formatting - Print leading zeros with C++ output operator?

Question

How can I format my output in C++? In other words, what is the C++ equivalent to the use of printf like this:

printf("%05d", zipCode);

I know I could just use printf in C++, but I would prefer the output operator <<.

Would you just use the following?

std::cout << "ZIP code: " << sprintf("%05d", zipCode) << std::endl;

Answer 1

This will do the trick, at least for non-negative numbers^(a) such as the ZIP codes^(b) mentioned in your question.

#include <iostream>
#include <iomanip>

using namespace std;
cout << setw(5) << setfill('0') << zipCode << endl;

// or use this if you don't like 'using namespace std;'
std::cout << std::setw(5) << std::setfill('0') << zipCode << std::endl;

The most common IO manipulators that control padding are:

• std::setw(width) sets the width of the field.
• std::setfill(fillchar) sets the fill character.
• std::setiosflags(align) sets the alignment, where align is ios::left or ios::right.

And just on your preference for using <<, I'd strongly suggest you look into the fmt library (see https://github.com/fmtlib/fmt). This has been a great addition to our toolkit for formatting stuff and is much nicer than massively length stream pipelines, allowing you to do things like:

cout << fmt::format("{:05d}", zipCode);

And it's currently being targeted by LEWG toward C++20 as well, meaning it will hopefully be a base part of the language at that point (or almost certainly later if it doesn't quite sneak in).

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^(a) If you do need to handle negative numbers, you can use std::internal as follows:

cout << internal << setw(5) << setfill('0') << zipCode << endl;

This places the fill character between the sign and the magnitude.

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^(b) This ("all ZIP codes are non-negative") is an assumption on my part but a reasonably safe one, I'd warrant :-)