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transform_view class (C++ Standard Library) |
API reference for the Standard Template Library (STL) <ranges> transform_view class, which is a view of an underlying sequence after applying a transformation function to each element. |
12/14/2022 |
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A view of elements, each of which is a transformation of an element in the specified range.
template<input_range V, move_constructible F>
requires view<V> && is_object_v<F> &&
regular_invocable<F&, range_reference_t<V>> &&
can-reference<invoke_result_t<F&, range_reference_t<V>>>
class transform_view : public view_interface<transform_view<V, F>>;F
The type of the function object that transforms the elements.
V
The type of the underlying view.
For a description of the following entries, see View class characteristics
| Characteristic | Description |
|---|---|
| Range adaptor | views::transform |
| Underlying range | Must satisfy input_range or higher |
| Element type | Same as the transformation function's return type. |
| View iterator category | Supports input_range up to random_access_range, depending on the underlying range |
| Sized | Only if the underlying range satisfies sized_range |
Is const-iterable |
Only if the underlying range is const iterable and the transformation works on const references. |
| Common range | Only if the underlying range satisfies common_range |
| Borrowed range | No |
| Member functions | Description |
|---|---|
| ConstructorsC++20 | Construct the view. |
baseC++20 |
Get the underlying range. |
beginC++20 |
Get an iterator to the first element. |
endC++20 |
Get the sentinel at the end of the view. |
sizeC++20 |
Get the number of elements. The underlying range must satisfy sized_range. |
Inherited from view_interface |
Description |
backC++20 |
Get the last element. |
emptyC++20 |
Test whether the view is empty. |
frontC++20 |
Get the first element. |
operator boolC++20 |
Test whether the view isn't empty. |
operator[]C++20 |
Get the element at the specified position. |
Header: <ranges> (since C++20)
Namespace: std::ranges
Compiler Option: /std:c++20 or later is required.
Construct an instance of a transform_view
1) transform_view() requires default_initializable<V>
&& default_initializable<F> = default;
2) constexpr transform_view(V base, F func);base
The underlying view.
func
The function that transforms each element.
For information about template parameter types, see Template parameters.
A transform_view instance.
The best way to create a transform_view is by using the views::transform range adaptor. Range adaptors are the intended way to create view classes. The view types are exposed in case you want to create your own custom view type.
1) Create a value-initialized transform_view. The transformation function and the underlying view must be default-initializable.
2) Move construct the transform_view from a base view and a transformation function func. Both base and func are moved via std::move().
// requires /std:c++20 or later
#include <ranges>
#include <iostream>
#include <vector>
#include <chrono>
using namespace std;
using namespace chrono;
void print(auto v)
{
for (auto x : v)
{
cout << x << ' ';
}
cout << '\n';
}
struct classes
{
string className;
weekday startDay;
};
int main()
{
std::vector<int> v{0, 1, 2, 3, -4, 5, 6};
// outputs 0 2 4 6 -8 10 12
print(v | std::views::transform([](int i) {return i * 2; }));
// ---- Modify the elements in the collection by returning a reference to the element to transform
std::vector<classes> theClasses = {
{"Math", Monday},
{"English", Wednesday},
{"History", Monday},
{"Science", Wednesday},
{"Art", Friday},
{"Music", Thursday}
};
// lambda to get a reference to the day of the week for a class
auto getDay = [](classes& c) -> weekday&
{
return c.startDay;
};
// If a class starts on Monday, change it to Tuesday
for (auto&& startDay : theClasses | std::views::transform(getDay))
{
// modify the startDay in the collection
if (startDay == Monday)
{
startDay = Tuesday;
}
}
// output classes and start times
for (auto c : theClasses)
{
std::cout << c.className << " : " << c.startDay << '\n';
}
}0 2 4 6 -8 10 12
Math : Tue
English : Wed
History : Tue
Science : Wed
Art : Fri
Music : Thu
Get the underlying view.
// Uses a copy constructor to return the underlying view
constexpr V base() const& requires std::copy_constructible<V>;
// Uses std::move() to return the underlying view
constexpr V base() &&;None.
The underlying view.
Get an iterator to the first element in the view.
constexpr auto begin();An iterator pointing at the first element in the view. The behavior is undefined if the view doesn't have a predicate.
:::image type="content" source="media/begin-end-sentinel.png" alt-text="Picture of a vector with the elements 10, 20, and 30. The first element contains 10 and is labeled begin(). The last element contains 30 and is labeled 'last element'. An imaginary box after the last element indicates the sentinel and is labeled end().":::
Get the sentinel at the end of the view.
constexpr auto end()The sentinel that follows the last element in the view:
:::image type="content" source="media/begin-end-sentinel.png" alt-text="Picture of a vector with the elements 10, 20, and 30. The first element contains 10 and is labeled begin(). The last element contains 30 and is labeled 'last element'. An imaginary box after the last element indicates the sentinel and is labeled end().":::
Get the number of elements in the view.
constexpr auto size() requires ranges::sized_range<V>;
constexpr auto size() const requires ranges::sized_range<const V>;None.
The number of elements in the view.