# Pointer Container Library

## Examples

Some examples are given here and in the accompanying test files:

- 1. Null pointers cannot be stored in the containers
- 2. Iterators and other operations return indirected values
- 3. Copy-semantics of pointer containers
- 4. Making a non-copyable type Clonable
- 5. Objects are cloned before insertion, inserted pointers are owned by the container
- 6. Transferring ownership of a single element
- 7. Transferring ownership of pointers between different pointer containers
- 8. Selected test files
- 9. A large example

# 1. Null pointers cannot be stored in the containers

    my_container.push_back( 0 );            // throws bad_ptr
    my_container.replace( an_iterator, 0 ); // throws bad_ptr
    my_container.insert( an_iterator, 0 );  // throws bad_ptr
    std::auto_ptr<T> p( 0 );
    my_container.push_back( p );            // throws bad_ptr

# 2. Iterators and other operations return indirected values

    ptr_vector<X> pvec;
    std::vector<X*> vec;
    *vec.begin()  = new X;   // fine, memory leak
    *pvec.begin() = new X;   // compile time error
    ( *vec.begin() )->foo(); // call X::foo(), a bit clumsy
    pvec.begin()->foo();     // no indirection needed
    *vec.front()  = X();     // overwrite first element
    pvec.front()  = X();     // no indirection needed

# 3. Copy-semantics of pointer containers

    ptr_vector<T> vec1;
    ...
    ptr_vector<T> vec2( vec1.clone() ); // deep copy objects of 'vec1' and use them to construct 'vec2', could be very expensive
    vec2 = vec1.release();              // give up ownership of pointers in 'vec1' and pass the ownership to 'vec2', rather cheap
    vec2.release();                     // give up ownership; the objects will be deallocated if not assigned to another container
    vec1 = vec2;                        // compile time error: 'operator=()' not defined
    ptr_vector<T> vec3( vec1 );         // compile time error: copy-constructor not defined

# 4. Making a non-copyable type Clonable

     // a class that has no normal copy semantics
    class X : boost::noncopyable { public: X* clone() const; ... };

    // this will be found by the library by argument dependent lookup (ADL)
    X* new_clone( const X& x )
    { return x.clone(); }

    // we can now use the interface that requires clonability
    ptr_vector<X> vec1, vec2;
    ...
    vec2 = vec1.clone();                                 // 'clone()' requires cloning <g>
    vec2.insert( vec2.end(), vec1.begin(), vec1.end() ); // inserting always means inserting clones

# 5. Objects are cloned before insertion, inserted pointers are owned by the container

    class X { ... };                     // assume 'X' is Clonable
    X x;                                 // and 'X' can be stack-allocated
    ptr_list<X> list;
    list.push_back( new_clone( x ) );    // insert a clone
    list.push_back( new X );             // always give the pointer directly to the container to avoid leaks
    list.push_back( &x );                // don't do this!!!
    std::auto_ptr<X> p( new X );
    list.push_back( p );                 // give up ownership
    BOOST_ASSERT( p.get() == 0 );

# 6. Transferring ownership of a single element

    ptr_deque<T>                    deq;
    typedef ptr_deque<T>::auto_type auto_type;

    // ... fill the container somehow

    auto_type ptr  = deq.release_back();             // remove back element from container and give up ownership
    auto_type ptr2 = deq.release( deq.begin() + 2 ); // use an iterator to determine the element to release
    ptr            = deq.release_front();            // supported for 'ptr_list' and 'ptr_deque'

    deq.push_back( ptr.release() );                  // give ownership back to the container

# 7. Transferring ownership of pointers between different pointer containers

    ptr_list<X> list; ptr_vector<X> vec;
    ...
    //
    // note: no cloning happens in these examples
    //
    list.transfer( list.begin(), vec.begin(), vec );           // make the first element of 'vec' the first element of 'list'
    vec.transfer( vec.end(), list.begin(), list.end(), list ); // put all the lists element into the vector

We can also transfer objects from `ptr_container<Derived>` to `ptr_container<Base` without any problems.

# 8. Selected test files

incomplete_type_test.cpp:
| Shows how to implement the Composite pattern.
simple_test.cpp:
| Shows how the usage of pointer container compares with a container of smart pointers
view_example.cpp:
| Shows how to use a pointer container as a view into other container
tree_test.cpp: | Shows how to make a tree-structure
array_test.cpp: | Shows how to make an n-ary tree

# 9. A large example

This examples shows many of the most common features at work. The example provide lots of comments.

    //

    // Boost.Pointer Container

    //

    //  Copyright Thorsten Ottosen 2003-2005. Use, modification and

    //  distribution is subject to the Boost Software License, Version

    //  1.0. (See accompanying file LICENSE_1_0.txt or copy at

    //  http://www.boost.org/LICENSE_1_0.txt)

    //

    // For more information, see http://www.boost.org/libs/ptr_container/

    //

    //

    // This example is intended to get you started.

    // Notice how the smart container

    //

    // 1. takes ownership of objects

    // 2. transfers ownership

    // 3. applies indirection to iterators

    // 4. clones objects from other smart containers

    //

    //

    // First we select which container to use.

    //

    #include <boost/ptr_container/ptr_deque.hpp>

    //

    // we need these later in the example

    //

    #include <boost/assert.hpp>

    #include <string>

    #include <exception>

    //

    // Then we define a small polymorphic class

    // hierarchy.

    //

    class animal : boost::noncopyable

    {

        virtual std::string do_speak() const = 0;

        std::string name_;

    protected:

        //

        // Animals cannot be copied...

        //

        animal( const animal& r ) : name_( r.name_ )           { }

        void operator=( const animal& );

    private:

        //

        // ...but due to advances in genetics, we can clone them!

        //

        virtual animal* do_clone() const = 0;

    public:

        animal( const std::string& name ) : name_(name)        { }

        virtual ~animal() throw()                              { }

        std::string speak() const

        {

            return do_speak();

        }

        std::string name() const

        {

            return name_;

        }

        animal* clone() const

        {

            return do_clone();

        }

    };

    //

    // An animal is still not Clonable. We need this last hook.

    //

    // Notice that we pass the animal by const reference

    // and return by pointer.

    //

    animal* new_clone( const animal& a )

    {

        return a.clone();

    }

    //

    // We do not need to define 'delete_clone()' since

    // since the default is to call the default 'operator delete()'.

    //

    const std::string muuuh = "Muuuh!";

    const std::string oiink = "Oiiink";

    class cow : public animal

    {

        virtual std::string do_speak() const

        {

            return muuuh;

        }

        virtual animal* do_clone() const

        {

            return new cow( *this );

        }

    public:

        cow( const std::string& name ) : animal(name)          { }

    };

    class pig : public animal

    {

        virtual std::string do_speak() const

        {

            return oiink;

        }

        virtual animal* do_clone() const

        {

            return new pig( *this );

        }

    public:

        pig( const std::string& name ) : animal(name)          { }

    };

    //

    // Then we, of course, need a place to put all

    // those animals.

    //

    class farm

    {

        //

        // This is where the smart containers are handy

        //

        typedef boost::ptr_deque<animal> barn_type;

        barn_type                        barn;

        //

        // An error type

        //

        struct farm_trouble : public std::exception           { };

    public:

        //

        // We would like to make it possible to

        // iterate over the animals in the farm

        //

        typedef barn_type::iterator  animal_iterator;

        //

        // We also need to count the farm's size...

        //

        typedef barn_type::size_type size_type;

        //

        // And we also want to transfer an animal

        // safely around. The easiest way to think

        // about '::auto_type' is to imagine a simplified

        // 'std::auto_ptr<T>' ... this means you can expect

        //

        //   T* operator->()

        //   T* release()

        //   deleting destructor

        //

        // but not more.

        //

        typedef barn_type::auto_type  animal_transport;

        //

        // Create an empty farm.

        //

        farm()                                                 { }

        //

        // We need a constructor that can make a new

        // farm by cloning a range of animals.

        //

        farm( animal_iterator begin, animal_iterator end )

         :

            //

            // Objects are always cloned before insertion

            // unless we explicitly add a pointer or

            // use 'release()'. Therefore we actually

            // clone all animals in the range

            //

            barn( begin, end )                               { }

        //

        // ... so we need some other function too

        //

        animal_iterator begin()

        {

            return barn.begin();

        }

        animal_iterator end()

        {

            return barn.end();

        }

        //

        // Here it is quite ok to have an 'animal*' argument.

        // The smart container will handle all ownership

        // issues.

        //

        void buy_animal( animal* a )

        {

            barn.push_back( a );

        }

        //

        // The farm can also be in economical trouble and

        // therefore be in the need to sell animals.

        //

        animal_transport sell_animal( animal_iterator to_sell )

        {

            if( to_sell == end() )

                throw farm_trouble();

            //

            // Here we remove the animal from the barn,

            // but the animal is not deleted yet...it's

            // up to the buyer to decide what

            // to do with it.

            //

            return barn.release( to_sell );

        }

        //

        // How big a farm do we have?

        //

        size_type size() const

        {

            return barn.size();

        }

        //

        // If things are bad, we might choose to sell all animals :-(

        //

        std::auto_ptr<barn_type> sell_farm()

        {

            return barn.release();

        }

        //

        // However, if things are good, we might buy somebody

        // else's farm :-)

        //

        void buy_farm( std::auto_ptr<barn_type> other )

        {

            //

            // This line inserts all the animals from 'other'

            // and is guaranteed either to succeed or to have no

            // effect

            //

            barn.transfer( barn.end(), // insert new animals at the end

                             *other );     // we want to transfer all animals,

                                           // so we use the whole container as argument

            //

            // You might think you would have to do

            //

            // other.release();

            //

            // but '*other' is empty and can go out of scope as it wants

            //

            BOOST_ASSERT( other->empty() );

        }

    }; // class 'farm'.

    int main()

    {

        //

        // First we make a farm

        //

        farm animal_farm;

        BOOST_ASSERT( animal_farm.size() == 0u );

        animal_farm.buy_animal( new pig("Betty") );

        animal_farm.buy_animal( new pig("Benny") );

        animal_farm.buy_animal( new pig("Jeltzin") );

        animal_farm.buy_animal( new cow("Hanz") );

        animal_farm.buy_animal( new cow("Mary") );

        animal_farm.buy_animal( new cow("Frederik") );

        BOOST_ASSERT( animal_farm.size() == 6u );

        //

        // Then we make another farm...it will actually contain

        // a clone of the other farm.

        //

        farm new_farm( animal_farm.begin(), animal_farm.end() );

        BOOST_ASSERT( new_farm.size() == 6u );

        //

        // Is it really clones in the new farm?

        //

        BOOST_ASSERT( new_farm.begin()->name() == "Betty" );

        //

        // Then we search for an animal, Mary (the Crown Princess of Denmark),

        // because we would like to buy her ...

        //

        typedef farm::animal_iterator iterator;

        iterator to_sell;

        for( iterator i   = animal_farm.begin(),

                      end = animal_farm.end();

             i != end; ++i )

        {

            if( i->name() == "Mary" )

            {

                to_sell = i;

                break;

            }

        }

        farm::animal_transport mary = animal_farm.sell_animal( to_sell );

        if( mary->speak() == muuuh )

            //

            // Great, Mary is a cow, and she may live longer

            //

            new_farm.buy_animal( mary.release() );

        else

            //

            // Then the animal would be destroyed (!)

            // when we go out of scope.

            //

            ;

        //

        // Now we can observe some changes to the two farms...

        //

        BOOST_ASSERT( animal_farm.size() == 5u );

        BOOST_ASSERT( new_farm.size()    == 7u );

        //

        // The new farm has however underestimated how much

        // it cost to feed Mary and its owner is forced to sell the farm...

        //

        animal_farm.buy_farm( new_farm.sell_farm() );

        BOOST_ASSERT( new_farm.size()    == 0u );

        BOOST_ASSERT( animal_farm.size() == 12u );

    }

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Copyright: | Thorsten Ottosen 2004-2006. Use, modification and distribution is subject to the Boost Software License, Version 1.0 (see LICENSE_1_0.txt).