cppdraft_translate/cppdraft/template/mask/array/overview.md

[template.mask.array.overview]

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 mask_array {public:using value_type = T; void operator= (const valarray&) const; void operator*= (const valarray&) const; void operator/= (const valarray&) const; void operator%= (const valarray&) const; void operator+= (const valarray&) const; void operator-= (const valarray&) const; void operator^= (const valarray&) const; void operator&= (const valarray&) const; void operator|= (const valarray&) const; void operator<<=(const valarray&) const; void operator>>=(const valarray&) 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 ofb to the masked elements in a (those for which the corresponding element inmask is true).