C++ Primer, Fifth Edition

5.6.1. A throw Expression

The detecting part of a program uses a throw expression to raise an exception. A throw consists of the keyword throw followed by an expression. The type of the expression determines what kind of exception is thrown. A throw expression is usually followed by a semicolon, making it into an expression statement.

As a simple example, recall the program in § 1.5.2 (p. 23) that added two objects of type Sales_item. That program checked whether the records it read referred to the same book. If not, it printed a message and exited.

Sales_item item1, item2;cin >> item1 >> item2;// first check that item1 and item2 represent the same bookif (item1.isbn() == item2.isbn()) {    cout << item1 + item2 << endl;    return 0;   // indicate ...