In source file (Selection.cpp), you have to qualify the method names with class name:
void Selection::setCurrentArray(std::string selectionEntries[])
{
//Copies a string array to a vector of struct entry
}
Otherwise these methods will be free function, and not definition of the class methods. As a result, your class methods remain undefined, and when linker tries to link with the methods, it can not find them, and shows undefined reference of the method error.