std::ranges::view_interface<D>:: front
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(C++23)
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(C++23)
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front
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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.
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.
derived
is
static_cast
<
const
D
&
>
(
*
this
)
.
Parameters
(none)
Return value
The first element in the view.
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.
Example
|
This section is incomplete
Reason: no example |
See also
|
(C++11)
(C++14)
|
returns an iterator to the beginning of a container or array
(function template) |
|
(C++20)
|
returns an iterator to the beginning of a range
(customization point object) |
|
(C++20)
|
returns an iterator to the beginning of a read-only range
(customization point object) |