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11.4.1 The Package Exceptions

Static Semantics

1
The following language-defined library package exists: 
2/2
with Ada.Streams;
package Ada.Exceptions is
    pragma Preelaborate(Exceptions);
    type Exception_Id is private;
    pragma Preelaborable_Initialization(Exception_Id);
    Null_Id : constant Exception_Id;
    function Exception_Name(Id : Exception_Id) return String;
    function Wide_Exception_Name(Id : Exception_Id) return Wide_String;
    function Wide_Wide_Exception_Name(Id : Exception_Id)
        return Wide_Wide_String;
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    type Exception_Occurrence is limited private;
    pragma Preelaborable_Initialization(Exception_Occurrence);
    type Exception_Occurrence_Access is access all Exception_Occurrence;
    Null_Occurrence : constant Exception_Occurrence;
4/3
    procedure Raise_Exception(E : in Exception_Id;
                              Message : in String := "")
        with No_Return;
    function Exception_Message(X : Exception_Occurrence) return String;
    procedure Reraise_Occurrence(X : in Exception_Occurrence);
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    function Exception_Identity(X : Exception_Occurrence)
                                return Exception_Id;
    function Exception_Name(X : Exception_Occurrence) return String;
        -- Same as Exception_Name(Exception_Identity(X)).
    function Wide_Exception_Name(X : Exception_Occurrence)
        return Wide_String;
        -- Same as Wide_Exception_Name(Exception_Identity(X)).
    function Wide_Wide_Exception_Name(X : Exception_Occurrence)
        return Wide_Wide_String;
        -- Same as Wide_Wide_Exception_Name(Exception_Identity(X)).
    function Exception_Information(X : Exception_Occurrence) return String;
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    procedure Save_Occurrence(Target : out Exception_Occurrence;
                              Source : in Exception_Occurrence);
    function Save_Occurrence(Source : Exception_Occurrence)
                             return Exception_Occurrence_Access;
6.1/2
    procedure Read_Exception_Occurrence
       (Stream : not null access Ada.Streams.Root_Stream_Type'Class;
        Item   : out Exception_Occurrence);
    procedure Write_Exception_Occurrence
       (Stream : not null access Ada.Streams.Root_Stream_Type'Class;
        Item   : in Exception_Occurrence);
6.2/2
    for Exception_Occurrence'Read use Read_Exception_Occurrence;
    for Exception_Occurrence'Write use Write_Exception_Occurrence;
6.3/2
private
   ... -- not specified by the language
end Ada.Exceptions;
7
Each distinct exception is represented by a distinct value of type Exception_Id. Null_Id does not represent any exception, and is the default initial value of type Exception_Id. Each occurrence of an exception is represented by a value of type Exception_Occurrence. Null_Occurrence does not represent any exception occurrence, and is the default initial value of type Exception_Occurrence.
8/1
For a prefix E that denotes an exception, the following attribute is defined: 
9
E'Identity
E'Identity returns the unique identity of the exception. The type of this attribute is Exception_Id.
10/2
 Raise_Exception raises a new occurrence of the identified exception.
10.1/4
   Exception_Message returns the message associated with the given Exception_Occurrence. For an occurrence raised by a call to Raise_Exception, the message is the Message parameter passed to Raise_Exception. For the occurrence raised by a raise_statement or raise_expression with an exception_name and a string_expression or string_simple_expression, the message is the string_expression or string_simple_expression. For the occurrence raised by a raise_statement or raise_expression with an exception_name but without a string_expression or string_simple_expression, the message is a string giving implementation-defined information about the exception occurrence. For an occurrence originally raised in some other manner (including by the failure of a language-defined check), the message is an unspecified string. In all cases, Exception_Message returns a string with lower bound 1. 
10.2/2
   Reraise_Occurrence reraises the specified exception occurrence.
11
Exception_Identity returns the identity of the exception of the occurrence.
12/2
 The Wide_Wide_Exception_Name functions return the full expanded name of the exception, in upper case, starting with a root library unit. For an exception declared immediately within package Standard, the defining_identifier is returned. The result is implementation defined if the exception is declared within an unnamed block_statement.
12.1/2
   The Exception_Name functions (respectively, Wide_Exception_Name) return the same sequence of graphic characters as that defined for Wide_Wide_Exception_Name, if all the graphic characters are defined in Character (respectively, Wide_Character); otherwise, the sequence of characters is implementation defined, but no shorter than that returned by Wide_Wide_Exception_Name for the same value of the argument. 
12.2/2
   The string returned by the Exception_Name, Wide_Exception_Name, and Wide_Wide_Exception_Name functions has lower bound 1.
13/2
 Exception_Information returns implementation-defined information about the exception occurrence. The returned string has lower bound 1. 
14/2
 Reraise_Occurrence has no effect in the case of Null_Occurrence. Raise_Exception and Exception_Name raise Constraint_Error for a Null_Id. Exception_Message, Exception_Name, and Exception_Information raise Constraint_Error for a Null_Occurrence. Exception_Identity applied to Null_Occurrence returns Null_Id.
15
The Save_Occurrence procedure copies the Source to the Target. The Save_Occurrence function uses an allocator of type Exception_Occurrence_Access to create a new object, copies the Source to this new object, and returns an access value designating this new object; the result may be deallocated using an instance of Unchecked_Deallocation.
15.1/2
   Write_Exception_Occurrence writes a representation of an exception occurrence to a stream; Read_Exception_Occurrence reconstructs an exception occurrence from a stream (including one written in a different partition). 
Paragraph 16 was deleted. 

Implementation Permissions

17
An implementation of Exception_Name in a space-constrained environment may return the defining_identifier instead of the full expanded name.
18
The string returned by Exception_Message may be truncated (to no less than 200 characters) by the Save_Occurrence procedure (not the function), the Reraise_Occurrence procedure, and the re-raise statement.

Implementation Advice

19
Exception_Message (by default) and Exception_Information should produce information useful for debugging. Exception_Message should be short (about one line), whereas Exception_Information can be long. Exception_Message should not include the Exception_Name. Exception_Information should include both the Exception_Name and the Exception_Message.

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