OatPP(Oat++) is an open-source lightweight C++ Web Framework.

OatPP is a well-structured, relatively easy-to-use, multithreaded, modern C++ web framework.

The learning curve is narrower and the project shares the same approach with all the modern language web frameworks like Node.JS, Go, Java, etc. which helps developers to adapt easily.

Project website: https://oatpp.io/

Github Repo: https://github.com/oatpp/oatpp

Although installing and building the Oat++ library is straightforward, the default installation folder structure can cause some dependency errors in gcc/g++ on (as I experienced) Linux systems.

You can follow the instructions to solve this issue whenever you face it.

1- Install pre-requirements
OatPP does not offer pre-built binaries. We must download the source code from the repository and compile it on the target computer. This process requires some dependencies, which we must install on our system before this compilation process.

apt install git cmake build-essential libatomic -y

2- Download the source code
Download the source code from the OatPP (Oat++) GitHub repository to any folder in your system. I prefer a temp folder for this purpose which we could delete completely after the installation.
Download the git repository in your preferred folder.

mkdir temp && cd temp
git clone https://github.com/oatpp/oatpp.git

3- Build the OatPP (Oat++) Library
Open the oatpp folder and create a build folder under this to collect the CMake build outputs. If you want to customize the building process please consider the OatPP (Oat++) CMake Options.

cd oatpp
mkdir build && cd build
cmake ..

4- Install the OatPP (Oat++) Library
Finally use make to start installation.

make install

This will complete the installation and, normally you are ready to use it including the header files to your project and link the library to your compiler.

If you have compiler errors in finding OatPP references like me, follow the next steps to change the system's static library and header files folder structures.

OatPP Installation script targets the /usr/local/include and /usr/local/lib folders as default but it uses version numbered root folder and deeply nested folder structures in the destination. Cause of this folder structure, header files in the library could not be found as default. We will bring the library files to the upper level in the folder structure to solve this problem.

Open the /usr/local/lib/oatpp-[version.number] folder and copy the oatpp folder under the /usr/local/lib folder with sudo previlages.
After copying the library folder, you can delete the bad structured library folder.

cd /usr/loca/lib/oatpp-[version.number]
sudo cp -R oatpp /usr/local/lib/

cd /usr/local/lib
sudo rm -R /usr/local/lib/oatpp-[version.number]

Open the /usr/local/include/oatpp-[version]/oatpp/ folder and copy the oatpp folder under the /usr/local/include/ folder with sudo previlages.
After copying the header folder, you can delete the bad structured header folder.

cd /usr/local/include/oatpp-[version.number]/oatpp/oatpp
sudo cp -R oatpp /usr/local/include/

cd /usr/local/include
sudo rm -R /usr/local/include/oatpp-[version.number]

This will solve the issue you face with “unknown reference” during the compilation.