std::mask_array

Defined in header <valarray> 
template< class T > class mask_array;

std::mask_array is a helper template used by the valarray subscript operator with std::valarray<bool> argument. It has reference semantics and provides access to the subset of the valarray consisting of the elements whose indices correspond to true values in the std::valarray<bool> mask.

Member types

Type Definition
value_type T

Member functions

constructs a mask_array
(public member function)
destroys a mask_array
(public member function)
assigns contents
(public member function)
performs arithmetic operation on the array referred by mask.
(public member function)

Example

#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 << n << ' ';
    std::cout << '\n';
 
    data[data > 5] = -1;
    // 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(int n: data) std::cout << n << ' ';
    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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