std::mask_array::operator=(3)

NAME
std::mask_array::operator= - std::mask_array::operator=

Synopsis
   void operator=( const T& value ) const; (1)
   void operator=( const std::valarray<T>& val_arr ) const; (2)
   const mask_array& operator=( const mask_array& other_arr ) const; (3)
   Assigns values to all referred elements.
   1) Assigns value to all of the elements.
   2) Assigns the elements of val_arr to the referred to elements of *this.
   3) Assigns the selected elements from other_arr to the referred to elements of
   *this.

Parameters
   value - a value to assign to all of the referred elements
   val_arr - std::valarray to assign
   other_arr - std::mask_array to assign

Return value
   1,2) (none)
   3) *this

Example
// Run this code
 #include <iomanip>
 #include <iostream>
 #include <valarray>
 void print(std::valarray<int> const& v)
 {
     for (int e : v)
         std::cout << std::setw(2) << e << ' ';
     std::cout << '\n';
 }
 int main()
 {
     const auto init = {1, 2, 3, 4, 5, 6, 7, 8};
     std::valarray<int> v;
     v = init;
     v[(v % 2) == 0] = 0; // (1)
     print(v);
     v = init;
     v[(v % 2) == 1] = std::valarray<int>{-1, -2, -3, -4}; // (2)
     print(v);
     v = init;
     v[(v % 2) == 0] = v[(v % 2) == 1]; // (3)
     print(v);
 }

Output:
  1 0 3 0 5 0 7 0
 -1 2 -2 4 -3 6 -4 8
  1 1 3 3 5 5 7 7
   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 123 C++98 overload (2) was non-const made const
   LWG 253 C++98 the copy assignment operator was private made public
   LWG 621 C++98 the copy assignment operator was non-const made const