17 Language support library [support]

17.7 Type identification [support.rtti]

17.7.1 General [support.rtti.general]

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The header <typeinfo> defines a type associated with type information generated by the implementation.
It also defines two types for reporting dynamic type identification errors.
The header <typeindex> defines a wrapper type for use as an index type in associative containers ([associative]) and in unordered associative containers ([unord]).

17.7.2 Header <typeinfo> synopsis [typeinfo.syn]

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// all freestanding namespace std { class type_info; class bad_cast; class bad_typeid; }

17.7.3 Class type_info [type.info]

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namespace std { class type_info { public: virtual ~type_info(); constexpr bool operator==(const type_info& rhs) const noexcept; bool before(const type_info& rhs) const noexcept; size_t hash_code() const noexcept; const char* name() const noexcept; type_info(const type_info&) = delete; // cannot be copied type_info& operator=(const type_info&) = delete; // cannot be copied }; }
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The class type_info describes type information generated by the implementation ([expr.typeid]).
Objects of this class effectively store a pointer to a name for the type, and an encoded value suitable for comparing two types for equality or collating order.
The names, encoding rule, and collating sequence for types are all unspecified and may differ between programs.
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constexpr bool operator==(const type_info& rhs) const noexcept;
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Effects: Compares the current object with rhs.
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Returns: true if the two values describe the same type.
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bool before(const type_info& rhs) const noexcept;
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Effects: Compares the current object with rhs.
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Returns: true if *this precedes rhs in the implementation's collation order.
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size_t hash_code() const noexcept;
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Returns: An unspecified value, except that within a single execution of the program, it shall return the same value for any two type_info objects which compare equal.
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Remarks: An implementation should return different values for two type_info objects which do not compare equal.
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const char* name() const noexcept;
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Returns: An implementation-defined ntbs.
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Remarks: The message may be a null-terminated multibyte string, suitable for conversion and display as a wstring ([string.classes], [locale.codecvt]).

17.7.4 Class bad_cast [bad.cast]

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namespace std { class bad_cast : public exception { public: // see [exception] for the specification of the special member functions const char* what() const noexcept override; }; }
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The class bad_cast defines the type of objects thrown as exceptions by the implementation to report the execution of an invalid dynamic_cast expression ([expr.dynamic.cast]).
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const char* what() const noexcept override;
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Returns: An implementation-defined ntbs.

17.7.5 Class bad_typeid [bad.typeid]

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namespace std { class bad_typeid : public exception { public: // see [exception] for the specification of the special member functions const char* what() const noexcept override; }; }
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The class bad_typeid defines the type of objects thrown as exceptions by the implementation to report a null pointer in a typeid expression ([expr.typeid]).
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const char* what() const noexcept override;
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Returns: An implementation-defined ntbs.

17.7.6 Header <typeindex> synopsis [type.index.synopsis]

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#include <compare> // see [compare.syn] namespace std { class type_index; template<class T> struct hash; template<> struct hash<type_index>; }

17.7.7 Class type_index [type.index]

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namespace std { class type_index { public: type_index(const type_info& rhs) noexcept; bool operator==(const type_index& rhs) const noexcept; bool operator< (const type_index& rhs) const noexcept; bool operator> (const type_index& rhs) const noexcept; bool operator<=(const type_index& rhs) const noexcept; bool operator>=(const type_index& rhs) const noexcept; strong_ordering operator<=>(const type_index& rhs) const noexcept; size_t hash_code() const noexcept; const char* name() const noexcept; private: const type_info* target; // exposition only // Note that the use of a pointer here, rather than a reference, // means that the default copy/move constructor and assignment // operators will be provided and work as expected. }; }
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The class type_index provides a simple wrapper for type_info which can be used as an index type in associative containers ([associative]) and in unordered associative containers ([unord]).
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type_index(const type_info& rhs) noexcept;
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Effects: Constructs a type_index object, the equivalent of target = &rhs.
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bool operator==(const type_index& rhs) const noexcept;
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Returns: *target == *rhs.target.
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bool operator<(const type_index& rhs) const noexcept;
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Returns: target->before(*rhs.target).
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bool operator>(const type_index& rhs) const noexcept;
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Returns: rhs.target->before(*target).
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bool operator<=(const type_index& rhs) const noexcept;
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Returns: !rhs.target->before(*target).
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bool operator>=(const type_index& rhs) const noexcept;
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Returns: !target->before(*rhs.target).
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strong_ordering operator<=>(const type_index& rhs) const noexcept;
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Effects: Equivalent to: if (*target == *rhs.target) return strong_ordering::equal; if (target->before(*rhs.target)) return strong_ordering::less; return strong_ordering::greater;
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size_t hash_code() const noexcept;
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Returns: target->hash_code().
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const char* name() const noexcept;
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Returns: target->name().
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template<> struct hash<type_index>;
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For an object index of type type_index, hash<type_index>()(index) shall evaluate to the same result as index.hash_code().