Stuff you should know about In- and Output with Streams

1 July 2019

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

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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;

}

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.

Input

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

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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

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.

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