[C++] Streams

First publication date: 2020/04/27

Characters streams

We will deal with basic handling of data streams with a focus on text files.

You will need to include the fstream header and do NOT forget to use the std:: prefix (or the proper use statement)

You are already using predefined in your programs :

std::cout : the standard output stream
std::cin : the standard input stream
std::cerr : the standard error output stream

These streams can be redirected to files if needed but we can also use these streams directly.

Writing in a stream

To see how you can write in a file, let's analyse the following piece of code :

std::string   name = "my_file.txt";
  std::ofstream file(name.c_str());

  if (!file.fail()) {
    file << amount << std::endl; 
    for (int i = 0; i < amount; ++i)
      file << i+1 << std::endl;

  file.close();

Line 1: declaration of a variable of string type referring to the name of the file
Line 2: declaration of a writing stream variable. The constructor takes the name of the file as a parameter. The same result can be achieved with open()
Line 4: if the file is not null, the file is opened otherwise it means a problem occured.
Lines 5 & 7: we write to the file the same way we use std::cout
Line 9: the stream is closed. It is optionnal in this case because the stream destructor closes it anyway.

If you do not know what nom.c_str() means, go back to the notice about strings.

Reading from a stream

std::ifstream file; // other way to open a file
int i = 0, max;

file.open(name.c_str());
// fail() not tested :-(

file >> max;

while(!file.eof() && i<max) {
  double reading;
  file >> reading;
  ++i;
  std::cout << reading << " ";
}
file.close();

Line 1: declaration of a input stream
Line 4: associates the stream with a file
Line 6: reading from the stream/file
Line 8: test of the eof() flag. This flag is only updated after reading.

Stream types

We just saw a first type of streams (characters streams for text files). The page on strings gives another type of stream, not associated to a file but to a string. Handy, isn't it ?

We can use binary streams as well but there is no serialization concept in C++, unfortunately.