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std::valarray<T>::operator[] - cppreference.com

Retrieve single elements or portions of the array.

The const overloads that return element sequences create a new std::valarray object. The non-const overloads return classes holding references to the array elements.

If pos < size() is false, the behavior is undefined.

If pos < size() is false:

• If the implementation is hardened, a contract violation occurs. Moreover, if the contract-violation handler returns under “observe” evaluation semantic, the behavior is undefined.
• If the implementation is not hardened, the behavior is undefined.

3-10) If the argument does not specify a valid subset of *this, the behavior is undefined.

1,2) A reference to the corresponding element.

A

object containing copies of the selected items.

4,6,8,10) The corresponding data structure containing references to the selected items.

May throw implementation-defined exceptions.

For proper std::valarray values a, b and proper std::size_t values i, j, all of the following expressions always evaluate to true:

1) (a[i] = q, a[i]) == q for non-const a

• This means that std::valarray elements are adjacent in memory.

for every objects

and

that are not aliases of one another

• This means that there are no aliases in the elements and this property can be used to perform some kinds of optimization.

References become invalid on resize() or when the array is destructed.

For overloads (3,5,7,9), The function can be implemented with the return type different from std::valarray. In this case, the replacement type has the following properties:

• All const member functions of std::valarray are provided.
• std::valarray, std::slice_array, std::gslice_array, std::mask_array and std::indirect_array can be constructed from the replacement type.
• For every function taking a const std::valarray<T>& except begin() and end()(since C++11), identical functions taking the replacement types shall be added;
• For every function taking two const std::valarray<T>& arguments, identical functions taking every combination of const std::valarray<T>& and replacement types shall be added.
• The return type does not add more than two levels of template nesting over the most deeply-nested argument type.

Slice/mask/indirect index accesses do not chain: v[v == n][std::slice(0, 5, 2)] = x; is an error because std::mask_array (the type of v[v == n]) does not have operator[].

#include <cstddef>
#include <iomanip>
#include <iostream>
#include <valarray>
 
int main() 
{
    std::valarray<int> data = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
 
    std::cout << "Initial valarray:   ";
    for (int n : data)
        std::cout << std::setw(3) << n;
    std::cout << '\n';
 
    data[data > 5] = -1; // valarray<bool> overload of operator[]
    // the type of data > 5 is std::valarray<bool>
    // the type of data[data > 5] is std::mask_array<int>
 
    std::cout << "After v[v > 5] = -1:";
    for (std::size_t n = 0; n < data.size(); ++n) 
        std::cout << std::setw(3) << data[n]; // regular operator[]
    std::cout << '\n';
}

Output:

Initial valarray:     0  1  2  3  4  5  6  7  8  9
After v[v > 5] = -1:  0  1  2  3  4  5 -1 -1 -1 -1

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

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