There are 3 C++ samples: Benchmark, Recognizer and RuntimeKey. Check here, here and here on how to build and use them.
The C++ samples are common to all platforms: Android, Windows, iOS, Raspberry Pi (3 or 4), Linux...
For Android, we recommend using the Java samples under android folder. These are complete samples with support for reatime recognition from video stream (camera) and require Android Studio to build. More info on how to install these samples is available here.
Every sample contain a single C++ source file and is easy to cross compile.
This section explain how to install Raspberry Pi 4 (Raspbian OS) toolchain to cross compile the samples. These instructions can be easily adapted to another target platform (e.g Android).
The toolchain for Raspberry Pi 4 could be found at http://sysprogs.com/getfile/566/raspberry-gcc8.3.0.exe and more toolchain versions are at https://gnutoolchains.com/raspberry/.
If you haven't changed the installation dir then, it should be installed at C:\SysGCC. To add the toolchain to the %PATH:
set PATH=%PATH%;C:\SysGCC\raspberry\bin
sudo apt-get update
sudo apt-get install crossbuild-essential-armhf