Ejemplo de DataFlow con Archivos de Binarios

El ejemplo se enfoca en crear un archivo de binario con los datos que estan definidos en el objeto persona.

Librerias utilizadas

• fstream: Input/output stream class to operate on files.

Unformatted Operations (binary files)

• When data is stored in a file in the binary format, reading and writing data is faster because no time is lost in converting the data from one format to another format. Such files are called binary files. This following program explains how to create binary files and also how to read, write, search, delete and modify data from binary files.
• The class ios_base is a multipurpose class that serves as the base class for all I/O stream classes.

Member types and constants, stream open mode type:

Constant	Explanation
app	seek to the end of stream before each write
binary	open in binary mode
in	open for reading
out	open for writing
trunc	discard the contents of the stream when opening
ate	seek to the end of stream immediately after open

File size and indexation

In C++, files are considered a stream or an array of uninterpreted bytes, each byte can also be considered a char, with the file contents considered as a char array: (char *)myFile.

The "array" of bytes stored in a file is indexed from zero to len-1, where len is the total number of bytes in the entire file.

Opening Files

There are two main ways of opening files in binary mode:

1. Set a file name and necessary flags in the constructor when declaring the object.

ifstream myReadFile(filename, ios::in | ios::binary);

1. Declare a stream object and use the open method to set the file name and necessary flags.

ifstream myFile;
myFile.open ("data2.dat", ios::out | ios::binary);

There are two main flags that need to be used when manipulating binary files:

1. The i/o mode ios::in or ios::out
2. The binary mode ios::binary

Read

The read method extracts a given number of bytes from the stream, and places them into the memory pointed to by the first parameter.

Person person;

std::string filename = "people.dat";
ifstream inFile;
inFile.open(filename, ios::in | ios::binary);

inFile.read((char*)&person, sizeof(person));

cout << person.toString << std::endl;

inFile.close();

Write

The write member function writes a given number of bytes on the given stream, starting at the position of the "put" pointer.

Person person1 = Person("Julio");

std::string filename = "people.dat";
ofstream outFile;
outFile.open(filename, ios::out | ios::binary);

outFile.write((char *)&person1, sizeof(person1));
outFile.close();

Accessing file positions

Each open file will have a "get" and a "put" pointer, these store a position in the file, and are part of the stream object.

The GET pointer

It is the current reading position, the index of the next byte that will be read from the file.

The get pointer can be repositioned with the istream& seekg(streampos pos) method.

To return the index of the get pointer on a given stream use istream& tellg().

Person person;

std::string filename = "people.dat";
ifstream inFile;
inFile.open(filename, ios::binary);

inFile.seekg (sizeof(person),ios::beg);
// Reading the second person in the fileinFile.read((char*)&person, sizeof(person));

cout << person.toString << std::endl;

inFile.close();

The PUT pointer

The put pointer can be repositioned with the ostream& seekp(streampos pos) method.

To return the index of the put pointer on a given stream use istream& tellp().

Person person;

std::string filename = "people.dat";
ofstream outFile;
outFile.open(filename, std::ios::binary | std::ios::app);

std::cout << "Adding in position: " << outFile.tellg() << " byte." << std::endl;
// Adding to the end of the file.outFile.write((char*)&person, sizeof(person));

outFile.close();