constexpr decltype(auto) front()
    requires ranges::forward_range<D>;
(1) (since C++20)
constexpr decltype(auto) front() const
    requires ranges::forward_range<const D>;
(2) (since C++20)

The default implementation of front() member function returns the first element in the view of the derived type. Whether the element is returned by value or by reference depends on the operator* of the iterator type.

1) Let derived be static_cast<D&>(*this). Equivalent to return *ranges::begin(derived);. The behavior is undefined if empty() is true (i.e. the beginning iterator compares equal to the sentinel), even if the iterator obtained in the same way is dereferenceable.
2) Same as (1), except that derived is static_cast<const D&>(*this).



Return value

The first element in the view.


In C++20, no type derived from std::ranges::view_interface in the standard library provides their own front() member function. Almost all of these types use the default implementation.

A notable exception is std::ranges::basic_istream_view. For it never satisfies forward_range, the view cannot use the inherited front().

The inherited front() member function is available for std::ranges::empty_view, but a call to it always results in undefined behavior.


See also

returns an iterator to the beginning of a container or array
(function template)
returns an iterator to the beginning of a range
(customization point object)
returns an iterator to the beginning of a read-only range
(customization point object)

© cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.