std::ranges::cbegin
Defined in header <ranges> |
||
---|---|---|
inline namespace /*unspecified*/ { inline constexpr /*unspecified*/ cbegin = /*unspecified*/; } |
(since C++20) (customization point object) |
|
Call signature | ||
template< class T > requires /* see below */ constexpr std::input_or_output_iterator auto cbegin( T&& t ); |
(since C++20) |
Returns an iterator to the first element of the const-qualified argument.
Let CT
be.
-
const std::remove_reference_t<T>&
if the argument is a lvalue (i.e.T
is an lvalue reference type), -
const T
otherwise,
a call to ranges::cbegin
is expression-equivalent to ranges::begin(static_cast<CT&&>(t))
.
The return type models std::input_or_output_iterator
in both cases.
Expression-equivalent
Expression e
is expression-equivalent to expression f
, if.
-
e
andf
have the same effects, and - either both are constant subexpressions or else neither is a constant subexpression, and
- either both are potentially-throwing or else neither is potentially-throwing (i.e.
noexcept(e) == noexcept(f)
).
Customization point objects
The name ranges::cbegin
denotes a customization point object, which is a const function object of a literal semiregular
class type. For exposition purposes, the cv-unqualified version of its type is denoted as __cbegin_fn
.
All instances of __cbegin_fn
are equal. The effects of invoking different instances of type __cbegin_fn
on the same arguments are equivalent, regardless of whether the expression denoting the instance is an lvalue or rvalue, and is const-qualified or not (however, a volatile-qualified instance is not required to be invocable). Thus, ranges::cbegin
can be copied freely and its copies can be used interchangeably.
Given a set of types Args...
, if std::declval<Args>()...
meet the requirements for arguments to ranges::cbegin
above, __cbegin_fn
models
.
-
std::invocable<__cbegin_fn, Args...>
, -
std::invocable<const __cbegin_fn, Args...>
, -
std::invocable<__cbegin_fn&, Args...>
, and -
std::invocable<const __cbegin_fn&, Args...>
.
Otherwise, no function call operator of __cbegin_fn
participates in overload resolution.
Example
#include <iostream> #include <ranges> #include <vector> int main() { std::vector<int> v = { 3, 1, 4 }; auto vi = std::ranges::cbegin(v); std::cout << *vi << '\n'; // *vi = 42; // Error: read-only variable is not assignable int a[] = { -5, 10, 15 }; auto ai = std::ranges::cbegin(a); std::cout << *ai << '\n'; // *ai = 42; // Error: read-only variable is not assignable }
Output:
3 -5
See also
(C++20)
|
returns an iterator to the beginning of a range (customization point object) |
(C++11)(C++14)
|
returns an iterator to the beginning of a container or array (function template) |
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