const_fmt/README.md

const_fmt

An extremely lightweight library, intentionally resembling fmtlib as much as possible. This is achieved by moving as much of the formatting process as possible to compile time.

Meant for systems with very few resources, such as embedded systems.

Overview

During compile-time, the string to be formatted is preprocessed to the point only the actual values to be formatted have to be written (If they are not available at compile time).

For example One number: {:03}; And another one: {:05.3} is preprocessed into One number: 000; And another one: 00.000. This is returned as a std::array<char, N>, where N is automatically evaluated. The only code executed at runtime then formats the numbers and writes them into their places in the array.

Disclaimer: The actual formatting code is largely shamelessly stolen from fmtlib.

Including in a project

In order to keep it as lightweight and optimizable as possible, const_fmt is implemented as a header-only library.

This means that using it in a project is as simple as cloning this repo (or e.g. adding it as a submodule) and adding the repository root folder to the compiler include directories.

Building and running the tests

1. Initialize the googletest framework submodule

$ git submodule update --init

2. Create the build directory

$ cmake -B build -S .

3. Build the project

$ cmake --build build/

4. Run the tests

$ ctest --test-dir build/

Limitations

For the compile time preprocessing of format strings non non-type template parameters are heavily relied upon, which means C++20 is required.

Only a relatively limited subset of the fmtlib syntax is recognized (for now anyway). In particular, float formatting in decimal and integer formatting in decimal, binary and hexadecimal are supported.

By nature of the library design, which forces compile-time preprocessing of the const_format string, no dynamic width or dynamic precision can be implemented.