How to write and parse "yaml" file for ROS?

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You can use yaml-cpp along with ROS2. Just add following codes to your package.xml, CMakeLists.txt and source file:

Package.xml:

<build_depend>yaml-cpp</build_depend>
<build_export_depend>yaml-cpp</build_export_depend>
<exec_depend>yaml-cpp</exec_depend>

CMakeLists.txt:

find_package(yaml-cpp REQUIRED)
include_directories(HEADERS_EXECUTABLE
    ${YAML_INCLUDE_DIRS}
)
target_link_libraries(${CMAKE_PROJECT_NAME}
    yaml-cpp
)

main.cpp:

#include <yaml-cpp/yaml.h>

int main(int argc, char const *argv[])
{
    YAML::Node config = YAML::LoadFile("[Name Of File].yaml");

    if (config["SOMETHING"]) {
      std::cout << config["SOMETHING"] << "\n";
    }
    return 0;
}

You can find more information about yaml-cpp here.

Here is an example yaml file you can use to specify topic names to be connected to:

topics:
    - topic1
    - topic2
    - topic3
    - topic4

What you see here is a map with one key "topics", its value is a list with 4 string elements. That's pretty much all you need to know about yaml syntax to cover 90% use cases in ROS (both reading and writing config files).

To read those topic names in a ROS node using roscpp, see this page. Scroll all the way down to "Retrieving Lists".

There are some libraries out there to parse yaml files in C/C++. Have a look at yaml.org in the C/C++ Projects section.

For C++ this Page shows how to parse an example yaml file with the yaml-cpp parser.