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std::data

Defined in header <iterator>
Defined in header <array>
Defined in header <deque>
Defined in header <forward_list>
Defined in header <list>
Defined in header <map>
Defined in header <regex>
Defined in header <set>
Defined in header <string>
Defined in header <unordered_map>
Defined in header <unordered_set>
Defined in header <vector> 
template <class C> 
constexpr auto data(C& c) -> decltype(c.data());
 (1) (since C++17) 
template <class C> 
constexpr auto data(const C& c) -> decltype(c.data());
 (2) (since C++17) 
template <class T, std::size_t N> 
constexpr T* data(T (&array)[N]) noexcept;
 (3) (since C++17) 
template <class E>
constexpr const E* data(std::initializer_list<E> il) noexcept;
 (4) (since C++17)

Returns a pointer to the block of memory containing the elements of the container.

1,2) returns c.data()
3) returns array
4) returns il.begin()

Parameters

c - a container with a data() method
array - an array of arbitrary type
il - an initializer list

Return value

A pointer to the block of memory containing the elements of the container.

Exceptions

3,4)
noexcept specification:
noexcept

Possible implementation

First version 
template <class C> 
constexpr auto data(C& c) -> decltype(c.data())
{
    return c.data();
}
Second version 
template <class C> 
constexpr auto data(const C& c) -> decltype(c.data())
{
    return c.data();
}
Third version 
template <class T, std::size_t N>
constexpr T* data(T (&array)[N]) noexcept
{
    return array;
}
Fourth version 
template <class E> 
constexpr const E* data(std::initializer_list<E> il) noexcept
{
    return il.begin();
}

Example

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