I have a custom container class for which I'd like to write the iterator
and const_iterator
classes.
I never did this before and I failed to find an appropriate how-to. What are the guidelines regarding iterator creation, and what should I be aware of ?
I'd also like to avoid code duplication (I feel that const_iterator
and iterator
share many things; should one subclass the other ?).
Foot note: I'm pretty sure Boost has something to ease this but I can't use it here, for many stupid reasons.
This question is related to
c++
iterator
const-iterator
There are plenty of good answers but I created a template header I use that is quite concise and easy to use.
To add an iterator to your class it is only necessary to write a small class to represent the state of the iterator with 7 small functions, of which 2 are optional:
#include <iostream>
#include <vector>
#include "iterator_tpl.h"
struct myClass {
std::vector<float> vec;
// Add some sane typedefs for STL compliance:
STL_TYPEDEFS(float);
struct it_state {
int pos;
inline void begin(const myClass* ref) { pos = 0; }
inline void next(const myClass* ref) { ++pos; }
inline void end(const myClass* ref) { pos = ref->vec.size(); }
inline float& get(myClass* ref) { return ref->vec[pos]; }
inline bool cmp(const it_state& s) const { return pos != s.pos; }
// Optional to allow operator--() and reverse iterators:
inline void prev(const myClass* ref) { --pos; }
// Optional to allow `const_iterator`:
inline const float& get(const myClass* ref) const { return ref->vec[pos]; }
};
// Declare typedef ... iterator;, begin() and end() functions:
SETUP_ITERATORS(myClass, float&, it_state);
// Declare typedef ... reverse_iterator;, rbegin() and rend() functions:
SETUP_REVERSE_ITERATORS(myClass, float&, it_state);
};
Then you can use it as you would expect from an STL iterator:
int main() {
myClass c1;
c1.vec.push_back(1.0);
c1.vec.push_back(2.0);
c1.vec.push_back(3.0);
std::cout << "iterator:" << std::endl;
for (float& val : c1) {
std::cout << val << " "; // 1.0 2.0 3.0
}
std::cout << "reverse iterator:" << std::endl;
for (auto it = c1.rbegin(); it != c1.rend(); ++it) {
std::cout << *it << " "; // 3.0 2.0 1.0
}
}
I hope it helps.
They often forget that iterator
must convert to const_iterator
but not the other way around. Here is a way to do that:
template<class T, class Tag = void>
class IntrusiveSlistIterator
: public std::iterator<std::forward_iterator_tag, T>
{
typedef SlistNode<Tag> Node;
Node* node_;
public:
IntrusiveSlistIterator(Node* node);
T& operator*() const;
T* operator->() const;
IntrusiveSlistIterator& operator++();
IntrusiveSlistIterator operator++(int);
friend bool operator==(IntrusiveSlistIterator a, IntrusiveSlistIterator b);
friend bool operator!=(IntrusiveSlistIterator a, IntrusiveSlistIterator b);
// one way conversion: iterator -> const_iterator
operator IntrusiveSlistIterator<T const, Tag>() const;
};
In the above notice how IntrusiveSlistIterator<T>
converts to IntrusiveSlistIterator<T const>
. If T
is already const
this conversion never gets used.
I'm going to show you how you can easily define iterators for your custom containers, but just in case I have created a c++11 library that allows you to easily create custom iterators with custom behavior for any type of container, contiguous or non-contiguous.
You can find it on Github
Here are the simple steps to creating and using custom iterators:
typedef blRawIterator< Type > iterator;
typedef blRawIterator< const Type > const_iterator;
iterator begin(){return iterator(&m_data[0]);};
const_iterator cbegin()const{return const_iterator(&m_data[0]);};
Finally, onto defining our custom iterator classes:
NOTE: When defining custom iterators, we derive from the standard iterator categories to let STL algorithms know the type of iterator we've made.
In this example, I define a random access iterator and a reverse random access iterator:
//-------------------------------------------------------------------
// Raw iterator with random access
//-------------------------------------------------------------------
template<typename blDataType>
class blRawIterator
{
public:
using iterator_category = std::random_access_iterator_tag;
using value_type = blDataType;
using difference_type = std::ptrdiff_t;
using pointer = blDataType*;
using reference = blDataType&;
public:
blRawIterator(blDataType* ptr = nullptr){m_ptr = ptr;}
blRawIterator(const blRawIterator<blDataType>& rawIterator) = default;
~blRawIterator(){}
blRawIterator<blDataType>& operator=(const blRawIterator<blDataType>& rawIterator) = default;
blRawIterator<blDataType>& operator=(blDataType* ptr){m_ptr = ptr;return (*this);}
operator bool()const
{
if(m_ptr)
return true;
else
return false;
}
bool operator==(const blRawIterator<blDataType>& rawIterator)const{return (m_ptr == rawIterator.getConstPtr());}
bool operator!=(const blRawIterator<blDataType>& rawIterator)const{return (m_ptr != rawIterator.getConstPtr());}
blRawIterator<blDataType>& operator+=(const difference_type& movement){m_ptr += movement;return (*this);}
blRawIterator<blDataType>& operator-=(const difference_type& movement){m_ptr -= movement;return (*this);}
blRawIterator<blDataType>& operator++(){++m_ptr;return (*this);}
blRawIterator<blDataType>& operator--(){--m_ptr;return (*this);}
blRawIterator<blDataType> operator++(int){auto temp(*this);++m_ptr;return temp;}
blRawIterator<blDataType> operator--(int){auto temp(*this);--m_ptr;return temp;}
blRawIterator<blDataType> operator+(const difference_type& movement){auto oldPtr = m_ptr;m_ptr+=movement;auto temp(*this);m_ptr = oldPtr;return temp;}
blRawIterator<blDataType> operator-(const difference_type& movement){auto oldPtr = m_ptr;m_ptr-=movement;auto temp(*this);m_ptr = oldPtr;return temp;}
difference_type operator-(const blRawIterator<blDataType>& rawIterator){return std::distance(rawIterator.getPtr(),this->getPtr());}
blDataType& operator*(){return *m_ptr;}
const blDataType& operator*()const{return *m_ptr;}
blDataType* operator->(){return m_ptr;}
blDataType* getPtr()const{return m_ptr;}
const blDataType* getConstPtr()const{return m_ptr;}
protected:
blDataType* m_ptr;
};
//-------------------------------------------------------------------
//-------------------------------------------------------------------
// Raw reverse iterator with random access
//-------------------------------------------------------------------
template<typename blDataType>
class blRawReverseIterator : public blRawIterator<blDataType>
{
public:
blRawReverseIterator(blDataType* ptr = nullptr):blRawIterator<blDataType>(ptr){}
blRawReverseIterator(const blRawIterator<blDataType>& rawIterator){this->m_ptr = rawIterator.getPtr();}
blRawReverseIterator(const blRawReverseIterator<blDataType>& rawReverseIterator) = default;
~blRawReverseIterator(){}
blRawReverseIterator<blDataType>& operator=(const blRawReverseIterator<blDataType>& rawReverseIterator) = default;
blRawReverseIterator<blDataType>& operator=(const blRawIterator<blDataType>& rawIterator){this->m_ptr = rawIterator.getPtr();return (*this);}
blRawReverseIterator<blDataType>& operator=(blDataType* ptr){this->setPtr(ptr);return (*this);}
blRawReverseIterator<blDataType>& operator+=(const difference_type& movement){this->m_ptr -= movement;return (*this);}
blRawReverseIterator<blDataType>& operator-=(const difference_type& movement){this->m_ptr += movement;return (*this);}
blRawReverseIterator<blDataType>& operator++(){--this->m_ptr;return (*this);}
blRawReverseIterator<blDataType>& operator--(){++this->m_ptr;return (*this);}
blRawReverseIterator<blDataType> operator++(int){auto temp(*this);--this->m_ptr;return temp;}
blRawReverseIterator<blDataType> operator--(int){auto temp(*this);++this->m_ptr;return temp;}
blRawReverseIterator<blDataType> operator+(const int& movement){auto oldPtr = this->m_ptr;this->m_ptr-=movement;auto temp(*this);this->m_ptr = oldPtr;return temp;}
blRawReverseIterator<blDataType> operator-(const int& movement){auto oldPtr = this->m_ptr;this->m_ptr+=movement;auto temp(*this);this->m_ptr = oldPtr;return temp;}
difference_type operator-(const blRawReverseIterator<blDataType>& rawReverseIterator){return std::distance(this->getPtr(),rawReverseIterator.getPtr());}
blRawIterator<blDataType> base(){blRawIterator<blDataType> forwardIterator(this->m_ptr); ++forwardIterator; return forwardIterator;}
};
//-------------------------------------------------------------------
Now somewhere in your custom container class:
template<typename blDataType>
class blCustomContainer
{
public: // The typedefs
typedef blRawIterator<blDataType> iterator;
typedef blRawIterator<const blDataType> const_iterator;
typedef blRawReverseIterator<blDataType> reverse_iterator;
typedef blRawReverseIterator<const blDataType> const_reverse_iterator;
.
.
.
public: // The begin/end functions
iterator begin(){return iterator(&m_data[0]);}
iterator end(){return iterator(&m_data[m_size]);}
const_iterator cbegin(){return const_iterator(&m_data[0]);}
const_iterator cend(){return const_iterator(&m_data[m_size]);}
reverse_iterator rbegin(){return reverse_iterator(&m_data[m_size - 1]);}
reverse_iterator rend(){return reverse_iterator(&m_data[-1]);}
const_reverse_iterator crbegin(){return const_reverse_iterator(&m_data[m_size - 1]);}
const_reverse_iterator crend(){return const_reverse_iterator(&m_data[-1]);}
.
.
.
// This is the pointer to the
// beginning of the data
// This allows the container
// to either "view" data owned
// by other containers or to
// own its own data
// You would implement a "create"
// method for owning the data
// and a "wrap" method for viewing
// data owned by other containers
blDataType* m_data;
};
Check this below code, it works
#define MAX_BYTE_RANGE 255
template <typename T>
class string
{
public:
typedef char *pointer;
typedef const char *const_pointer;
typedef __gnu_cxx::__normal_iterator<pointer, string> iterator;
typedef __gnu_cxx::__normal_iterator<const_pointer, string> const_iterator;
string() : length(0)
{
}
size_t size() const
{
return length;
}
void operator=(const_pointer value)
{
if (value == nullptr)
throw std::invalid_argument("value cannot be null");
auto count = strlen(value);
if (count > 0)
_M_copy(value, count);
}
void operator=(const string &value)
{
if (value.length != 0)
_M_copy(value.buf, value.length);
}
iterator begin()
{
return iterator(buf);
}
iterator end()
{
return iterator(buf + length);
}
const_iterator begin() const
{
return const_iterator(buf);
}
const_iterator end() const
{
return const_iterator(buf + length);
}
const_pointer c_str() const
{
return buf;
}
~string()
{
}
private:
unsigned char length;
T buf[MAX_BYTE_RANGE];
void _M_copy(const_pointer value, size_t count)
{
memcpy(buf, value, count);
length = count;
}
};
Boost has something to help: the Boost.Iterator library.
More precisely this page: boost::iterator_adaptor.
What's very interesting is the Tutorial Example which shows a complete implementation, from scratch, for a custom type.
template <class Value> class node_iter : public boost::iterator_adaptor< node_iter<Value> // Derived , Value* // Base , boost::use_default // Value , boost::forward_traversal_tag // CategoryOrTraversal > { private: struct enabler {}; // a private type avoids misuse public: node_iter() : node_iter::iterator_adaptor_(0) {} explicit node_iter(Value* p) : node_iter::iterator_adaptor_(p) {} // iterator convertible to const_iterator, not vice-versa template <class OtherValue> node_iter( node_iter<OtherValue> const& other , typename boost::enable_if< boost::is_convertible<OtherValue*,Value*> , enabler >::type = enabler() ) : node_iter::iterator_adaptor_(other.base()) {} private: friend class boost::iterator_core_access; void increment() { this->base_reference() = this->base()->next(); } };
The main point, as has been cited already, is to use a single template implementation and typedef
it.
I don't know if Boost has anything that would help.
My preferred pattern is simple: take a template argument which is equal to value_type
, either const qualified or not. If necessary, also a node type. Then, well, everything kind of falls into place.
Just remember to parameterize (template-ize) everything that needs to be, including the copy constructor and operator==
. For the most part, the semantics of const
will create correct behavior.
template< class ValueType, class NodeType >
struct my_iterator
: std::iterator< std::bidirectional_iterator_tag, T > {
ValueType &operator*() { return cur->payload; }
template< class VT2, class NT2 >
friend bool operator==
( my_iterator const &lhs, my_iterator< VT2, NT2 > const &rhs );
// etc.
private:
NodeType *cur;
friend class my_container;
my_iterator( NodeType * ); // private constructor for begin, end
};
typedef my_iterator< T, my_node< T > > iterator;
typedef my_iterator< T const, my_node< T > const > const_iterator;
Source: Stackoverflow.com