C++ properties and observer pattern

xproperty is a C++ library providing traitlets-style properties.

xproperty provides an implementation of the observer patterns relying on C++ template and preprocessor metaprogramming techniques.

Properies of observed objects have no additional memory footprint than the value they hold. The assignment of a new value is simply replaced at compiled time by

• the call to the validator for that property
• the actual underlying assigment
• the call to the observor for that property.

We also provide the implementation of an xobserved class whose static validator and observer are bound to a dynamic unordered map of callbacks that can be registered dynamically.

xproperty requires a modern C++ compiler supporting C++14. The following C++ compilers are supported:

• On Windows platforms, Visual C++ 2015 Update 2, or more recent
• On Unix platforms, gcc 4.9 or a recent version of Clang

xproperty is a header-only library. We provide a package for the mamba (or conda) package manager.

mamba install -c conda-forge xproperty

Or you can directly install it from the sources:

cmake -DCMAKE_INSTALL_PREFIX=your_install_prefix
make install

To get started with using xproperty, check out the full documentation

http://xproperty.readthedocs.io/

xproperty depends on nlohmann_json.

Versions prior to 0.12.0, also depend on the xtl library.

• Declaring an observed object Foo with two properties named bar and baz of type double.
• Registering a validator, executed prior to assignment, which can potentially coerce the proposed value.
• Registering a notifier, executed after the assignement.

#include <iostream>
#include <stdexcept>

#include "xproperty/xobserved.hpp"

struct Foo : public xp::xobserved<Foo>
{
    XPROPERTY(double, Foo, bar);
    XPROPERTY(double, Foo, baz);
};

Registering an observer and a validator

Foo foo;

XOBSERVE(foo, bar, [](Foo& f)
{
    std::cout << "Observer: New value of bar: " << f.bar << std::endl;
});

XVALIDATE(foo, bar, [](Foo&, double& proposal)
{
    std::cout << "Validator: Proposal: " << proposal << std::endl;
    if (proposal < 0)
    {
        throw std::runtime_error("Only non-negative values are valid.");
    }
    return proposal;
});

Testing the validated and observed properties

foo.bar = 1.0;                           // Assigning a valid value
                                         // The notifier prints "Observer: New value of bar: 1"
std::cout << foo.bar << std::endl;       // Outputs 1.0

try
{
    foo.bar = -1.0;                      // Assigning an invalid value
}
catch (...)
{
    std::cout << foo.bar << std::endl;   // Still outputs 1.0
}

Shortcuts to link properties of observed objects

// Create two observed objects
Foo source, target;
source.bar = 1.0;

// Link `source.bar` and `target.baz`
XDLINK(source, bar, target, baz);

source.bar = 2.0;
std::cout << target.baz << std::endl;    // Outputs 2.0

Building the tests requires the GTest testing framework and cmake.

gtest and cmake are available as a packages for most linux distributions. Besides, they can also be installed from conda-forge.

mamba install -c conda-forge gtest cmake

Once gtest and cmake are installed, you can build and run the tests:

mkdir build
cd build
cmake -DBUILD_TESTS=ON ..
make -j2 xtest

In the context of continuous integration with Travis CI, tests are run in an environment, which can be activated with

cd test
mamba env create -f ./test-environment.yml
source activate test-xproperty
cd ..
cmake -DBUILD_TESTS=ON .
make -j2 xtest

xpropery's documentation is built with three tools

• doxygen
• sphinx
• breathe

While doxygen must be installed separately, you can install breathe by typing

pip install breathe

Breathe can also be installed with mamba or conda

mamba install -c conda-forge breathe

Finally, build the documentation with

make html

from the docs subdirectory.

We use a shared copyright model that enables all contributors to maintain the copyright on their contributions.

This software is licensed under the BSD-3-Clause license. See the LICENSE file for details.