29 Numerics library [numerics]

29.6 Numeric arrays [numarray]

29.6.8 Class template mask_array [template.mask.array]

29.6.8.1 Overview [template.mask.array.overview]

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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 }; }
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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>&);
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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).