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464. Suggestion for new member functions in standard containers

Section: 24.3.11 [vector], 24.4.4 [map] Status: CD1 Submitter: Thorsten Ottosen Opened: 2004-05-12 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

To add slightly more convenience to vector<T> and map<Key,T> we should consider to add

1. add vector<T>::data() member (const and non-const version) semantics: if( empty() ) return 0; else return buffer_;
2. add map<Key,T>::at( const Key& k ) member (const and non-const version) semantics: iterator i = find( k ); if( i != end() ) return *i; else throw range_error();

Rationale:

• To obtain a pointer to the vector's buffer, one must use either operator[]() (which can give undefined behavior for empty vectors) or at() (which will then throw if the vector is empty).
• tr1::array<T,sz> already has a data() member
• e cannot use operator[]() when T is not DefaultDonstructible
• Neither when the map is const.
• when we want to make sure we don't add an element accidently
• when it should be considered an error if a key is not in the map

Proposed resolution:

In 24.3.11 [vector], add the following to the vector synopsis after "element access" and before "modifiers":

  // [lib.vector.data] data access
  pointer       data();
  const_pointer data() const;

Add a new subsection of 24.3.11 [vector]:

23.2.4.x vector data access

   pointer       data();
   const_pointer data() const;

Returns: A pointer such that [data(), data() + size()) is a valid range. For a non-empty vector, data() == &front().

Complexity: Constant time.

Throws: Nothing.

In 24.4.4 [map], add the following to the map synopsis immediately after the line for operator[]:

  T&       at(const key_type& x);
  const T& at(const key_type& x) const;

Add the following to 24.4.4.3 [map.access]:

  T&       at(const key_type& x);
  const T& at(const key_type& x) const;

Returns: A reference to the element whose key is equivalent to x, if such an element is present in the map.

Throws: out_of_range if no such element is present.

Rationale:

Neither of these additions provides any new functionality but the LWG agreed that they are convenient, especially for novices. The exception type chosen for at, std::out_of_range, was chosen to match vector::at.