Boost C++ Libraries

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Boost Pointer Container Library

Examples

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

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).