std::begin(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

meet the requirements of mutable LegacyRandomAccessIterator,

model contiguous_iterator,

(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

meet the requirements of constant LegacyRandomAccessIterator,

model contiguous_iterator,

(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.

Parameters

v

-

a numeric array

Return value

Iterator to the first value in the numeric array.

Exceptions

May throw implementation-defined exceptions.

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.

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++!

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

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