NAME¶

std::ranges::cbegin - std::ranges::cbegin

Synopsis¶

Defined in header <ranges>
inline namespace /*unspecified*/ {

inline constexpr /*unspecified*/ cbegin = (since C++20)
/*unspecified*/; (customization point object)

}
Call signature
template< class T >

requires /* see below */ (since C++20)

constexpr std::input_or_output_iterator auto cbegin(
T&& t );

Returns an iterator to the first element of the const-qualified argument.

range-begin-end.svg

Let CT be

1. const std::remove_reference_t<T>& if the argument is a lvalue (i.e. T is an
lvalue reference type),
2. 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 and f 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¶

// Run this code

#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¶

ranges::begin returns an iterator to the beginning of a range
(C++20) (customization point object)
begin
cbegin returns an iterator to the beginning of a container or array
(C++11) (function template)
(C++14)