C++ File I/O – binary files using fstream

C++ File I/O with binary files using fstream class is a simple task. fstream class has the capability to do both Input as well as Output operations i.e., read and write. All types of operations like reading/ writing of characters, strings, lines and not to mention buffered I/O are supported by fstream.

Operating systems store the files in binary file format. Computers can deal with only binary numbers. But binary files are not readable by humans. Our level of comfort lies only with proper ASCII or UNICODE characters. This article deals with how C++ File I/O class fstream can be used for reading and writing binary files. For ASCII file operations in C++, refer to C++ Text file I/O article.

For our C++ File I/O binary file examples, we assume a struct WebSites with two members as follows.

// Struct for C++ File I/O binary file sample
struct WebSites
char SiteName[100];
int Rank;

Write operations in C++ Binary File I/O:

There are some important points to be noted while doing a write operation.

  • The file has to be opened in output and binary mode using the flags ios::out (output mode) and ios::binary( binary mode)
  • The function write takes two parameters. The first parameter is of type char * for the data to be written and the second is of type int asking for the size of data to be written to the binary file.
  • File has to be closed at the end.
// Sample for C++ File I/O binary file write

void write_to_binary_file(WebSites p_Data)
fstream binary_file("c:\test.dat",ios::out|ios::binary|ios::app);
binary_file.write(reinterpret_cast<char *>(&p_Data),sizeof(WebSites));

The above C++ File I/O binary sample function writes some data to the function. The file is opened in output and binary mode with ios::out and ios::binary. There is one more specifier ios::app, which tells the Operating system that the file is also opened in append mode. This means any new set of data will be appended to the end of file.

Also the write function used above, needs the parameter as a character pointer type. So we use a type converter reinterpret_cast to typecast the structure into char* type.

Read operations in C++ Binary File I/O:

This also has a similar flow of operations as above. The only difference is to open the file using ios::in, which opens the file in read mode.

// Sample for C++ File I/O binary file read

void read_from_binary_file()
WebSites p_Data;
fstream binary_file("c:\test.dat",ios::binary|ios::in);
binary_file.read(reinterpret_cast<char *>(&p_Data),sizeof(WebSites));

cout<<"Rank :"<< p_Data.Rank<<endl;


Most of the programs will usually go for ASCII text mode files only. But there will be some occasions where the C++ File I/O with binary files will be very useful.

This ” C++ File I/O using binary files” article only gives some very basic samples for read and write. Advanced C++ File I/O operations like seek, checking the file pointer validity etc., are also needed to be learnt while writing bigger programs.