Document Number

P1328R0
Date

2018-10-28
Project

Programming Language C++
Audience

Evolution Working Group
Summary

This paper proposes making std::type_info::operator== and operator!= constexpr.
Table of Contents

Summary

typeid (with the exception of its application on polymorphic glvalues, a restriction that P1327 proposes to lift) 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== and operator!= be made constexpr, enabling practical, rather than theoretical, use of typeid in constant expressions.

Proposed Changes

(All edits are relative to N4778.)

Change 16.7.2 [type.info] as follows:

  • namespace std {
  class type_info {
  public:
    virtual ~type_info();
    constexpr bool operator==(const type_info& rhs) const noexcept;
    constexpr bool operator!=(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;
    constexpr bool operator!=(const type_info& rhs) const noexcept;

    -- end