std::valarray::operator[](3)

💖 std::valarray::operator[](3) 💖

Okay bestie, welcome to the page for std::valarray::operator[](3) from the C++ Standard Libary manual ✨ This one is all about grabbing single elements or whole cute little portions of a valarray, and yes, there are ten overloads — we love a main character with range 💅

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✨ NAME ✨

std::valarray::operator[] — std::valarray::operator[]

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🌸 Synopsis 🌸

       const T&               operator[]( std::size_t pos ) const;
       (1)
          T&                     operator[]( std::size_t pos );
       (2)
          std::valarray<T>       operator[]( std::slice slicearr ) const;
       (3)
          std::slice_array<T>    operator[]( std::slice slicearr );
       (4)
          std::valarray<T>       operator[]( const std::gslice& gslicearr )
       const;       (5)
          std::gslice_array<T>   operator[]( const std::gslice& gslicearr );
       (6)
          std::valarray<T>       operator[]( const std::valarray<bool>& boolarr
       ) const; (7)
          std::mask_array<T>     operator[]( const std::valarray<bool>& boolarr
       );       (8)
          std::valarray<T>       operator[]( const std::valarray<std::size_t>&
       indarr )  (9)
          const;
          std::indirect_array<T> operator[]( const std::valarray<std::size_t>&
       indarr ); (10)

So what’s the vibe? You’re retrieving 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.

The selected element(s) must exist — no cap, that part is serious:

• for overloads (1,2), if pos is not less than size(), the behavior is undefined;
• and for overloads (3-10), if the argument does not specify a valid subset of *this, the behavior is undefined.

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🎀 Parameters 🎀

• pos — position of the element to return.
• slicearr — slice of the elements to return.
• gslicearr — gslice of the elements to return.
• boolarr — mask of the elements to return.
• indarr — indices of the elements to return.

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💜 Return value 💜

• 1,2) A reference to the corresponding element.
• 3,5,7,9) A std::valarray object containing copies of the selected items.
• 4,6,8,10) The corresponding data structure containing references to the selected items.

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😭 Exceptions 😭

May throw implementation-defined exceptions.

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🔥 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:

1. (a[i] = q, a[i]) == q for non-const a
2. &a[i + j] == &a[i] + j
   • This means that std::valarray elements are adjacent in memory.
3. &a[i] != &b[j] for every objects a and b 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[].

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🦋 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';
        }

🌟 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

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💀 Defect reports 💀

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

• LWG 389, applied to C++98: the return type of overload (1) was T; corrected to const T&.
• LWG 430, applied to C++98: the behavior was unclear for overloads (3-10) if an invalid subset is specified; the behavior is undefined in this case.

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🙈 Hidden category 🙈

• Pages with unreviewed LWG DR marker

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🫶 Cute sign-off: source is http://cppreference.com, version/date is 2024.06.10, and the page is std::valarray::operator[](3). Stay iconic, bestie 💅✨