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std::ranges::cbegin

From cppreference.com
< cpp‎ | ranges
 
 
Ranges library
Range access
cbegin
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(C++23)
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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.

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.

Contents

[edit] 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)).

[edit] 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

Otherwise, no function call operator of __cbegin_fn participates in overload resolution.

[edit] 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

[edit] See also

returns an iterator to the beginning of a range
(customization point object) [edit]
(C++11)(C++14)
returns an iterator to the beginning of a container or array
(function template) [edit]