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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`.