std::valarray::operator[]

	(1)
T operator[]( std::size_t pos ) const;		(until C++11)
const T& operator[]( std::size_t pos ) const;		(since C++11)
T& operator[]( std::size_t pos );		(2)
std::valarray<T> operator[]( std::slice slicearr ) const;		(3)
std::slice_array<T> operator[]( std::slice slicearr );		(4)
std::valarray<T> operator[]( const std::gslice& gslicearr ) const;		(5)
std::gslice_array<T> operator[]( const std::gslice& gslicearr );		(6)
std::valarray<T> operator[]( const valarray<bool>& boolarr ) const;		(7)
std::mask_array<T> operator[]( const valarray<bool>& boolarr );		(8)
std::valarray<T> operator[]( const valarray<std::size_t>& indarr ) const;		(9)
std::indirect_array<T> operator[]( const valarray<std::size_t>& indarr );		(10)

Retrieve single elements or portions of the array.

The const overloads that return element sequences create a new std::valarray object. The non-const overloads return classes holding references to the array elements.

Parameters

Return value

1,2) A reference to the corresponding element

3,5,7,9) A std::valarray object containing copies of the selected items

4,6,8,10) The corresponding data structure containing references to the selected items

Exceptions

(none).

Precondition

The selected elements must exist.

Notes

For proper values of i and j, the following properties are true:

1) (a[i] = q, a[i]) == q For a non-const a.

2) &a[i+j] == &a[i] + j This means that valarray elements are adjacent in memory.

3) &a[i] != &b[j]

This holds for every objects a and b that are not aliases of one another.

This means that there are no aliases in the elements and this property can be used to perform some kinds of optimization.

References become invalid on resize or when the array is destructed.

For overloads (3,5,7,9), The function can be implemented with the return type different from std::valarray. In this case, the replacement type has the following properties:

All const member functions of std::valarray are provided.
std::valarray, std::slice_array, std::gslice_array, std::mask_array and std::indirect_array can be constructed from the replacement type.
All functions accepting an argument of type const std::valarray& except begin() and end() (since C++14) should also accept the replacement type.
All functions accepting two arguments of type const std::valarray& should accept every combination of const std::valarray& and the replacement type.
The return type does not add more than two levels of template nesting over the most deeply-nested argument type.

Example

Links:

http://en.cppreference.com/w/cpp/numeric/valarray/operator_at

doc_CPP

2016-10-11 10:08:14

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