std::exception::exception (3) - Linux Man Pages
exception() throw(); (until C++11)
exception() noexcept; (1) (since C++11)
exception( const exception& other ) throw(); (2) (until C++11)
exception( const exception& other ) noexcept; (since C++11)
Constructs new exception object.
1) Default constructor. what() returns an implementation-defined string.
2) Copy constructor. Initializes the contents with those of other.
If *this and other both have dynamic type std::exception then std::strcmp(what(), other.what()) == 0.
other - another exception to assign the contents of
Because copying std::exception is not permitted to throw exceptions, when derived classes (such as std::runtime_error) have to manage a user-defined diagnostic message, it is typically implemented as a copy-on-write string.