Compilable/runnable example of having main() in a namespace

main_namespace.cpp

#include <iostream>
#include <string>
#include <vector>

/**************************************************************************
* Example of having main() in a namespace                                 *
* ----------------------------------------------------------------------- *
* By using extern "C", we avoid C++'s name mangling of main()'s symbol,   *
* despite the fact that it is hidden in a bird's nest of namespaces in    *
* our code.                                                               *
* Normally, it would contain semi-garbage characters that may or may not  *
* look like the namespace(s) the symbol is contained within.              *
* With extern "C", we get a clean reference to main that ld can link.     *
* ----------------------------------------------------------------------- *
* Run with:                                                               *
* $ g++ main_namespace.cpp -o mn                                          *
* $ ./mn                                                                  *
* And you should see the bird forage in the orchard.                      *
* Try commenting out the extern "C". The code will compile but not link:  *
* $ g++ -c main_namespace.cpp  # succeeds                                 *
* $ g++ main_namespace.o -o mn # fails                                    *
* ----------------------------------------------------------------------- *
* Why would you do this? You really shouldn't.                            *
**************************************************************************/

namespace birds
{
    enum Location {
        HOME = 0,
        TREE = 10,
        ORCHARD = 20,
        WINTER_HOME = 30
    };

    enum Status {
        RESTING = 0,
        FORAGING = 10,
        BUILDING_NEST = 20,
        LAYING_EGG = 30
    };

    class Bird {
    public:
        Bird(std::string &species) : species_name(species),
                                     current_location(HOME),
                                     current_status(RESTING) {}
        std::string get_name() const { return this->species_name; }
        Location get_location() const { return this->current_location; }
        Status get_status() const { return this->current_status; }
        void set_location(Location l) { this->current_location = l; }
        void set_status(Status s) { this->current_status = s; }
    private:
        std::string species_name;
        Location current_location;
        Status current_status;
    };

    std::ostream &operator<<(std::ostream &os, const Location &l)
    {
        switch(l) {
            case HOME: os << "at home"; break;
            case TREE: os << "on another branch in the tree"; break;
            case ORCHARD: os << "nearby in the orchard"; break;
            case WINTER_HOME: os << "in another hemisphere for winter"; break;
            default: os << "somewhere unclear"; break;
        }
    }

    std::ostream &operator<<(std::ostream &os, const Status &s)
    {
        switch(s) {
            case RESTING: os << "resting"; break;
            case FORAGING: os << "foraging"; break;
            case BUILDING_NEST: os << "building its nest"; break;
            case LAYING_EGG: os << "laying an egg"; break;
            default: os << "something unclear"; break;
        }
    }

    std::ostream &operator<<(std::ostream &os, const Bird *b)
    {
        os << b->get_name() << " is ";
        os << b->get_status() << " " << b->get_location();
    }

    namespace nest
    {
        class Nest {
        public:
            Nest(Bird *b) : owner(b) {}
            bool occupied() const { return (this->owner->get_location() == HOME); }
            Bird *get_owner() const { return this->owner; }
        private:
            Bird *owner;
        };

        std::ostream &operator<<(std::ostream &os, const Nest *n)
        {
            if(n->occupied()) {
                os << "This nest is occupied by " << n->get_owner()->get_name();
                os << ", who is " << n->get_owner()->get_status() << " ";
                os << n->get_owner()->get_location();
            }
            else {
                os << "This nest is empty";
            }
        }

        extern "C"
        int main(int argc, char **argv)
        {
            std::string american_robin("Turdus migratorius"); // unfortunate
            Bird *b = new Bird(american_robin);
            Nest *n = new Nest(b);

            std::cout << n << std::endl;

            b->set_location(ORCHARD);
            b->set_status(FORAGING);

            std::cout << b << std::endl;
            std::cout << n << std::endl;
        }
    }
}