C++ Centering String With Padding Characters

Introduction

In C++ programming, we often want to print a string that is centered and padded with some characters so that the print out looks nice.

In this blog post, we will show how to center a string with padding characters in C++.

Centering String With Padding Characters

In the following example, we will show how to center a string with padding characters in C++ using the std::format function in C++20 and the std::string function in C++14.

centered_string.cpp

#include <iostream>
#include <string>
#include <iomanip>
// The format header might not be available in some compilers even if they support C++20.
#if __cplusplus >= 202002L && __has_include(<format>)
#include <format>
#endif

std::string std_string_centered(std::string const& s, size_t width, char pad = ' ')
{
    size_t const l{s.length()};
    // Throw an exception if width is too small.
    if (width < l)
    {
        throw std::runtime_error("Width is too small.");
    }
#if __cplusplus >= 202002L && __has_include(<format>)
    // In C++20, center the string s and pad it with the pad character using runtime format.
    // We cannot use the std::format function here because it does not support runtime fmt string.
    // Use std::vformat instead.
    std::string const s_template{std::string("{:") + std::string(1, pad) + std::string("^{}}")};
    std::string const s_centered{std::vformat(s_template, std::make_format_args(s, width))};
#else
    size_t const left_pad{(width - l) / 2};
    size_t const right_pad{width - l - left_pad};
    std::string const s_centered{std::string(left_pad, pad) + s + std::string(right_pad, pad)};
#endif
    return s_centered;
}

int main()
{
    size_t const width{40};
    std::string const s{"Hello, Underworld!"};

    std::cout << std_string_centered("", width, '*') << std::endl;
    std::cout << std_string_centered(s, width, ' ') << std::endl;
    std::cout << std_string_centered("", width, '*') << std::endl;

    return 0;
}

The code can be compiled using G++ using C++14 or C++20. We could see that the string Hello, Underworld! was centered and padded to 40 characters using the character.

$ g++ centered_string.cpp -o centered_string -std=c++14
$ ./centered_string
****************************************
           Hello, Underworld!
****************************************
$ g++ centered_string.cpp -o centered_string -std=c++20
$ ./centered_string
****************************************
           Hello, Underworld!
****************************************

It’s possible that the format header is not available in some compilers even if they support C++20. To use a compiler that supports both C++20 and the format header, we can use the Docker container from the GCC Docker Hub. For example, we can use the GCC 13.2.0 Docker container gcc:13.2.0 pull the following command.

$ docker pull gcc:13.2.0

References

• std::formatter - CPP Reference
• std::format - CPP Reference
• std::vformat - CPP Reference