Introduction to Programming with c++ 13-7 BinaryIO

这一目主要是对BinaryIO的一些基本概念坐下总结。代码虽然不难写，不过这些基本概念倒是值得好好理解理解。

基本概念

Files can be classified into text and binary.

text file and binary file

A file that can be processed(read, created or modified) using a text editor is called a text file.All the files are called binary files. For example, the C++ source code are stored in text files and can be read by a text editor, but the c++ executable files are stored in binary files and are read by the operating sysytems.

（PS：这一段并没有从本质上揭示二者区别，只是给了一个形象的描述。）

more about text file and binary file

You can envision a text file as consisting of a sequence of characters and a bainary file as consisting of a sequence of bits.

（PS：这一段进一步说明了，一个是字节序列，一个是比特序列。）

文章此时举了一个199的例子，如果是存在text file, 那么199是 ‘1’, ‘9’, ‘9’这三个字符。如果是binary file, 就是199在计算机内存中表示。

内存表示如下：用表格代替下，每个表格看做内存中的一个字节。字符用ascii编码的形式存储。那么’1’(49),’9’(57)

199 （text file）

Memory
00110001
00111011
00111011

199（binary file）

Memory
11000111

从内存表示来看，显然binary file具有更高的存储效率。

Note

Computer do not differentiate binary files and text files. All files are stored in binary format, and all files are essentially binary files.Text I/O is built upon binary I/O to provide a level of abstraction for character encoding and decoding.

（PS：这一部分说的非常的精髓，二者本质上都是binary file，只不过text file 在此基础上做了一层抽象，对字符做了编码和解码，从而使得text file变成有结构数据，就和网络通信过程中下层对上层的封装一样。）

回到本章开始的地方，再总结一些基本概念。

stream

C++ uses the term stream to describe a flow of data.If it flows to your programme, the stream is called input stream. If it flows out from your programme, it it called an output stream.

C++ uses objects to read/write a stream of data.For convenience, an input object is called input stream which processing and manipulating input streams , output object is called output stream which processing and manipulating output streams .

代码

BinaryIO不常用，接口如下：

streamObject.write( char* address, int size );
streamObject.read( char* address, int size );
//这个感觉和c的也比较像，就是从address这个地址，读取size个字节。
//对于第二个函数，size是期望读取的字节数，但是实际读取的用函数gcount获得。

下面这部分代码针对string类型的，所以参数传递的时候不牵扯到转换。

#include <iostream>
#include <fstream>
#include <string>

int write_binary_file( const std::string& word );
std::string read_binary_file();

int main()
{
std::string s = "China";
write_binary_file( s );

std::string t = read_binary_file();
std::cout << t << std::endl;

return 0;
}

int write_binary_file( const std::string& word )
{
std::ofstream fout;
fout.open( "input.dat", std::ios::binary );
if( !fout.is_open() )
{
std::cerr << "Can not open the file!" << std::endl;
return -1;
}

fout.write( word.c_str(), word.size() );

fout.close();
return 0;
}
std::string read_binary_file()
{
static const int maxn = 32;
char tmp[maxn];

std::ifstream fin;
fin.open( "input.dat", std::ios::binary );
if( !fin.is_open() )
{
std::cerr << "Can not open the file!" << std::endl;
return NULL;
}

fin.read( tmp, maxn - 1 ); // the last character for '\0'
tmp[fin.gcount()] = '\0';

fin.close();

std::string ret( tmp, tmp + fin.gcount() );
return ret;
}

下面补充针对非string类型的代码，以int类型为例。

reinterpret_cast这个关键字之前没有接触过，感觉用在地址强转上。这个后序再总结吧。

int write_binary_file1( int val )
{
std::ofstream fout;
fout.open( "input.dat", std::ios::binary );
if( !fout.is_open() )
{
std::cerr << "Can not open the file!" << std::endl;
return -1;
}

fout.write( reinterpret_cast<char*>(&val), sizeof(int) );

fout.close();
return 0;
}
int read_binary_file1()
{
int ret = 0;

std::ifstream fin;
fin.open( "input.dat", std::ios::binary );
if( !fin.is_open() )
{
std::cerr << "Can not open the file!" << std::endl;
return -1;
}

fin.read( reinterpret_cast<char*>(&ret) , sizeof(int) );

return ret;
}