Difference between revisions of "cpp/utility/format/format kind"

Latest revision as of 06:10, 24 September 2024

Defined in header `<format>`
template< class R > constexpr /* unspecified / format_kind = / unspecified */;	(1)	(since C++23)
template< ranges::input_range R > requires std::same_as<R, std::remove_cvref_t<R>> constexpr range_format format_kind<R> = /* see description */;	(2)	(since C++23)

The variable template format_kind selects an appropriate std::range_format for a range R.

std::format_kind<R> is defined as follows:

If std::same_as<std::remove_cvref_t<ranges::range_reference_t<R>>, R> is true, std::format_kind<R> is std::range_format::disabled.
Otherwise, if R::key_type is valid and denotes a type:
- If R::mapped_type is valid and denotes a type, let U be std::remove_cvref_t<ranges::range_reference_t<R>>.

If either U is a specialization of std::pair or U is a specialization of std::tuple and std::tuple_size_v<U> == 2, std::format_kind<R> is std::range_format::map.

Otherwise, std::format_kind<R> is std::range_format::set.

Otherwise, std::format_kind<R> is std::range_format::sequence.

A program that instantiates a primary template of the format_kind variable template is ill-formed.

Given a cv-unqualified program-defined type T that models input_range, a program can specialize format_kind for T. Such specializations are usable in constant expressions, and have type const std::range_format.

[edit] Possible implementation

namespace detail
{
    template< typename >
    constexpr bool is_pair_or_tuple_2 = false;
 
    template< typename T, typename U >
    constexpr bool is_pair_or_tuple_2<std::pair<T, U>> = true;
 
    template< typename T, typename U >
    constexpr bool is_pair_or_tuple_2<std::tuple<T, U>> = true;
 
    template < typename T >
        requires std::is_reference_v<T> || std::is_const_v<T>
    constexpr bool is_pair_or_tuple_2<T> =
        is_pair_or_tuple_2<std::remove_cvref_t<T>>;
}
 
template< class R >
constexpr range_format format_kind = []
{
    static_assert(false, "instantiating a primary template is not allowed");
    return range_format::disabled;
}();
 
template< ranges::input_range R >
    requires std::same_as<R, std::remove_cvref_t<R>>
constexpr range_format format_kind<R> = []
{
    if constexpr (std::same_as<std::remove_cvref_t<std::ranges::range_reference_t<R>>, R>)
        return range_format::disabled;
    else if constexpr (requires { typename R::key_type; })
    {
        if constexpr (requires { typename R::mapped_type; } &&
                      detail::is_pair_or_tuple_2<std::ranges::range_reference_t<R>>)
            return range_format::map;
        else
            return range_format::set;
    }
    else
        return range_format::sequence;
}();

[edit] Example

Run this code

#include <filesystem>
#include <format>
#include <map>
#include <set>
#include <vector>
 
struct A {};
 
static_assert(std::format_kind<std::vector<int>> == std::range_format::sequence);
static_assert(std::format_kind<std::map<int>> == std::range_format::map);
static_assert(std::format_kind<std::set<int>> == std::range_format::set);
static_assert(std::format_kind<std::filesystem::path> == std::range_format::disabled);
// ill-formed:
// static_assert(std::format_kind<A> == std::range_format::disabled);
 
int main() {}

[edit] See also

range_format

(C++23)

specifies how a range should be formatted
(enum) [edit]

@@ Line 6: / Line 6: @@
 {{dcl|num=1|since=c++23|1=
 template< class R >
-inline constexpr /* unspecified */ format_kind = /* unspecified */;
+constexpr /* unspecified */ format_kind = /* unspecified */;
 }}
 {{dcl|num=2|since=c++23|1=
 template< ranges::input_range R >
      requires std::same_as<R, std::remove_cvref_t<R>>
-inline constexpr range_format format_kind<R> = /* see description */;
+constexpr range_format format_kind<R> = /* see description */;
 }}
 {{dcl end}}
@@ Line 22: / Line 22: @@
 ** If {{tt|R::mapped_type}} is valid and denotes a type, let {{tt|U}} be {{c|std::remove_cvref_t<ranges::range_reference_t<R>>}}.
 :: If either {{tt|U}} is a specialization of {{lc|std::pair}} or {{tt|U}} is a specialization of {{lc|std::tuple}} and {{c|1=std::tuple_size_v<U> == 2}}, {{c|std::format_kind<R>}} is {{c|std::range_format::map}}.
-** Otherwise, {{c|std::format_kind<R>}} is {{c|std::range_format::set}}.
+:* Otherwise, {{c|std::format_kind<R>}} is {{c|std::range_format::set}}.
 * Otherwise, {{c|std::format_kind<R>}} is {{c|std::range_format::sequence}}.
 A program that instantiates a primary template of the {{tt|format_kind}} variable template is ill-formed.
-Given a cv-unqualified {{lsd|cpp/language/type#Program-defined type}} {{tt|T}} that models {{lconcept|input_range}}, a program can spcialize {{tt|format_kind}} for {{tt|T}}. Such specializations are usable in constant expressions, and have type {{c/core|const std::range_format}}.
+Given a cv-unqualified {{lsd|cpp/language/type#Program-defined type}} {{tt|T}} that models {{lconcept|input_range}}, a program can specialize {{tt|format_kind}} for {{tt|T}}. Such specializations are usable in constant expressions, and have type {{c/core|const std::range_format}}.
 ===Possible implementation===
@@ Line 35: / Line 35: @@
 {
      template< typename >
-     inline constexpr bool is_pair_or_tuple_2 = false;
+     constexpr bool is_pair_or_tuple_2 = false;
      template< typename T, typename U >
-     inline constexpr bool is_pair_or_tuple_2<std::pair<T, U>> = true;
+     constexpr bool is_pair_or_tuple_2<std::pair<T, U>> = true;
-     template< typename... Ts >
+     template< typename T, typename U >
-     inline constexpr bool is_pair_or_tuple_2<std::tuple<Ts...>> = sizeof...(Ts) == 2;
+     constexpr bool is_pair_or_tuple_2<std::tuple<T, U>> = true;
      template < typename T >
          requires std::is_reference_v<T> {{!!}} std::is_const_v<T>
-     inline constexpr bool is_pair_or_tuple_2<T> =
+     constexpr bool is_pair_or_tuple_2<T> =
          is_pair_or_tuple_2<std::remove_cvref_t<T>>;
 }
 template< class R >
-inline constexpr range_format format_kind = []
+constexpr range_format format_kind = []
 {
      static_assert(false, "instantiating a primary template is not allowed");
@@ Line 58: / Line 58: @@
 template< ranges::input_range R >
      requires std::same_as<R, std::remove_cvref_t<R>>
-inline constexpr range_format format_kind<R> = []
+constexpr range_format format_kind<R> = []
 {
      if constexpr (std::same_as<std::remove_cvref_t<std::ranges::range_reference_t<R>>, R>)

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Standard format specification
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format (C++20)
format_to (C++20)
format_to_n (C++20)
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vformat (C++20)
vformat_to (C++20)
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format_kind (C++23)
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make_format_argsmake_wformat_args (C++20)(C++20)
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format_error (C++20)

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Difference between revisions of "cpp/utility/format/format kind"

Latest revision as of 06:10, 24 September 2024

[edit] Possible implementation

[edit] Example

[edit] See also

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