A.10.7 Input-Output of Characters and Strings
Static Semantics
For an item of type
Character the following procedures are provided:
procedure Get(File : in File_Type; Item : out Character);
procedure Get(Item : out Character);
After skipping any line terminators and any page
terminators, reads the next character from the specified input file and
returns the value of this character in the out parameter Item.
The exception End_Error
is propagated if an attempt is made to skip a file terminator.
procedure Put(File : in File_Type; Item : in Character);
procedure Put(Item : in Character);
If the line length
of the specified output file is bounded (that is, does not have the conventional
value zero), and the current column number exceeds it, has the effect
of calling New_Line with a spacing of one. Then, or otherwise, outputs
the given character to the file.
procedure Look_Ahead (File : in File_Type;
Item : out Character;
End_Of_Line : out Boolean);
procedure Look_Ahead (Item : out Character;
End_Of_Line : out Boolean);
{
AI05-0038-1}
{
AI05-0264-1}
Status_Error is propagated if the file is not open. Mode_Error is propagated
if the mode of the file is not In_File. Sets End_Of_Line to True if at
end of line, including if at end of page or at end of file; in each of
these cases the value of Item is not specified.
Otherwise,
End_Of_Line is set to False and Item is set to the next character (without
consuming it) from the file.
procedure Get_Immediate(File : in File_Type;
Item : out Character);
procedure Get_Immediate(Item : out Character);
{
AI05-0038-1}
Reads the next character, either control or graphic, from the specified
File or the default input file. Status_Error is propagated if the file
is not open. Mode_Error is propagated if the mode of the file is not
In_File. End_Error is propagated if at the end of the file. The current
column, line and page numbers for the file are not affected.
procedure Get_Immediate(File : in File_Type;
Item : out Character;
Available : out Boolean);
procedure Get_Immediate(Item : out Character;
Available : out Boolean);
{
AI05-0038-1}
If a character, either control or graphic, is available from the specified
File or the default input file, then the character is read; Available
is True and Item contains the value of this character. If a character
is not available, then Available is False and the value of Item is not
specified.
Status_Error is propagated if the file
is not open. Mode_Error is propagated if the mode of the file is not
In_File. End_Error is propagated if at the end of the file. The current
column, line and page numbers for the file are not affected.
{
AI95-00301-01}
For an item of type String the following subprograms are provided:
procedure Get(File : in File_Type; Item : out String);
procedure Get(Item : out String);
Determines the length
of the given string and attempts that number of Get operations for successive
characters of the string (in particular, no operation is performed if
the string is null).
procedure Put(File : in File_Type; Item : in String);
procedure Put(Item : in String);
Determines the length
of the given string and attempts that number of Put operations for successive
characters of the string (in particular, no operation is performed if
the string is null).
function Get_Line(File : in File_Type) return String;
function Get_Line return String;
{
AI95-00301-01}
Returns a result string constructed by reading successive characters
from the specified input file, and assigning them to successive characters
of the result string. The result string has a lower bound of 1 and an
upper bound of the number of characters read. Reading stops when the
end of the line is met; Skip_Line is then (in effect) called with a spacing
of 1.
{
AI95-00301-01}
Constraint_Error is raised if the length of the line exceeds Positive'Last;
in this case, the line number and page number are unchanged, and the
column number is unspecified but no less than it was before the call.
The exception End_Error is propagated if an attempt is made to skip a
file terminator.
Ramification: {
AI95-00301-01}
Precisely what is left in the file is unspecified if Constraint_Error
is raised because the line doesn't fit in a String; it should be consistent
with column number. This allows implementers to use whatever buffering
scheme makes sense. But the line terminator is not skipped in this case.
procedure Get_Line(File : in File_Type;
Item : out String;
Last : out Natural);
procedure Get_Line(Item : out String;
Last : out Natural);
Reads successive characters from the specified
input file and assigns them to successive characters of the specified
string. Reading stops if the end of the string is met. Reading also stops
if the end of the line is met before meeting the end of the string; in
this case Skip_Line is (in effect) called with a spacing of 1.
The
values of characters not assigned are not specified.
If characters are
read, returns in Last the index value such that Item(Last) is the last
character assigned (the index of the first character assigned is Item'First).
If no characters are read, returns in Last an index value that is one
less than Item'First. The exception End_Error is propagated if an attempt
is made to skip a file terminator.
procedure Put_Line(File : in File_Type; Item : in String);
procedure Put_Line(Item : in String);
Calls the procedure Put for the given string,
and then the procedure New_Line with a spacing of one.
Implementation Advice
The Get_Immediate procedures should be implemented
with unbuffered input. For a device such as a keyboard, input should
be “available” if a key has already been typed, whereas for
a disk file, input should always be available except at end of file.
For a file associated with a keyboard-like device, any line-editing features
of the underlying operating system should be disabled during the execution
of Get_Immediate.
Implementation Advice: Get_Immediate
should be implemented with unbuffered input; input should be available
immediately; line-editing should be disabled.
31 Get_Immediate can be used to read a
single key from the keyboard “immediately”; that is, without
waiting for an end of line. In a call of Get_Immediate without the parameter
Available, the caller will wait until a character is available.
32 In a literal string parameter of Put,
the enclosing string bracket characters are not output. Each doubled
string bracket character in the enclosed string is output as a single
string bracket character, as a consequence of the rule for string literals
(see
2.6).
33 A string read by Get or written by Put
can extend over several lines. An implementation is allowed to assume
that certain external files do not contain page terminators, in which
case Get_Line and Skip_Line can return as soon as a line terminator is
read.
Incompatibilities With Ada 95
{
AI95-00301-01}
{
AI05-0005-1}
The Get_Line functions are added to Ada.Text_IO.
If Ada.Text_IO is referenced in a
use_clause,
and a function Get_Line is defined in a package that is also referenced
in a
use_clause,
the user-defined Get_Line may no longer be use-visible, resulting in
errors. This should be rare and is easily fixed if it does occur.
Extensions to Ada 95
Wording Changes from Ada 2005
{
AI05-0038-1}
Correction: Added missing wording about raising Status_Error to
Look_Ahead and Get_Immediate.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
