Document Number

P1328R1
Date

2021-05-01
Project

Programming Language C++
Audience

Library Working Group
Summary

This paper proposes making std::type_info::operator== constexpr.

Revision History

Revision R1

  • Rebased against N4885

  • Added a feature macro

Summary

typeid is currently allowed in constant expressions, but the resulting std::type_info object is unusable as it has no constexpr member functions.

This paper proposes std::type_info::operator== be made constexpr, enabling practical, rather than theoretical, use of typeid in constant expressions.

Proposed Changes

(All edits are relative to N4885.)

Change 17.7.3 [type.info] as follows:

  • 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& rhs) = delete; // cannot be copied
    type_info& operator=(const type_info& rhs) = delete; // cannot be copied
  };
}
    constexpr bool operator==(const type_info& rhs) const noexcept;

Add to 17.3.2 [version.syn]:

  • #define __cpp_lib_constexpr_typeinfo   202105L  // also in <typeinfo>

with the value appropriately replaced with the date of adoption.

-- end