Ada Reference Manual — Legal Information

11.2 Exception Handlers

The response to one or more exceptions is specified by an exception_handler.

Syntax

handled_sequence_of_statements ::=
     sequence_of_statements
  [exception
     exception_handler
    {exception_handler}]

exception_handler ::=
when [choice_parameter_specification:] exception_choice {| exception_choice} =>
sequence_of_statements

choice_parameter_specification ::= defining_identifier

exception_choice ::= exception_name | others

Legality Rules

A choice with an exception_name covers the named exception. A choice with others covers all exceptions not named by previous choices of the same handled_sequence_of_statements. Two choices in different exception_handlers of the same handled_sequence_of_statements shall not cover the same exception.

A choice with others is allowed only for the last handler of a handled_sequence_of_statements and as the only choice of that handler.

An exception_name of a choice shall not denote an exception declared in a generic formal package.

Static Semantics

A choice_parameter_specification declares a choice parameter, which is a constant object of type Exception_Occurrence (see 11.4.1). During the handling of an exception occurrence, the choice parameter, if any, of the handler represents the exception occurrence that is being handled.

Dynamic Semantics

The execution of a handled_sequence_of_statements consists of the execution of the sequence_of_statements. The optional handlers are used to handle any exceptions that are propagated by the sequence_of_statements.

Examples

Example of an exception handler:

begin
   Open(File, In_File, "input.txt");   -- see A.8.2
exception
   when E : Name_Error =>
      Put("Cannot open input file : ");
      Put_Line(Exception_Message(E));  -- see 11.4.1
      raise;
end;