20 General utilities library [utilities]

20.20 Formatting [format]

20.20.6 Arguments [format.arguments]

20.20.6.1 Class template basic_­format_­arg [format.arg]

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namespace std { template<class Context> class basic_format_arg { public: class handle; private: using char_type = typename Context::char_type; // exposition only variant<monostate, bool, char_type, int, unsigned int, long long int, unsigned long long int, float, double, long double, const char_type*, basic_string_view<char_type>, const void*, handle> value; // exposition only template<class T> explicit basic_format_arg(const T& v) noexcept; // exposition only explicit basic_format_arg(float n) noexcept; // exposition only explicit basic_format_arg(double n) noexcept; // exposition only explicit basic_format_arg(long double n) noexcept; // exposition only explicit basic_format_arg(const char_type* s); // exposition only template<class traits> explicit basic_format_arg( basic_string_view<char_type, traits> s) noexcept; // exposition only template<class traits, class Allocator> explicit basic_format_arg( const basic_string<char_type, traits, Allocator>& s) noexcept; // exposition only explicit basic_format_arg(nullptr_t) noexcept; // exposition only template<class T> explicit basic_format_arg(const T* p) noexcept; // exposition only public: basic_format_arg() noexcept; explicit operator bool() const noexcept; }; }
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An instance of basic_­format_­arg provides access to a formatting argument for user-defined formatters.
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The behavior of a program that adds specializations of basic_­format_­arg is undefined.
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basic_format_arg() noexcept;
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Postconditions: !(*this).
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template<class T> explicit basic_format_arg(const T& v) noexcept;
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Constraints: The template specialization typename Context::template formatter_type<T> meets the Formatter requirements ([formatter.requirements]).
The extent to which an implementation determines that the specialization meets the Formatter requirements is unspecified, except that as a minimum the expression typename Context::template formatter_type<T>() .format(declval<const T&>(), declval<Context&>()) shall be well-formed when treated as an unevaluated operand.
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Effects:
  • (5.1)
    if T is bool or char_­type, initializes value with v;
  • (5.2)
    otherwise, if T is char and char_­type is wchar_­t, initializes value with static_­cast<wchar_­t>(v);
  • (5.3)
    otherwise, if T is a signed integer type ([basic.fundamental]) and sizeof(T) <= sizeof(int), initializes value with static_­cast<int>(v);
  • (5.4)
    otherwise, if T is an unsigned integer type and sizeof(T) <= sizeof(unsigned int), initializes value with static_­cast<unsigned int>(v);
  • (5.5)
    otherwise, if T is a signed integer type and sizeof(T) <= sizeof(long long int), initializes value with static_­cast<long long int>(v);
  • (5.6)
    otherwise, if T is an unsigned integer type and sizeof(T) <= sizeof(unsigned long long int), initializes value with static_­cast<unsigned long long int>(v);
  • (5.7)
    otherwise, initializes value with handle(v).
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explicit basic_format_arg(float n) noexcept; explicit basic_format_arg(double n) noexcept; explicit basic_format_arg(long double n) noexcept;
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Effects: Initializes value with n.
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explicit basic_format_arg(const char_type* s);
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Preconditions: s points to a NTCTS ([defns.ntcts]).
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Effects: Initializes value with s.
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template<class traits> explicit basic_format_arg(basic_string_view<char_type, traits> s) noexcept;
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Effects: Initializes value with s.
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template<class traits, class Allocator> explicit basic_format_arg( const basic_string<char_type, traits, Allocator>& s) noexcept;
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Effects: Initializes value with basic_­string_­view<char_­type>(s.data(), s.size()).
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explicit basic_format_arg(nullptr_t) noexcept;
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Effects: Initializes value with static_­cast<const void*>(nullptr).
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template<class T> explicit basic_format_arg(const T* p) noexcept;
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Constraints: is_­void_­v<T> is true.
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Effects: Initializes value with p.
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[Note 1:
Constructing basic_­format_­arg from a pointer to a member is ill-formed unless the user provides an enabled specialization of formatter for that pointer to member type.
— end note]
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explicit operator bool() const noexcept;
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Returns: !holds_­alternative<monostate>(value).
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The class handle allows formatting an object of a user-defined type.
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namespace std { template<class Context> class basic_format_arg<Context>::handle { const void* ptr_; // exposition only void (*format_)(basic_format_parse_context<char_type>&, Context&, const void*); // exposition only template<class T> explicit handle(const T& val) noexcept; // exposition only friend class basic_format_arg<Context>; // exposition only public: void format(basic_format_parse_context<char_type>&, Context& ctx) const; }; }
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template<class T> explicit handle(const T& val) noexcept;
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Effects: Initializes ptr_­ with addressof(val) and format_­ with [](basic_format_parse_context<char_type>& parse_ctx, Context& format_ctx, const void* ptr) { typename Context::template formatter_type<T> f; parse_ctx.advance_to(f.parse(parse_ctx)); format_ctx.advance_to(f.format(*static_cast<const T*>(ptr), format_ctx)); }
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void format(basic_format_parse_context<char_type>& parse_ctx, Context& format_ctx) const;
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Effects: Equivalent to: format_­(parse_­ctx, format_­ctx, ptr_­);
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template<class Visitor, class Context> decltype(auto) visit_format_arg(Visitor&& vis, basic_format_arg<Context> arg);
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Effects: Equivalent to: return visit(forward<Visitor>(vis), arg.value);