C++ for Scientists - Technische Universität Dresden

46 CHAPTER 2. C++ BASICS
The first statement declares a variable of type int called age, and the second one waits for an
input from cin (the keyboard) in order to store it in this integer variable. The input from the
keyboard is processed once the RETURN key has been pressed.
You can also use cin to request more than one datum input from the user:
std::cin ≫a ≫b;
is equivalent to:
std::cin ≫a;
std::cin ≫b;
In both cases the user must give two data, one for variable a and another one for variable b that
may be separated by any valid blank separator: a space, a tab character or a newline.
2.7.3 Input/Output with files
C ++ provides the following classes to perform output and input of characters to/from files:
• std::ofstream: used to write to files
• std::ifstream: used to read from files
• std::fstream: used to both read and write from/to files.
We can use file streams the same way we are already used cin and cout, with the only difference
that we have to associate these streams with physical files. Here is an example:
#include
#include
int main () {
std::ofstream myfile;
myfile.open (”example.txt”);
myfile ≪ ”Writing this to a file. ” ≪ std::endl;
myfile.close();
return 0;
}
This code creates a file called example.txt (or overwrites it if it already exists) and inserts a
sentence into it in a way that is similar to the use of cout. C ++ has the concept of an output
streams that is satisfied by an output file as well as be std::cout. That means that everything
that can be written to std::cout can also be written to a file, and vice versa. If you define yourself
the operator ≪ for a new type you do not need to program it for different output type but
only once for a general output stream, see 18
Alternatively, one can give the file stream object the file name as argument. This opens the file
implicitly. The file is also implicitly closed when myfile at some point, in this case at the end
of the main function. The mechanisms that control such implicit actions will become clear in
§ 2.2.3. The bottom line is that you only in few cases must close your files explicitly. The short
version of the previous listing is
18 TODO: Where? New section needed.