formula-cpp

Documentation · Tutorial · API reference

Declarative, traceable, self-documenting formulas for C++23. Header-only, no dependencies.

Software that implements a test method (a standard's procedure for testing a material) usually keeps the formula in one place, its units in another, where it comes from in a comment, and its audit trail in code written afterwards. Those drift apart. In formula-cpp they are one declaration: write the formula once, with ordinary operators, and get the number, its units, its derivation and its documentation from it.

Example

The compressive strength of a concrete specimen: the load that crushed it, 675 kN, over the area that carried it, 22500 mm².

#include <formula-cpp/format.hpp>
#include <formula-cpp/formula.hpp>

#include <print>

// A quantity is a type: a symbol, a description and a unit.
using Load = formula::Quantity<struct LoadTag, "F", "maximum load", formula::unit::Kilonewton>;
using Area = formula::Quantity<struct AreaTag, "A_c", "loaded area", formula::unit::SquareMillimetre>;
using Strength = formula::Quantity<struct StrengthTag, "f_c", "compressive strength", formula::unit::Megapascal>;

// The formula, written once with ordinary operators.
constexpr auto strength = formula::yields<Strength>(formula::var<Load> / formula::var<Area>);

int main()
{
    auto const specimen = formula::environment(formula::Measured<Load> { 675 }, formula::Measured<Area> { 22500 });

    auto const result = formula::checked_evaluate(strength, specimen);
    if (!result)
    {
        std::println("cannot calculate: {}", result.error());
        return 1;
    }
    std::println("{} = {}", formula::symbol_of<Strength>(), *result);
}

f_c = 30 MPa

• Units are part of the type. Kilonewtons over square millimetres arrive in megapascals with no conversion written. (Units and dimensions)
• Mistakes are compile errors. formula::var<Load> + formula::var<Area> does not compile. (Units and dimensions)
• Arithmetic is exact. The 30 is an exact rational, not a double. (Exact numbers)

What you get

• Dimensional analysis at compile time, with exact unit conversion. (Dimensions and units)
• Exact arithmetic, rounded only where you say, in the mode you name. (Exact numbers)
• Missing and entered values that never pass for computed ones. (Quantities and measurements, Missing and entered values)
• The method's own rounding, constraints and tables, as part of the formula. (Rounding and conditionals, Constraints, Lookup tables)
• Traces that show how every number was reached. (Tracing)
• Rendering and generated documentation: text, LaTeX, symbol tables and citations from the same declaration. (Citations and rendering)

New to the library? Start with the tutorial.

Installation

CPM

CPMAddPackage("gh:LASTRADA-Software/formula-cpp@0.4.0")
target_link_libraries(your_target PRIVATE formula-cpp::formula-cpp)

CMake, from an install tree

cmake -S . -B build -DFORMULA_INSTALL=ON -DCMAKE_INSTALL_PREFIX=/your/prefix
cmake --install build

find_package(formula-cpp CONFIG REQUIRED)
target_link_libraries(your_target PRIVATE formula-cpp::formula-cpp)

Copy the headers

include/ is self-contained and depends on nothing outside the standard library. formula.hpp is the umbrella header. format.hpp, render.hpp, document.hpp, trace.hpp and trace_render.hpp are separate, because they need <string>, <vector> or <format>: include them by name when you print, render, document or trace.

Requirements

• C++23
• CMake 3.23 or newer
• GCC 14 or newer, if you build with GCC

CI builds and tests every push to master and every pull request with MSVC cl and clang-cl on Windows, Clang and GCC 14 on Linux, and AppleClang on macOS.

Build options

Option	Default	Effect
FORMULA_BUILD_TESTS	ON when top-level	Build the test suite (fetches Catch2)
FORMULA_BUILD_EXAMPLES	ON when top-level	Build the examples
FORMULA_BUILD_DOCS	ON when top-level	Configure the Doxygen API-reference target (never in the default build)
FORMULA_TOOLS	ON when top-level	Build the project's own tooling
FORMULA_INSTALL	ON when top-level	Generate install and export rules
FORMULA_PEDANTIC	ON	Strict warnings on the project's own targets
FORMULA_WERROR	OFF	Treat warnings as errors

Contributing

See CONTRIBUTING.md.

Licence

Apache-2.0. See LICENSE.