typed_io(C++)

Synopsis

   // For each record type defined in
   // the corresponding .g file:
       typedef type_specifier T;
       istream& operator>>(istream& is,T& t);
       ostream& operator<<(ostream& os,const T& t);

Description

The G2++ compiler, g2++comp(CP), must be used prior to compiling client code that uses G2++ typed input/output routines. g2++comp(1C++) reads a collection of G2++ record definitions from one or more .g files (each definition is itself a G2++ record) and writes a pair of files, one .h file and one .c file, for each .g file. The .c file must be compiled and linked with the rest of the application. The .h file must be included by client programs. The .h file contains a set of three declarations for each record type defined in the .g file. The three declarations are:

typedef type_specifier T; Used by the client to declare variables suitable as sources or targets of the typed I/O operators. T is the upper case version of the record name in the .g file. For simple records (records with no indented groups), type_specifier will be a simple type; otherwise, type_specifier will be a structure type.

istream& operator>>(istream& is,T& t); Stream extraction operator. Scans istream is until it encounters a G2++ record of type T and maps the record contents into t. If a record of type T cannot be found before the stream is exhausted, the stream will test as null after the operation and the contents of t will be undefined. The mapping obeys the following rules (see G2++(4C++) for definitions of italicized terms):

• A group with an unknown name will be ignored.

• If a group is missing, each member of the corresponding group will be assigned the null value of its respective type.

• If a value has been defined as a fixed size string, characters in excess of the defined size will be ignored.

• If a value has been defined as a fixed size array, elements with an out-of-bounds index will be ignored.

• An out-of-order name will be recognized as long as it occurs within its proper group.

• A string is terminated by the first non-printable character. If the remaining characters of the string still cannot be converted to a value of its defined type, the null value for that type will be assigned to the corresponding structure member. Otherwise, the string will be converted to the appropriate type and assigned to the corresponding structure member.

• Only members containing non-null strings or non-zero integers will be inserted.

• Fixed size strings and arrays will be truncated if they exceed their defined sizes.

Example

Using g2++comp(1C++) to compile the following person.g file:

           Time    USER
                   .header Time.h
                   .header Timeio.h
                   .null   Time::MIN
           person
                   id      *
                   age     SHORT
                   b_day   Time
                   hobbies
                           *       *

produces the following person.h file:

           #ifndef PERSONH
           #define PERSONH
           #include <g2++.h>
           #include <Vblock.h>
           #include <String.h>
           #include "Time.h"
           #include "Timeio.h"
           class istream;
           class ostream;
           typedef struct PERSON{
                   String id;
                   short   age;
                   Time    b_day;
                   Vblock<String> hobbies;  see Vblock(3C++)
                   PERSON();
           }PERSON;
           istream& operator>>(istream& is, PERSON& x);
           ostream& operator<<(ostream& os, const PERSON& x);
           #endif

The following "batch" update program adds 1 to the age of every person whose birthday is today.

           #include "person.h"
           #include <time.h>
           #include <Time.h>
           main(){
               PERSON p;
               Time today = make_time(time(0));
               while(cin >> p){
                   if(p.b_day==today){
                        p.age += 1;
                   }
                   cout << p;
               }
           }

Notes

Applications without a priori knowledge of record types must use untyped I/O (see untyped_io(C++)).

References