28.3 — Output with ostream and ios

In this lesson, you'll learn how to control output formatting and use various features of C++ output streams. You'll discover how to format numbers, control precision, align text, and customize output to meet your specific needs.

Understanding std::ostream

std::ostream is the base class for output streams. The most commonly used output stream is std::cout, but the formatting techniques you'll learn apply to all output streams, including file streams and string streams.

#include <iostream>

int main()
{
    std::cout << "Hello, World!" << std::endl;
    std::cerr << "This is an error message" << std::endl;
    std::clog << "This is a log message" << std::endl;
    
    return 0;
}

Output:

Hello, World!
This is an error message
This is a log message

Basic output formatting

Setting field width

Use std::setw() to set the minimum width of the next output:

#include <iostream>
#include <iomanip>

int main()
{
    std::cout << "Without setw:" << std::endl;
    std::cout << 1 << std::endl;
    std::cout << 22 << std::endl;
    std::cout << 333 << std::endl;
    
    std::cout << "\nWith setw(5):" << std::endl;
    std::cout << std::setw(5) << 1 << std::endl;
    std::cout << std::setw(5) << 22 << std::endl;
    std::cout << std::setw(5) << 333 << std::endl;
    
    return 0;
}

Output:

Without setw:
1
22
333

With setw(5):
    1
   22
  333

Text alignment

Control how text is aligned within the specified width:

#include <iostream>
#include <iomanip>

int main()
{
    std::cout << "Left aligned:" << std::endl;
    std::cout << std::left;
    std::cout << std::setw(10) << "Apple" << "|" << std::endl;
    std::cout << std::setw(10) << "Banana" << "|" << std::endl;
    
    std::cout << "\nRight aligned:" << std::endl;
    std::cout << std::right;
    std::cout << std::setw(10) << "Apple" << "|" << std::endl;
    std::cout << std::setw(10) << "Banana" << "|" << std::endl;
    
    std::cout << "\nCenter aligned:" << std::endl;
    std::cout << std::internal;  // For numbers, affects +/- sign placement
    std::cout << std::setw(10) << -123 << "|" << std::endl;
    
    return 0;
}

Output:

Left aligned:
Apple     |
Banana    |

Right aligned:
     Apple|
    Banana|

Center aligned:
      -123|

Numeric formatting

Setting precision for floating-point numbers

#include <iostream>
#include <iomanip>

int main()
{
    double pi = 3.141592653589793;
    
    std::cout << "Default precision: " << pi << std::endl;
    
    std::cout << std::setprecision(3) << "Precision 3: " << pi << std::endl;
    std::cout << std::setprecision(6) << "Precision 6: " << pi << std::endl;
    std::cout << std::setprecision(10) << "Precision 10: " << pi << std::endl;
    
    return 0;
}

Output:

Default precision: 3.14159
Precision 3: 3.14
Precision 6: 3.14159
Precision 10: 3.141592654

Fixed vs scientific notation

#include <iostream>
#include <iomanip>

int main()
{
    double number = 1234.56789;
    
    std::cout << "Default: " << number << std::endl;
    
    std::cout << std::fixed;
    std::cout << "Fixed notation: " << number << std::endl;
    
    std::cout << std::scientific;
    std::cout << "Scientific notation: " << number << std::endl;
    
    // Reset to default
    std::cout.unsetf(std::ios::fixed | std::ios::scientific);
    std::cout << "Back to default: " << number << std::endl;
    
    return 0;
}

Output:

Default: 1234.57
Fixed notation: 1234.567890
Scientific notation: 1.234568e+03
Back to default: 1234.57

Controlling decimal places

#include <iostream>
#include <iomanip>

int main()
{
    double price = 19.9;
    double tax = 0.08;
    double total = price * (1 + tax);
    
    std::cout << std::fixed << std::setprecision(2);
    std::cout << "Price: $" << price << std::endl;
    std::cout << "Tax: $" << (price * tax) << std::endl;
    std::cout << "Total: $" << total << std::endl;
    
    return 0;
}

Output:

Price: $19.90
Tax: $1.59
Total: $21.49

Formatting integers

Number base (hexadecimal, octal, decimal)

#include <iostream>
#include <iomanip>

int main()
{
    int number = 255;
    
    std::cout << "Decimal: " << std::dec << number << std::endl;
    std::cout << "Hexadecimal: " << std::hex << number << std::endl;
    std::cout << "Octal: " << std::oct << number << std::endl;
    
    // Show base prefix
    std::cout << std::showbase;
    std::cout << "With base prefix:" << std::endl;
    std::cout << "Decimal: " << std::dec << number << std::endl;
    std::cout << "Hexadecimal: " << std::hex << number << std::endl;
    std::cout << "Octal: " << std::oct << number << std::endl;
    
    return 0;
}

Output:

Decimal: 255
Hexadecimal: ff
Octal: 377
With base prefix:
Decimal: 255
Hexadecimal: 0xff
Octal: 0377

Fill characters and padding

#include <iostream>
#include <iomanip>

int main()
{
    int number = 42;
    
    std::cout << "Default padding:" << std::endl;
    std::cout << std::setw(8) << number << std::endl;
    
    std::cout << "Zero padding:" << std::endl;
    std::cout << std::setfill('0') << std::setw(8) << number << std::endl;
    
    std::cout << "Star padding:" << std::endl;
    std::cout << std::setfill('*') << std::setw(8) << number << std::endl;
    
    // Reset fill character
    std::cout << std::setfill(' ');
    
    return 0;
}

Output:

Default padding:
      42
Zero padding:
00000042
Star padding:
******42

Boolean output formatting

#include <iostream>
#include <iomanip>

int main()
{
    bool flag1 = true;
    bool flag2 = false;
    
    std::cout << "Default boolean output:" << std::endl;
    std::cout << "flag1: " << flag1 << std::endl;
    std::cout << "flag2: " << flag2 << std::endl;
    
    std::cout << std::boolalpha;  // Show as text
    std::cout << "Text boolean output:" << std::endl;
    std::cout << "flag1: " << flag1 << std::endl;
    std::cout << "flag2: " << flag2 << std::endl;
    
    return 0;
}

Output:

Default boolean output:
flag1: 1
flag2: 0
Text boolean output:
flag1: true
flag2: false

Creating formatted tables

#include <iostream>
#include <iomanip>
#include <string>

int main()
{
    // Data to display
    std::string names[] = {"Alice", "Bob", "Charlie"};
    int ages[] = {25, 30, 35};
    double salaries[] = {50000.50, 65000.75, 80000.00};
    
    // Table header
    std::cout << std::left << std::setfill(' ');
    std::cout << std::setw(10) << "Name" 
              << std::setw(6) << "Age" 
              << std::setw(12) << "Salary" << std::endl;
    std::cout << std::setw(28) << std::setfill('-') << "" << std::endl;
    std::cout << std::setfill(' ');
    
    // Table data
    for (int i = 0; i < 3; ++i)
    {
        std::cout << std::left << std::setw(10) << names[i]
                  << std::right << std::setw(6) << ages[i]
                  << std::fixed << std::setprecision(2) 
                  << std::setw(12) << salaries[i] << std::endl;
    }
    
    return 0;
}

Output:

Name      Age       Salary
----------------------------
Alice        25    50000.50
Bob          30    65000.75
Charlie      35    80000.00

Stream manipulators

Persistent vs non-persistent manipulators

#include <iostream>
#include <iomanip>

int main()
{
    // setw is non-persistent (affects only the next output)
    std::cout << std::setw(5) << 1 << std::setw(5) << 2 << std::endl;
    std::cout << 3 << 4 << std::endl;  // No width formatting
    
    // setprecision is persistent (affects all subsequent output)
    std::cout << std::setprecision(2);
    std::cout << 3.14159 << " " << 2.71828 << std::endl;
    
    return 0;
}

Output:

    1    2
34
3.1 2.7

Custom manipulators

You can create your own stream manipulators:

#include <iostream>
#include <iomanip>

// Custom manipulator to format currency
std::ostream& currency(std::ostream& os)
{
    return os << std::fixed << std::setprecision(2) << "$";
}

int main()
{
    double price = 19.9;
    
    std::cout << "Price: " << currency << price << std::endl;
    std::cout << "Tax: " << currency << (price * 0.08) << std::endl;
    
    return 0;
}

Output:

Price: $19.90
Tax: $1.59

Using ios flags directly

You can also manipulate formatting using ios flags:

#include <iostream>

int main()
{
    double number = 1234.5;
    
    // Set flags using setf()
    std::cout.setf(std::ios::fixed);
    std::cout.setf(std::ios::showpoint);
    std::cout.precision(3);
    
    std::cout << "With flags: " << number << std::endl;
    
    // Unset flags using unsetf()
    std::cout.unsetf(std::ios::fixed);
    std::cout << "Without fixed: " << number << std::endl;
    
    return 0;
}

Output:

With flags: 1234.500
Without fixed: 1.23e+03

Output stream states and error handling

#include <iostream>

int main()
{
    std::cout << "Before output, stream state: " << std::cout.good() << std::endl;
    
    std::cout << "Hello, World!" << std::endl;
    
    // Check stream state after output
    if (std::cout.good())
    {
        std::cout << "Output operation was successful" << std::endl;
    }
    else if (std::cout.fail())
    {
        std::cout << "Output operation failed" << std::endl;
    }
    
    return 0;
}

Output:

Before output, stream state: 1
Hello, World!
Output operation was successful

Formatting for different data displays

Progress bar example

#include <iostream>
#include <iomanip>

void displayProgress(int percentage)
{
    int barWidth = 40;
    int filled = (percentage * barWidth) / 100;
    
    std::cout << "[";
    std::cout << std::setfill('=') << std::setw(filled) << "";
    std::cout << std::setfill(' ') << std::setw(barWidth - filled) << "";
    std::cout << "] " << std::setw(3) << percentage << "%\r";
    std::cout.flush();
}

int main()
{
    for (int i = 0; i <= 100; i += 10)
    {
        displayProgress(i);
        // In a real program, you'd have some work here
    }
    std::cout << std::endl;
    
    return 0;
}

Output:

[========================================] 100%

Financial report formatting

#include <iostream>
#include <iomanip>

int main()
{
    std::cout << std::fixed << std::setprecision(2);
    std::cout << std::right;
    
    std::cout << "Financial Report" << std::endl;
    std::cout << std::setfill('=') << std::setw(30) << "" << std::endl;
    std::cout << std::setfill(' ');
    
    std::cout << std::setw(20) << "Revenue:" << std::setw(10) << 150000.00 << std::endl;
    std::cout << std::setw(20) << "Expenses:" << std::setw(10) << 95000.50 << std::endl;
    std::cout << std::setfill('-') << std::setw(30) << "" << std::endl;
    std::cout << std::setfill(' ');
    std::cout << std::setw(20) << "Net Profit:" << std::setw(10) << (150000.00 - 95000.50) << std::endl;
    
    return 0;
}

Output:

Financial Report
==============================
             Revenue:  150000.00
            Expenses:   95000.50
------------------------------
          Net Profit:   54999.50

Best practices for output formatting

✅ Good practices:

#include <iostream>
#include <iomanip>

int main()
{
    // Save original format state
    std::ios_base::fmtflags originalFlags = std::cout.flags();
    int originalPrecision = std::cout.precision();
    
    // Apply temporary formatting
    std::cout << std::fixed << std::setprecision(2);
    std::cout << "Formatted output: " << 3.14159 << std::endl;
    
    // Restore original format state
    std::cout.flags(originalFlags);
    std::cout.precision(originalPrecision);
    
    std::cout << "Back to original: " << 3.14159 << std::endl;
    
    return 0;
}

❌ Common mistakes:

#include <iostream>
#include <iomanip>

int main()
{
    // Bad: Not considering persistent manipulators
    std::cout << std::setprecision(2);
    std::cout << 3.14159 << std::endl;  // Precision affects this
    std::cout << 2.71828 << std::endl;  // And this too!
    
    // Bad: Inconsistent table formatting
    std::cout << std::setw(5) << "Name" << "Age" << std::endl;  // Only first column formatted
    
    return 0;
}

Summary

Effective output formatting with std::ostream includes:

• Width control with std::setw() for aligned output
• Precision control with std::setprecision() for floating-point numbers
• Alignment options with std::left, std::right, and std::internal
• Number base formatting with std::hex, std::oct, and std::dec
• Fill characters with std::setfill() for custom padding
• Boolean formatting with std::boolalpha for text output
• Stream state management for maintaining format consistency

Mastering output formatting allows you to create professional-looking displays, reports, and user interfaces that enhance the usability of your programs.

Quiz

1. What's the difference between persistent and non-persistent manipulators?
2. How do you display a floating-point number with exactly 2 decimal places?
3. What's the purpose of std::setfill()?
4. How can you display a boolean value as "true"/"false" instead of 1/0?
5. Why might you want to save and restore stream format flags?

Practice exercises

Apply your output formatting skills with these exercises:

1. Student Report Card: Create a formatted table showing student names, grades for 3 subjects, and average grades with proper alignment.

2. Number Converter: Write a program that displays the same number in decimal, hexadecimal, and octal formats with proper labeling.

3. Invoice Generator: Create a formatted invoice with item names, quantities, prices, and totals properly aligned with currency formatting.

4. Progress Monitor: Build a progress indicator that shows completion percentage with both a progress bar and numeric percentage.