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Difference between revisions of "cpp/ranges/range adaptor closure"

From cppreference.com
< cpp‎ | ranges
m (langlink:zh)
m (fmt; warning in #Example is peculiarity of gcc-13 (trunk and others deal fine with "missing initializer"))
Line 17: Line 17:
 
===Example===
 
===Example===
 
{{example
 
{{example
−
|code=
+
|code=
−
#include <iostream>
+
 
#include <ranges>
 
#include <ranges>
−
#include <string>
 
 
#include <string_view>
 
#include <string_view>
  
−
// define Slice as a range adaptor closure
+
// Define Slice as a range adaptor closure
−
struct Slice : std::ranges::range_adaptor_closure<Slice> {
+
struct Slice : std::ranges::range_adaptor_closure<Slice>
 +
{
 
     std::size_t start = 0;
 
     std::size_t start = 0;
 
     std::size_t end = std::string_view::npos;
 
     std::size_t end = std::string_view::npos;
  
−
     std::string_view operator()(std::string_view sv) const {
+
     constexpr std::string_view operator()(std::string_view sv) const
 +
    {
 
         return sv.substr(start, end - start);
 
         return sv.substr(start, end - start);
 
     }
 
     }
 
};
 
};
  
−
int main() {
+
int main()
−
     std::string str = "01234567";
+
{
 +
     constexpr std::string_view str = "01234567";
  
−
     Slice slicer{.start = 1, .end = 6};
+
     constexpr Slice slicer{.start = 1, .end = 6};
  
−
     auto sv1 = slicer(str); // use slicer as a normal function object
+
     constexpr auto sv1 = slicer(str); // use slicer as a normal function object
−
     auto sv2 = str {{!}} slicer; // use slicer as a range adaptor closure object
+
     constexpr auto sv2 = str {{!}} slicer; // use slicer as a range adaptor closure object
−
     std::cout << sv1 << '\n';
+
     static_assert(sv1 == "12345");
−
     std::cout << sv2 << '\n';
+
     static_assert(sv2 == "12345");
  
 
     // range adaptor closures can be composed
 
     // range adaptor closures can be composed
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     auto slice_and_drop = slicer
+
     constexpr auto slice_and_drop
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                        {{!}} std::views::drop_while([](auto ch) { return ch != '3'; });
+
        = slicer
−
     std::cout << std::string_view(str {{!}} slice_and_drop) << '\n';
+
        {{!}} std::views::drop_while([](char ch) { return ch != '3'; });
 +
     static_assert(std::string_view(str {{!}} slice_and_drop) == "345");
 
}
 
}
−
|output=
 
−
12345
 
−
12345
 
−
345
 
 
}}
 
}}
  
−
{{langlinks|de|zh|es|ja|ru}}
+
{{langlinks|de|es|ja|ru|zh}}

Revision as of 10:19, 28 September 2023

 
C++
 
Ranges library
Range adaptors
Range adaptor closure objects
range_adaptor_closure
(C++23)    
 
Defined in header <ranges>
template< class D >

    requires std::is_object_v<D> && std::same_as<D, std::remove_cv_t<D>>

class range_adaptor_closure {};
 (since C++23)

std::ranges::range_adaptor_closure is a helper class template for defining a RangeAdaptorClosureObject.

Let t be the object of type T, the implementation ensures that t is a range adaptor closure object if all the requirements are met:

  • t is a unary function object that takes one range argument.
  • T has exactly one public base class ranges::range_adaptor_closure<T>, and T has no base classes of type ranges::range_adaptor_closure<U> for any other type U.
  • T does not satisfy range.

Example

Run this code
#include <ranges>
#include <string_view>
 
// Define Slice as a range adaptor closure
struct Slice : std::ranges::range_adaptor_closure<Slice>
{
    std::size_t start = 0;
    std::size_t end = std::string_view::npos;
 
    constexpr std::string_view operator()(std::string_view sv) const
    {
        return sv.substr(start, end - start);
    }
};
 
int main()
{
    constexpr std::string_view str = "01234567";
 
    constexpr Slice slicer{.start = 1, .end = 6};
 
    constexpr auto sv1 = slicer(str); // use slicer as a normal function object
    constexpr auto sv2 = str | slicer; // use slicer as a range adaptor closure object
    static_assert(sv1 == "12345");
    static_assert(sv2 == "12345");
 
    // range adaptor closures can be composed
    constexpr auto slice_and_drop
        = slicer
        | std::views::drop_while([](char ch) { return ch != '3'; });
    static_assert(std::string_view(str | slice_and_drop) == "345");
}
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