextFunctionPtr  m_GetNextFunc;

    // Use container GetFirst & GetNext functions to set to first element, or end() if not found
    void init() 
    {
        m_Pos = 0;
        m_End = true;

        if (m_pCont && m_GetFirstFunc != 0)
        {
            m_Pos = (m_pCont->*m_GetFirstFunc)();
            advance();
        }
    }

    // Use container GetNext function to find next element in container
    void advance()
    {
        m_End = m_Pos ? false : true;
        m_Item = (m_Pos && m_pCont && m_GetNextFunc != 0) ? 
            (m_pCont->*m_GetNextFunc)(m_Pos) : Item();
    }
};
    struct Container : public CObList
    {
        myCObject*       GetNext(POSITION& rPosition)
        {
            return dynamic_cast<myCObject*>(CObList::GetNext(rPosition));
        }
        myCObject const* GetNext(POSITION& rPosition) const
        { 
            return dynamic_cast<const myCObject*>(CObList::GetNext(rPosition)); 
        }
    };


    class ListIter : public BaseMFCIter < const myCObject*, Container, POSITION  >
    {
    public:
        ListIter( Container* pObj = 0)  
            : BaseMFCIter< const myCObject*, Container, POSITION >
                (pObj, &CObList::GetHeadPosition, &Container::GetNext)
        {
        }
    };

    struct Comparator
    {
        std::string stringToCompare;
        bool operator() ( const myCObject* lhs )
        {
            return (bool) lhs->x.compare( stringToCompare );
        }
    };

    void main( )
    {


        myCObject* m = new myCObject( "one" );
        myCObject* n = new myCObject( "two" );
        myCObject* p = new myCObject( "three" );
        myCObject* q = new myCObject( "four" );

        Container cont;
        cont.AddHead( m );
        cont.AddHead( n );
        cont.AddHead( p );
        cont.AddHead( q );

        Comparator pred;
        pred.stringToCompare = "1";
        ListIter iter = ListIter( &cont );
        ListIter endIter = ListIter( );
        ListIter foundIter = std::find_if( iter, endIter, pred );

        std::cout << "foundIter x is: " << foundIter->x.c_str() << std::endl;

    }

gives me foundIter x is: four. This propably happens because of the way the end position is defined so

_InIt _Find_if(_InIt _First, _InIt _Last, _Pr _Pred)
    {   // find first satisfying _Pred
    _DEBUG_RANGE(_First, _Last);
    _DEBUG_POINTER(_Pred);
    for (; _First != _Last; ++_First)
        if (_Pred(*_First))
            break;
    return (_First);
    }

doesn’t iterate properly but i can’t figure out how to fix it.

Answer 1

A number of issues fixed:

• (bool) lhs->x.compare( stringToCompare ) returns true _whenever the string don’t match** (see string::compare)
• you were searching for “1”, which doesn’t exist
• since the predicate was wrong, you received the first match, which was the first element, also inserted the last, and the name was "four" 🙂
• you didn’t check whether a valid match was found (dereferencing the end-iterator is illegal and may crash your program or do worse things: undefined behaviour)
• you had a superflous x.c_str() in the output statement

• I changed the Compare predicate around to be more idiomatic:

  • initialize stringToCompare from the constructor
  • make the field const
  • make the operator() a const method

This should do the trick (untested code, I’m not near a compiler the coming hours)

Update

After arriving home, I finally broke out the debugger to track that strange behaviour (see comments).

To my dismay, I found out that the BaseMFCIter was designed by someone with very limited understanding of what an iterator is: the copy constructor and assignment operator were completely wrong: they had the effect of creating a new begin iterator – for the same collection. This however, means that an iterator could never be returned from a function.

Therefore, I fixed it (first by implementing it right, later by removing the now-redundant constructor and operator= in favour of the compiler-generated default implementations).

See the full history of my and your edits:

git clone git://gist.github.com/1353471.git

• sehe 11 minutes ago rely on default generated copy constructor and assignment instead
• sehe 12 minutes ago fixed broken copy constructor and assignment
• sehe 65 minutes ago tentative
• Dmitry 73 minutes ago Attempt at find_if with predicate
• sehe Heeren 25 hours ago Fixed and Tested (VS2010)
• sehe 25 hours ago ( STL iterator for MFC container CObList )

 

#include "afxwin.h"
#include "afxtempl.h"
#include <iostream>
#include <algorithm>
#include <cstdlib> 
#include <string>
#include <iterator>

class myCObject : public CObject
{
public:
    myCObject( const std::string& val ) { x = val; }
    std::string x;
};

template < typename Item, class Cont, class Key = POSITION >
class BaseMFCIter : public std::iterator < std::input_iterator_tag, Item >
{
public:
    // Define types for the 2 member functions to be used:
    typedef Key  (Cont::*GetFirstFunctionPtr) ()     const;
    typedef Item    (Cont::*GetNextFunctionPtr)  (Key&) const;

    // Default constructor, makes a null iterator, equal to BaseMFCIter::end()
    BaseMFCIter() : m_pCont(0), m_Pos(0), m_GetFirstFunc(0), m_GetNextFunc(0), m_End(true) {}

    // Constructor taking pointer to container and the iteration functions
    BaseMFCIter(Cont* pCont, GetFirstFunctionPtr pFF, GetNextFunctionPtr pNF) 
        : m_pCont(pCont), m_Pos(0), m_GetFirstFunc(pFF), m_GetNextFunc(pNF)
    { init(); }

    bool operator == (const BaseMFCIter& rhs) const
    { return (m_Pos == rhs.m_Pos && m_End == rhs.m_End); }

    bool operator != (const BaseMFCIter& rhs) const 
    { return !operator==(rhs); }

    BaseMFCIter&    operator ++ ()    { advance(); return *this; }
    BaseMFCIter&    operator ++ (int) { BaseMFCIter ret(*this); advance(); return ret; }
    Item            operator *  ()    { return m_Item; }
    Item            operator -> ()    { return m_Item; }

    static BaseMFCIter end   ()    { return BaseMFCIter(); }    // end() returns default null iterator

private:
    Item  m_Item;   // Current item from container
    Cont* m_pCont;  // Pointer to container
    Key   m_Pos;    // Key to item in container
    bool  m_End;    // Flag to indicate end of container reached

    // Pointers to container iteration functions
    GetFirstFunctionPtr m_GetFirstFunc;
    GetNextFunctionPtr  m_GetNextFunc;

    // Use container GetFirst & GetNext functions to set to first element, or end() if not found
    void init() 
    {
        m_Pos = 0;
        m_End = true;

        if (m_pCont && m_GetFirstFunc != 0)
        {
            m_Pos = (m_pCont->*m_GetFirstFunc)();
            advance();
        }
    }

    // Use container GetNext function to find next element in container
    void advance()
    {
        m_End = m_Pos ? false : true;
        m_Item = (m_Pos && m_pCont && m_GetNextFunc != 0) ? 
            (m_pCont->*m_GetNextFunc)(m_Pos) : Item();
    }
};

struct Container : public CObList
{
    myCObject* GetNext(POSITION& rPosition)
    {
        return dynamic_cast<myCObject*>(CObList::GetNext(rPosition));
    }
    myCObject const* GetNext(POSITION& rPosition) const
    { 
        return dynamic_cast<const myCObject*>(CObList::GetNext(rPosition)); 
    }
};


class ListIter : public BaseMFCIter < const myCObject*, Container, POSITION  >
{
public:
    ListIter( Container* pObj = 0)  
        : BaseMFCIter< const myCObject*, Container, POSITION >
            (pObj, &CObList::GetHeadPosition, &Container::GetNext)
    {
    }
};

struct Comparator
{
    Comparator(const std::string& compareTo) : stringToCompare(compareTo) {}

    bool operator() ( const myCObject* lhs ) const
    {
        return 0 == lhs->x.compare( stringToCompare );
    }
  private:
    const std::string stringToCompare;
};

void main( )
{
    myCObject* m = new myCObject( "one" );
    myCObject* n = new myCObject( "two" );
    myCObject* p = new myCObject( "three" );
    myCObject* q = new myCObject( "four" );

    Container cont;
    cont.AddHead( m );
    cont.AddHead( n );
    cont.AddHead( p );
    cont.AddHead( q );

    Comparator pred("three");
    ListIter iter = ListIter(&cont),
             endIter = ListIter( );

    ListIter foundIter = std::find_if( iter, endIter, pred );

    if (endIter != foundIter)
    {
        std::cout << "foundIter x is: " << foundIter->x << std::endl;
    }
    else
    {
        std::cout << "not found" << std::endl;
    }
}