21 Language support library [language.support]

21.8 Exception handling [support.exception]

21.8.2 Class exception [exception]

namespace std {
  class exception {
  public:
    exception() noexcept;
    exception(const exception&) noexcept;
    exception& operator=(const exception&) noexcept;
    virtual ~exception();
    virtual const char* what() const noexcept;
  };
}
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The class exception defines the base class for the types of objects thrown as exceptions by C++ standard library components, and certain expressions, to report errors detected during program execution.
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Each standard library class T that derives from class exception shall have a publicly accessible copy constructor and a publicly accessible copy assignment operator that do not exit with an exception.
These member functions shall meet the following postcondition: If two objects lhs and rhs both have dynamic type T and lhs is a copy of rhs, then strcmp(lhs.what(), rhs.what()) shall equal 0.
exception() noexcept;
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Effects: Constructs an object of class exception.
exception(const exception& rhs) noexcept; exception& operator=(const exception& rhs) noexcept;
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Effects: Copies an exception object.
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Postconditions: If *this and rhs both have dynamic type exception then the value of the expression strcmp(what(), rhs.what()) shall equal 0.
virtual ~exception();
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Effects: Destroys an object of class exception.
virtual const char* what() 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 ([multibyte.strings]), suitable for conversion and display as a wstring ([string.classes], [locale.codecvt]).
The return value remains valid until the exception object from which it is obtained is destroyed or a non-const member function of the exception object is called.