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std::begin(std::valarray)

From cppreference.com
< cpp‎ | numeric‎ | valarray
 
 
Numerics library
 
std::valarray
 
template< class T >
/* see below */ begin( valarray<T>& v );
(1) (since C++11)
template< class T >
/* see below */ begin( const valarray<T>& v );
(2) (since C++11)

The overload of std::begin for valarray returns an iterator of unspecified type referring to the first element in the numeric array.

1) The return type must
(since C++20)
  • have a member type value_type, which is T, and
  • have a member type reference, which is T&.
2) The return type must
(since C++20)
  • have a member type value_type, which is T, and
  • have a member type reference, which is const T&.

The iterator returned from this function is invalidated when the member function resize() is called on v or when the lifetime of v ends, whichever comes first.

[edit] Parameters

v - a numeric array

[edit] Return value

Iterator to the first value in the numeric array.

[edit] Exceptions

May throw implementation-defined exceptions.

[edit] Notes

Unlike other functions that take std::valarray arguments, begin() cannot accept the replacement types (such as the types produced by expression templates) that may be returned from expressions involving valarrays: std::begin (v1 + v2) is not portable, std::begin (std::valarray <T>(v1 + v2)) has to be used instead.

The intent of this function is to allow range for loops to work with valarrays, not to provide container semantics.

[edit] Example

Run this code
#include <algorithm>
#include <iostream>
#include <valarray>
 
void show(const std::valarray <int>& v)
{
 std::for_each (std::begin (v), std::end (v), [](int c)
 {
 std::cout << c << ' ';
 });
 std::cout << '\n';
};
 
int main()
{
 const std::valarray <int> x{47, 70, 37, 52, 90, 23, 17, 33, 22, 16, 21, 4};
 const std::valarray <int> y{25, 31, 71, 56, 21, 21, 15, 34, 21, 27, 12, 6};
 
 show(x); 
 show(y); 
 
 const std::valarray <int> z{x + y};
 
 for (char c : z)
 std::cout << c;
}

Output:

47 70 37 52 90 23 17 33 22 16 21 4 
25 31 71 56 21 21 15 34 21 27 12 6 
Hello, C++!

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 2058 C++11 1. begin() was required to support replacement types
2. it was unspecified when the returned iterators will be invalidated
1. not required
2. specified

[edit] See also

specializes std::end
(function template) [edit]

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