Ranges library
Range access
Range conversions

Range primitives

Dangling iterator handling
Range concepts

Range factories
Range adaptors
Range generators
Range adaptor objects
Range adaptor closure objects
Helper items
(until C++23)(C++23)

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).


[edit] Parameters


[edit] Return value

The first element in the view.

[edit] Notes

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.

[edit] Example

[edit] See also

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