temple 1717088 1280

Stuff you should know about In- and Output with Streams

Today’s post is about what you should know about Iostreams. In particular, I write about formatted and unformatted In- and Output.

 

temple 1717088 1280

Input and Output Functions

C++ has four predefined stream objects for the convenience of dealing with the keyboard and the monitor.

PredefinedStreamObject

The stream objects can be used to write a program that reads from the command line and returns the sum.

// Iostreams.cpp

#include <iostream>

int main(){

  std::cout << std::endl;
  
  std::cout << "Type in your numbers(Quit with an arbitrary character): " << std::endl;
  
  int sum{0};
  int val;

  while ( std::cin >> val ) sum += val;

  std::cout << "Sum: " << sum << std::endl;
  
  std::cout << std::endl;

}

 

 

Rainer D 6 P2 500x500Modernes C++ Mentoring

Be part of my mentoring programs:

  • "Fundamentals for C++ Professionals" (open)
  • "Design Patterns and Architectural Patterns with C++" (open)
  • "C++20: Get the Details" (open)
  • "Concurrency with Modern C++" (starts March 2024)
  • Do you want to stay informed: Subscribe.

     

    Iostreams

    The program uses the stream operators << and >> and the stream manipulator std::endl.

    • The insert operator << pushes characters onto the output stream std::cout.
    • The extract operator >> pulls the characters from the input stream std::cin.
    • You can build chains of insert or extract operators because both operators return a reference to themselves.

    std::endl is a stream manipulator because it puts a ‘\n’ character onto std::cout and flushes the output buffer.

    Here are the most frequently used stream manipulators.

    StreamManipulators Input

    You can read in two ways from the input stream: Formatted with the extractor >> and unformatted with explicit methods.

    Formatted Input

    The extract operator >>

    • is predefined for all built-in types and strings,
    • can be implemented for user-defined data types,
    • can be configured by format specifiers.

    The following code snippet shows a straightforward way to read two int‘s.

    #include <iostream>
    ...
    int a, b;
    std::cout << "Two natural numbers: " << std::endl;
    std::cin >> a >> b; // < 2000 11>
    std::cout << "a: " << a << " b: " << b;
    

     

    std::cin ignores by default leading whitespace.

    Unformatted Input

    An input stream supports a few methods for unformatted input.

    UnformattedInput

    • std::string has a getline function

    The getline function of std::string has a big advantage over the getline function of the istream. The std::string automatically takes care of its memory. On the contrary, you have to reserve the memory for the buffer buf in the call is.get(buf, num). Using the getline function  is quite convenient because you can also specify a delimiter:

     

    // inputUnformatted.cpp
    
    #include <fstream>
    #include <iostream>
    #include <string>
    
    int main(){
    
      std::cout << std::endl;
    
      std::string line;
      std::cout << "Write a line: " << std::endl;
      std::getline(std::cin, line);                       // (1)
      std::cout << line << std::endl;
    
      std::cout << std::endl;
      
      std::ifstream inputFile("test.txt"); 
      while ( std::getline(inputFile, line, ';') ) {      // (2)
          std::cout << line << std::endl;
      }
    
    }
    

     

    First, the program reads in line (1) for std::cin; second, it reads in line (2) from the file test.txt.

    For simplicity reasons, the code does no error handling. You can read the error handling details in my last post:  C++ Core Guidelines: iostreams. The file test.txt contains numbers, which are separated by “;”

     inputUnformatted

    Output

    As promised in my last post C++ Core Guidelines: iostreams, here are the format specifiers for iostreams; you should know, or at least know, where to find them.

    Important Format Specifiers

    I often hear students who are experienced C++ programmers in my classes complain that arithmetic in C++ is not precise enough. The reason is mostly not C++ but the default format specifiers for the Iostreams. Let’s see what you should know:

    First of all. You can use manipulators or flags that specify the format.

    Manipulators and Flags

     

    // formatSpecifier.cpp
    
    #include <iostream>
    
    int main(){
    
      std::cout << std::endl;
    
      int num{2011};
    
      std::cout << "num: " << num << "\n\n";
    
      std::cout.setf(std::ios::hex, std::ios::basefield);     // (1)
      std::cout << "hex: " << num << std::endl;
      std::cout.setf(std::ios::dec, std::ios::basefield);     // (1)
      std::cout << "dec: " << num << std::endl;
    
      std::cout << std::endl;
    
      std::cout << std::hex << "hex: " << num << std::endl;   // (2)
      std::cout << std::dec << "dec: " << num << std::endl;   // (2)
    
      std::cout << std::endl;
    
    }
    

     

    Lines (1) use flags and lines (2) manipulators to format the output.

    formatSpecifier

    From the readability and maintainability point of view, I strongly prefer manipulators.

    Manipulators for the Iostreams

    Okay, let me start with the essential manipulators.

    The following tables present the relevant format specifiers. The format specifiers are sticky except for the field width, which is reset after each application.
    The manipulators without arguments need the header <iostream>, and those with arguments need the header <iomanip>.

    • Boolean Values

    BooleanValue

    • Field With and Fill Characters

    FieldWithAndFillCharacters

    • Alignment of Text

    Alignment

    • Positive Signs and Upper/Lower Case

    FloatingPointNumbers

    • Numeric Base

    NumericBase

    • Floating Point Numbers

    FloatingPointNumbers

     There are special rules for floating-point numbers:

    • The number of significant digits (digits after the comma) is, by default, 6.
    • If the number of significant digits is not big enough, the number is displayed in scientific notation.
    • Leading and trailing zeros are not displayed.
    • If possible, the decimal point is not displayed.

     After so much theory, here are the format specifiers in action.

     

    // formatSpecifierOutput.cpp
    
    #include <iomanip>
    #include <iostream>
    
    int main(){
    
      std::cout << std::endl;
    
      std::cout << "std::setw, std::setfill and std::left, right and internal: " << std::endl;
    
      std::cout.fill('#');
      std::cout << -12345 << std::endl;
      std::cout << std::setw(10) << -12345 << std::endl;
      std::cout << std::setw(10) << std::left << -12345 << std::endl;
      std::cout << std::setw(10) << std::right << -12345 << std::endl;
      std::cout << std::setw(10) << std::internal << -12345 << std::endl;
    
      std::cout << std::endl;
    
      std::cout << "std::showpos:" << std::endl;
    
      std::cout << 2011 << std::endl;
      std::cout << std::showpos << 2011 << std::endl;
    
    
      std::cout << std::noshowpos << std::endl;
    
      std::cout << "std::uppercase: "  << std::endl;
      std::cout << 12345678.9 << std::endl;
      std::cout << std::uppercase << 12345678.9 << std::endl;
    
      std::cout << std::nouppercase << std::endl;
    
      std::cout << "std::showbase and std::oct, dec and hex: " << std::endl;
      std::cout << 2011 << std::endl;
      std::cout << std::oct << 2011 << std::endl;
      std::cout << std::hex << 2011 << std::endl;
    
      std::cout << std::endl;
    
      std::cout << std::showbase;
      std::cout << std::dec << 2011 << std::endl;
      std::cout << std::oct << 2011 << std::endl;
      std::cout << std::hex << 2011 << std::endl;
    
      std::cout << std::dec << std::endl;
    
      std::cout << "std::setprecision, std::fixed and std::scientific: " << std::endl;
    
      std::cout << 123.456789 << std::endl;
      std::cout << std::fixed << std::endl;
      std::cout << std::setprecision(3) << 123.456789 << std::endl;
      std::cout << std::setprecision(4) << 123.456789 << std::endl;
      std::cout << std::setprecision(5) << 123.456789 << std::endl;
      std::cout << std::setprecision(6) << 123.456789 << std::endl;
      std::cout << std::setprecision(7) << 123.456789 << std::endl;
      std::cout << std::setprecision(8) << 123.456789 << std::endl;
      std::cout << std::setprecision(9) << 123.456789 << std::endl;
    
      std::cout << std::endl;
      std::cout << std::setprecision(6) << 123.456789 << std::endl;
      std::cout << std::scientific << std::endl;
      std::cout << std::setprecision(6) << 123.456789 << std::endl;
      std::cout << std::setprecision(3) << 123.456789 << std::endl;
      std::cout << std::setprecision(4) << 123.456789 << std::endl;
      std::cout << std::setprecision(5) << 123.456789 << std::endl;
      std::cout << std::setprecision(6) << 123.456789 << std::endl;
      std::cout << std::setprecision(7) << 123.456789 << std::endl;
      std::cout << std::setprecision(8) << 123.456789 << std::endl;
      std::cout << std::setprecision(9) << 123.456789 << std::endl;
    
      std::cout << std::endl;
    
    }
    

     

     The output should be sufficient to explain the program formatSpecifierOutput.cpp.

    formatSpecifierOutput

    What’s next?

    When you synchronize too much, you lose. In the case of the Iostreams, you will lose performance. I will show you the numbers in my next post.

     

     

    Thanks a lot to my Patreon Supporters: Matt Braun, Roman Postanciuc, Tobias Zindl, G Prvulovic, Reinhold Dröge, Abernitzke, Frank Grimm, Sakib, Broeserl, António Pina, Sergey Agafyin, Андрей Бурмистров, Jake, GS, Lawton Shoemake, Jozo Leko, John Breland, Venkat Nandam, Jose Francisco, Douglas Tinkham, Kuchlong Kuchlong, Robert Blanch, Truels Wissneth, Kris Kafka, Mario Luoni, Friedrich Huber, lennonli, Pramod Tikare Muralidhara, Peter Ware, Daniel Hufschläger, Alessandro Pezzato, Bob Perry, Satish Vangipuram, Andi Ireland, Richard Ohnemus, Michael Dunsky, Leo Goodstadt, John Wiederhirn, Yacob Cohen-Arazi, Florian Tischler, Robin Furness, Michael Young, Holger Detering, Bernd Mühlhaus, Matthieu Bolt, Stephen Kelley, Kyle Dean, Tusar Palauri, Dmitry Farberov, Juan Dent, George Liao, Daniel Ceperley, Jon T Hess, Stephen Totten, Wolfgang Fütterer, Matthias Grün, Phillip Diekmann, Ben Atakora, Ann Shatoff, Rob North, Bhavith C Achar, Marco Parri Empoli, moon, and Philipp Lenk.

    Thanks, in particular, to Jon Hess, Lakshman, Christian Wittenhorst, Sherhy Pyton, Dendi Suhubdy, Sudhakar Belagurusamy, Richard Sargeant, Rusty Fleming, John Nebel, Mipko, Alicja Kaminska, Slavko Radman, and David Poole.

    My special thanks to Embarcadero
    My special thanks to PVS-Studio
    My special thanks to Tipi.build 
    My special thanks to Take Up Code
    My special thanks to SHAVEDYAKS

    Seminars

    I’m happy to give online seminars or face-to-face seminars worldwide. Please call me if you have any questions.

    Standard Seminars (English/German)

    Here is a compilation of my standard seminars. These seminars are only meant to give you a first orientation.

    • C++ – The Core Language
    • C++ – The Standard Library
    • C++ – Compact
    • C++11 and C++14
    • Concurrency with Modern C++
    • Design Pattern and Architectural Pattern with C++
    • Embedded Programming with Modern C++
    • Generic Programming (Templates) with C++
    • Clean Code with Modern C++
    • C++20

    Online Seminars (German)

    Contact Me

    Modernes C++ Mentoring,

     

     

    0 replies

    Leave a Reply

    Want to join the discussion?
    Feel free to contribute!

    Leave a Reply

    Your email address will not be published. Required fields are marked *