`std::iterator` Is Deprecated

The Standard Library API requires that each iterator type has to expose five typedefs:

• iterator_category - the type of the iterator
• value_type - type stored in the iterator
• difference_type - the type that is the result of subtracting two iterators
• pointer - pointer type of the stored type
• reference - the reference type of the stored type

iterator_category must be one of input_iterator_tag, forward_iterator_tag, bidirectional_iterator_tag or random_access_iterator_tag.

Before C++17, if you wanted to define a custom iterator, you could use std::iterator as a base class. In C++14 it’s defined as:

template<
    class Category,
    class T,
    class Distance = std::ptrdiff_t,
    class Pointer = T*,
    class Reference = T&
> struct iterator;

This helper class made defining the typedefs in a short way:

class ColumnIterator 
 : public std::iterator<std::random_access_iterator_tag, Column>
{
    // ...
};

In C++17 you must not derive from std::iterator and you need to write the trait typedefs explicitly:

class ColumnIterator {
public:
    using iterator_category = std::random_iterator_tag;
    using value_type = Column;
    using difference_type = std::ptrdiff_t;
    using pointer = Column*;
    using reference = Column&;

    // ...
};

While you have to write more code, it’s much easier to read, and it’s less error-prone.

For example, the referencing paper mentions the following example from The Standard Library:

template <class T, class charT = char, class traits = char_traits<charT> >
class ostream_iterator:
  public iterator<output_iterator_tag, void, void, void, void>;

In the above code, it’s not clear what all of those four void types mean in the definition.

Additionally, std::iterator could lead to complicated errors in the name lookup, especially if you happen to use the same name in the derived iterator as in the base class.

Extra Info: You can read more information in the paper P0174R2 - Deprecating Vestigial Library Parts in C++17.

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Let’s have a look at some smaller elements that were altered in the Standard Library.