GeezerVison is showing cplusplus.com — std::mask_array
class template
<valarray>
std::mask_array
template <class T> mask_array;
Valarray mask selection
This class is used as an intermediate type returned by valarray's subscript operator (operator[]) when used with masks.
It references the elements in the valarray object that are selected by a mask (a valarray<bool>), and overloads the assignment and compound assignment operators, allowing direct access to the elements in the selection.
The type is convertible to a valarray (see valarray constructor), producing a new object with copies of the referred elements.
Objects of this type are obtained as the return value of a call to
valarray::operator[]. They cannot be directly
constructed nor copied. It is declared as:
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template <class T> class mask_array {
public:
typedef T value_type;
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;
void operator=(const T&);
~mask_array();
private:
mask_array();
mask_array(const mask_array&);
mask_array& operator= (const mask_array&);
};
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Objects of this type are obtained as the return value of a call to
valarray::operator[]. They cannot be
default-constructed, but can be copied. It is declared as:
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template <class T> class mask_array {
public:
typedef T value_type;
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;
void operator=(const T&) const;
mask_array() = delete;
mask_array(const mask_array&);
const mask_array& operator= (const mask_array&) const;
~mask_array();
};
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Example
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// mask_array example
#include <iostream> // std::cout
#include <cstddef> // std::size_t
#include <valarray> // std::valarray
int main ()
{
std::valarray<int> foo (10);
for (int i=0; i<10; ++i) foo[i]=i; // 0 1 2 3 4 5 6 7 8 9
std::valarray<bool> mymask (10);
for (int i=0; i<10; ++i)
mymask[i]= ((i%2)==1); // f t f t f t f t f t
foo[mymask] *= std::valarray<int>(10,5); // 0 10 2 30 4 50 6 70 8 90
foo[!mymask] = 0; // 0 10 0 30 0 50 0 70 0 90
std::cout << "foo:";
for (std::size_t i=0; i<foo.size(); ++i)
std::cout << foo[i] << ' ';
std::cout << '\n';
return 0;
}
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Output:
foo: 0 10 0 30 0 50 0 70 0 90
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