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// 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);
The G2++ compiler, g2++comp(1C++), 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):
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; } }
Applications without a priori knowledge of record types must use untyped I/O (see untyped_io(3C++)).