26 Numerics library [numerics]

26.7 Numeric arrays [numarray]

26.7.8 Class template mask_­array [template.mask.array]

26.7.8.1 Overview [template.mask.array.overview]

namespace std {
  template<class T> class mask_array {
  public:
    using value_type = T;

    void operator=  (const valarray<T>&) const;
    void operator*= (const valarray<T>&) const;
    void operator/= (const valarray<T>&) const;
    void operator%= (const valarray<T>&) const;
    void operator+= (const valarray<T>&) const;
    void operator-= (const valarray<T>&) const;
    void operator^= (const valarray<T>&) const;
    void operator&= (const valarray<T>&) const;
    void operator|= (const valarray<T>&) const;
    void operator<<=(const valarray<T>&) const;
    void operator>>=(const valarray<T>&) const;

    mask_array(const mask_array&);
    ~mask_array();
    const mask_array& operator=(const mask_array&) const;
    void operator=(const T&) const;

    mask_array() = delete;      // as implied by declaring copy constructor above
  };
}
1
#
This template is a helper template used by the mask subscript operator:
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mask_array<T> valarray<T>::operator[](const valarray<bool>&).
2
#
It has reference semantics to a subset of an array specified by a boolean mask.
Thus, the expression a[mask] = b; has the effect of assigning the elements of b to the masked elements in a (those for which the corresponding element in mask is true).

26.7.8.2 Assignment [mask.array.assign]

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void operator=(const valarray<T>&) const; const mask_array& operator=(const mask_array&) const;
1
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These assignment operators have reference semantics, assigning the values of the argument array elements to selected elements of the valarray<T> object to which it refers.

26.7.8.3 Compound assignment [mask.array.comp.assign]

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void operator*= (const valarray<T>&) const; void operator/= (const valarray<T>&) const; void operator%= (const valarray<T>&) const; void operator+= (const valarray<T>&) const; void operator-= (const valarray<T>&) const; void operator^= (const valarray<T>&) const; void operator&= (const valarray<T>&) const; void operator|= (const valarray<T>&) const; void operator<<=(const valarray<T>&) const; void operator>>=(const valarray<T>&) const;
1
#
These compound assignments have reference semantics, applying the indicated operation to the elements of the argument array and selected elements of the valarray<T> object to which the mask object refers.

26.7.8.4 Fill function [mask.array.fill]

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void operator=(const T&) const;
1
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This function has reference semantics, assigning the value of its argument to the elements of the valarray<T> object to which the mask_­array object refers.