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std::vector<T,Allocator>:: front

From cppreference.net

C++
Containers library
std::vector
Member types
Member functions
Non-member functions
(until C++20) (until C++20) (until C++20) (until C++20) (until C++20)
Deduction guides (C++17)
reference front ( ) ;
(1) (constexpr since C++20)
const_reference front ( ) const ;
(2) (constexpr since C++20)

Returns a reference to the first element in the container.

If empty() is true , the behavior is undefined.

(until C++26)

If empty() is true :

  • If the implementation is hardened , a contract violation occurs. Moreover, if the contract-violation handler returns under “observe” evaluation semantic, the behavior is undefined.
  • If the implementation is not hardened, the behavior is undefined.
(since C++26)

Contents

Return value

Reference to the first element.

Complexity

Constant.

Notes

For a container c , the expression c. front ( ) is equivalent to * c. begin ( ) .

Example

The following code uses front to display the first element of a std:: vector < char > :

Run this code 
#include <cassert>
#include <vector>
int main()
{
    std::vector<char> letters{'a', 'b', 'c', 'd'};
    assert(letters.front() == 'a');
}

See also

access the last element
(public member function)
(C++11)
returns a reverse iterator to the end
(public member function)
(C++11)
returns an iterator to the beginning
(public member function)
direct access to the underlying contiguous storage
(public member function)