A.10.6 Get and Put Procedures

The procedures Get and Put for items of the type Character, String, numeric types, and enumeration types are described in subsequent subclauses. Features of these procedures that are common to most of these types are described in this subclause. The Get and Put procedures for items of type Character and String deal with individual character values; the Get and Put procedures for numeric and enumeration types treat the items as lexical elements.

All procedures Get and Put have forms with a file parameter, written first. Where this parameter is omitted, the appropriate (input or output) current default file is understood to be specified. Each procedure Get operates on a file of mode In_File. Each procedure Put operates on a file of mode Out_File or Append_File.

All procedures Get and Put maintain the current column, line, and page numbers of the specified file: the effect of each of these procedures upon these numbers is the result of the effects of individual transfers of characters and of individual output or skipping of terminators. Each transfer of a character adds one to the current column number. Each output of a line terminator sets the current column number to one and adds one to the current line number. Each output of a page terminator sets the current column and line numbers to one and adds one to the current page number. For input, each skipping of a line terminator sets the current column number to one and adds one to the current line number; each skipping of a page terminator sets the current column and line numbers to one and adds one to the current page number. Similar considerations apply to the procedures Get_Line, Put_Line, and Set_Col.

Several Get and Put procedures, for numeric and enumeration types, have format parameters which specify field lengths; these parameters are of the nonnegative subtype Field of the type Integer.

Input-output of enumeration values uses the syntax of the corresponding lexical elements. Any Get procedure for an enumeration type begins by skipping any leading blanks, or line or page terminators. A blank is defined as a space or a horizontal tabulation character. Next, characters are input only so long as the sequence input is an initial sequence of an identifier or of a character literal (in particular, input ceases when a line terminator is encountered). The character or line terminator that causes input to cease remains available for subsequent input.

For a numeric type, the Get procedures have a format parameter called Width. If the value given for this parameter is zero, the Get procedure proceeds in the same manner as for enumeration types, but using the syntax of numeric literals instead of that of enumeration literals. If a nonzero value is given, then exactly Width characters are input, or the characters up to a line terminator, whichever comes first; any skipped leading blanks are included in the count. The syntax used for numeric literals is an extended syntax that allows a leading sign (but no intervening blanks, or line or page terminators) and that also allows (for real types) an integer literal as well as forms that have digits only before the point or only after the point.

Any Put procedure, for an item of a numeric or an enumeration type, outputs the value of the item as a numeric literal, identifier, or character literal, as appropriate. This is preceded by leading spaces if required by the format parameters Width or Fore (as described in later subclauses), and then a minus sign for a negative value; for an enumeration type, the spaces follow instead of leading. The format given for a Put procedure is overridden if it is insufficiently wide, by using the minimum necessary width.

Two further cases arise for Put procedures for numeric and enumeration types, if the line length of the specified output file is bounded (that is, if it does not have the conventional value zero). If the number of characters to be output does not exceed the maximum line length, but is such that they cannot fit on the current line, starting from the current column, then (in effect) New_Line is called (with a spacing of one) before output of the item. Otherwise, if the number of characters exceeds the maximum line length, then the exception Layout_Error is propagated and nothing is output.

The exception Status_Error is propagated by any of the procedures Get, Get_Line, Put, and Put_Line if the file to be used is not open. The exception Mode_Error is propagated by the procedures Get and Get_Line if the mode of the file to be used is not In_File; and by the procedures Put and Put_Line, if the mode is not Out_File or Append_File.

The exception End_Error is propagated by a Get procedure if an attempt is made to skip a file terminator. The exception Data_Error is propagated by a Get procedure if the sequence finally input is not a lexical element corresponding to the type, in particular if no characters were input; for this test, leading blanks are ignored; for an item of a numeric type, when a sign is input, this rule applies to the succeeding numeric literal. The exception Layout_Error is propagated by a Put procedure that outputs to a parameter of type String, if the length of the actual string is insufficient for the output of the item. 

In the examples, here and in A.10.8 and A.10.9, the string quotes and the lower case letter b are not transferred: they are shown only to reveal the layout and spaces.

Example of overridden width parameter: