std::valarray<T>::operator[]

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.

[edit] Parameters

[edit] Return value

[edit] Exceptions

May throw implementation-defined exceptions.

[edit] Notes

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

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[].

[edit] Example

#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';
}

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

[edit] Defect reports

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