CODEWITHSUNDEEP
c++stltypedef
user258367
user258367

Reputation: 3407

multiple typedef for same type

Below code is class initializer list which is provided in Ubuntu standard package. In this iterator and const_iterator are typedef of same type. I just wanted to know why would we like to have same typedef for different type of Iterator ? Ideally for Iterator it should have typedef _E* iterator.

// In the class initializer list:
namespace std 
{ 
  /// initializer_list 
  template<class _E> 
    class initializer_list 
    { 
    public: 
      typedef _E                value_type; 
      typedef const _E&         reference; 
      typedef const _E&         const_reference; 
      typedef size_t            size_type; 
      typedef const _E*         iterator; 
      typedef const _E*         const_iterator;

P.S: I am not able to think of suitable title so I have given this title

Upvotes: 1

Views: 2193

Answers (3)

Joseph Mansfield
Joseph Mansfield

Reputation: 110658

To meet the requirements for a Container, both types iterator an const_iterator must exist. Many algorithms and lots of code rely on containers having both of these types available. There's no reason the iterator type has to be a non-const (mutable) iterator, however. In this case, they've decided that both iterator and const_iterator are const (immutable) iterators.

The reason they have both iterators as const is because they clearly don't want to you to be able to change the values in the initializer_list.

As another example of this, take a look at std::set whose iterator and const_iterator types are also both constant iterators. Since std::sets contain ordered elements, if you were to be able to change the contents of the std::set, that ordering would be invalidated. To prevent this, both iterator types are made immutable.

Upvotes: 5

bstamour
bstamour

Reputation: 7766

It's probably because you don't ever want to change the values stored inside of an initializer list. They're meant to be temporary objects to be passed up into constructors.

C++ has an (informal, for now) ideal called Concepts that apply to generic types. The Container concept requires certain typedefs to be available for your object. Since std::initializer_list is a container, it requires both ::iterator and ::const_iterator typedefs. However, like std::set, you don't want to let the user modify the elements inside of the initializer_list, and so the ::iterator is the same as ::const_iterator: they're both read-only.

Upvotes: 0

Oliver Charlesworth
Oliver Charlesworth

Reputation: 272517

Because these fields are required by any type that satisfies the requirements of Container. It just so happens that in the case of std::initializer_list, the iterator and the const_iterator are the same.

Upvotes: 2

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