Boost C++ Libraries

All containers offered by Boost.Container can store movable-only types and actual requirements for value_type depend on each container operations. Following C++11 requirements even for C++03 compilers, many operations now require movable or default constructible types instead of just copy constructible types.

Containers themselves are also movable, with no-throw guarantee if allocator or predicate (if present) copy operations are no-throw. This allows high performance operations when transferring data between vectors. Let's see an example:

#include <boost/container/vector.hpp>
#include <boost/move/utility_core.hpp>
#include <cassert>

//Non-copyable class
class non_copyable
{
   BOOST_MOVABLE_BUT_NOT_COPYABLE(non_copyable)

   public:
   non_copyable(){}
   non_copyable(BOOST_RV_REF(non_copyable)) {}
   non_copyable& operator=(BOOST_RV_REF(non_copyable)) { return *this; }
};

int main ()
{
   using namespace boost::container;

   //Store non-copyable objects in a vector
   vector<non_copyable> v;
   non_copyable nc;
   v.push_back(boost::move(nc));
   assert(v.size() == 1);

   //Reserve no longer needs copy-constructible
   v.reserve(100);
   assert(v.capacity() >= 100);

   //This resize overload only needs movable and default constructible
   v.resize(200);
   assert(v.size() == 200);

   //Containers are also movable
   vector<non_copyable> v_other(boost::move(v));
   assert(v_other.size() == 200);
   assert(v.empty());

   return 0;
}

All containers offered by Boost.Container implement placement insertion, which means that objects can be built directly into the container from user arguments without creating any temporary object. For compilers without variadic templates support placement insertion is emulated up to a finite (10) number of arguments.

Expensive to move types are perfect candidates emplace functions and in case of node containers (list, set, ...) emplace allows storing non-movable and non-copyable types in containers! Let's see an example:

#include <boost/container/list.hpp>
#include <cassert>

//Non-copyable and non-movable class
class non_copy_movable
{
   non_copy_movable(const non_copy_movable &);
   non_copy_movable& operator=(const non_copy_movable &);

   public:
   non_copy_movable(int = 0) {}
};

int main ()
{
   using namespace boost::container;

   //Store non-copyable and non-movable objects in a list
   list<non_copy_movable> l;
   non_copy_movable ncm;

   //A new element will be built calling non_copy_movable(int) constructor
   l.emplace(l.begin(), 0);
   assert(l.size() == 1);

   //A new element will be value initialized
   l.emplace(l.begin());
   assert(l.size() == 2);
   return 0;
}

Most Boost.Container containers can be used to define recursive containers:

#include <boost/container/vector.hpp>
#include <boost/container/stable_vector.hpp>
#include <boost/container/deque.hpp>
#include <boost/container/list.hpp>
#include <boost/container/map.hpp>
#include <boost/container/string.hpp>

using namespace boost::container;

struct data
{
   int               i_;
   //A vector holding still undefined class 'data'
   vector<data>      v_;
   vector<data>::iterator vi_;
   //A stable_vector holding still undefined class 'data'
   stable_vector<data> sv_;
   stable_vector<data>::iterator svi_;
   //A stable_vector holding still undefined class 'data'
   deque<data> d_;
   deque<data>::iterator di_;
   //A list holding still undefined 'data'
   list<data>        l_;
   list<data>::iterator li_;
   //A map holding still undefined 'data'
   map<data, data>   m_;
   map<data, data>::iterator   mi_;

   friend bool operator <(const data &l, const data &r)
   { return l.i_ < r.i_; }
};

struct tree_node
{
   string name;
   string value;

   //children nodes of this node
   list<tree_node>            children_;
   list<tree_node>::iterator  selected_child_;
};



int main()
{
   //a container holding a recursive data type
   stable_vector<data> sv;
   sv.resize(100);

   //Let's build a tree based in
   //a recursive data type
   tree_node root;
   root.name  = "root";
   root.value = "root_value";
   root.children_.resize(7);
   root.selected_child_ = root.children_.begin();
   return 0;
}

Containers of incomplete types are useful to break header file dependencies and improve compilation types. With Boost.Container, you can write a header file defining a class with containers of incomplete types as data members, if you carefully put all the implementation details that require knowing the size of the value_type in your implementation file:

In this header file we define a class (MyClassHolder) that holds a vector of an incomplete type (MyClass) that it's only forward declared.

#include <boost/container/vector.hpp>

//MyClassHolder.h

//We don't need to include "MyClass.h"
//to store vector<MyClass>
class MyClass;

class MyClassHolder
{
   public:

   void AddNewObject(const MyClass &o);
   const MyClass & GetLastObject() const;

   private:
   ::boost::container::vector<MyClass> vector_;
};

Then we can define MyClass in its own header file.

//MyClass.h

class MyClass
{
   private:
   int value_;

   public:
   MyClass(int val = 0) : value_(val){}

   friend bool operator==(const MyClass &l, const MyClass &r)
   {  return l.value_ == r.value_;  }
   //...
};

And include it only in the implementation file of MyClassHolder

//MyClassHolder.cpp

#include "MyClassHolder.h"

//In the implementation MyClass must be a complete
//type so we include the appropriate header
#include "MyClass.h"

void MyClassHolder::AddNewObject(const MyClass &o)
{  vector_.push_back(o);  }

const MyClass & MyClassHolder::GetLastObject() const
{  return vector_.back();  }

Finally, we can just compile, link, and run!

//Main.cpp

#include "MyClassHolder.h"
#include "MyClass.h"

#include <cassert>

int main()
{
   MyClass mc(7);
   MyClassHolder myclassholder;
   myclassholder.AddNewObject(mc);
   return myclassholder.GetLastObject() == mc ? 0 : 1;
}