C++ Basics: Pad Each Output Field Explicitly with setw

Separate one-field width settings from persistent fill characters in formatted stream output.

Prepare a scratch directory

Tested on Linux x86_64 with GCC 14.2.0. Install a C++20-capable compiler and standard-library development headers, with g++ on PATH. You also need Bash, a text editor, and the mktemp, rm, and rmdir utilities. No third-party library is required.

Keep the same terminal open. Stop if setup fails, before saving the source:

start_directory="$PWD"
work_directory="$(mktemp -d)" || exit 1
cd -- "$work_directory" || exit 1

Run the example

Save this as setw-field-padding-demo.cpp:

#include <iomanip>
#include <iostream>

int main()
{
    std::cout << std::setfill('0');
    std::cout << "One width: " << std::setw(4) << 7 << ' ' << 12 << '\n';
    std::cout << "Two widths: " << std::setw(4) << 7 << ' '
              << std::setw(4) << 12 << '\n';
    std::cout << "Long value: " << std::setw(4) << 12345 << '\n';
    std::cout << std::setfill(' ');
    std::cout << "Restored: [" << std::setw(4) << 7 << "]\n";
}

Compile and run it. The program runs only after compilation succeeds:

g++ -std=c++20 -Wall -Wextra -Werror -pedantic setw-field-padding-demo.cpp -o setw-field-padding-demo &&
./setw-field-padding-demo

Observed output:

One width: 0007 12
Two widths: 0007 0012
Long value: 12345
Restored: [   7]

Read the result

The first setw applies to 7 only, leaving 12 unpadded. Setting the width again pads both numbers. A five-digit value remains intact because width is a minimum, not a truncation limit. Restoring the fill character prevents later output from unexpectedly receiving zero padding.

Apply it carefully

For these formatted integer insertions, width resets after each value; the fill character persists. Put setw immediately before the field it belongs to, since another formatted insertion can consume it. Width measures stream character units, not terminal display columns, so it is insufficient for general Unicode table alignment. Negative values may need an explicit alignment policy when padding around a sign.

The C++ working draft describes the relevant rules. This example builds in C++20 mode; the current working draft also documents later additions.

Clean up

Remove only the source and executable created here, then return to the starting directory:

rm -- setw-field-padding-demo.cpp setw-field-padding-demo
cd -- "$start_directory" || exit 1
rmdir -- "$work_directory"
unset start_directory work_directory

If the directory is not empty, inspect its remaining contents before removing anything else.

Acknowledgements

This post was created with assistance from AI.