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Showing content from https://en.cppreference.com/w/cpp/language/../named_req/../numeric/valarray/begin2.html below:

std::begin(std::valarray) - cppreference.com

template< class T >
/* see below */ begin( valarray<T>& v );

template< class T >
/* see below */ begin( const valarray<T>& v );

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

The return type must

• meet the requirements of mutable LegacyRandomAccessIterator,

• have a member type value_type, which is T, and
• have a member type reference, which is T&.

The return type must

• meet the requirements of constant LegacyRandomAccessIterator,

• 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.

Iterator to the first value in the numeric array.

May throw implementation-defined exceptions.

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.

#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++!

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

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