View Compilation Error

1_549_1311_28486.cpp: In constructor `Node::Node(int, int)':
1_549_1311_28486.cpp:16: error: `nullptr' undeclared (first use this function)
1_549_1311_28486.cpp:16: error: (Each undeclared identifier is reported only once for each function it appears in.)
1_549_1311_28486.cpp: In constructor `Chat::Chat()':
1_549_1311_28486.cpp:24: error: `nullptr' undeclared (first use this function)
1_549_1311_28486.cpp: In member function `int Chat::add(int)':
1_549_1311_28486.cpp:33: error: `nullptr' undeclared (first use this function)
1_549_1311_28486.cpp: In member function `int Chat::close(int)':
1_549_1311_28486.cpp:63: error: `nullptr' undeclared (first use this function)
1_549_1311_28486.cpp: In member function `int Chat::chat(int)':
1_549_1311_28486.cpp:71: error: `nullptr' undeclared (first use this function)
1_549_1311_28486.cpp: In member function `int Chat::Untop()':
1_549_1311_28486.cpp:92: error: `nullptr' undeclared (first use this function)
1_549_1311_28486.cpp: In member function `void Chat::pushFront(Node*)':
1_549_1311_28486.cpp:114: error: `nullptr' undeclared (first use this function)
1_549_1311_28486.cpp: In member function `void Chat::bye()':
1_549_1311_28486.cpp:127: error: `nullptr' undeclared (first use this function)
1_549_1311_28486.cpp: In member function `void Chat::print()':
1_549_1311_28486.cpp:141: error: `nullptr' undeclared (first use this function)