Reference documentation for deal.II version Git ca6ed5c 2017-11-20 14:53:00 +0100
Classes | Public Member Functions | Static Public Member Functions | List of all members
VectorMemory< VectorType > Class Template Referenceabstract

#include <deal.II/lac/vector_memory.h>

Inheritance diagram for VectorMemory< VectorType >:
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Classes

class  Pointer
 

Public Member Functions

virtual ~VectorMemory ()=default
 
virtual VectorType * alloc ()=0
 
virtual void free (const VectorType *const)=0
 
- Public Member Functions inherited from Subscriptor
 Subscriptor ()
 
 Subscriptor (const Subscriptor &)
 
 Subscriptor (Subscriptor &&)
 
virtual ~Subscriptor ()
 
Subscriptoroperator= (const Subscriptor &)
 
Subscriptoroperator= (Subscriptor &&)
 
void subscribe (const char *identifier=nullptr) const
 
void unsubscribe (const char *identifier=nullptr) const
 
unsigned int n_subscriptions () const
 
void list_subscribers () const
 
template<class Archive >
void serialize (Archive &ar, const unsigned int version)
 

Static Public Member Functions

static::ExceptionBase & ExcNotAllocatedHere ()
 
- Static Public Member Functions inherited from Subscriptor
static::ExceptionBase & ExcInUse (int arg1, char *arg2, std::string &arg3)
 
static::ExceptionBase & ExcNoSubscriber (char *arg1, char *arg2)
 

Detailed Description

template<typename VectorType = ::Vector<double>>
class VectorMemory< VectorType >

Memory management base class for vectors. This is an abstract base class used, among other places, by all iterative methods to allocate space for auxiliary vectors.

The purpose of this class is as follows: in iterative solvers and other places, one needs to allocate temporary storage for vectors, for example for auxiliary vectors. One could allocate and release them anew every time, but this may be expensive in some situations if it has to happen very frequently. A common case for this is when an iterative method is used to invert a matrix in each iteration of an outer solver, such as when inverting a matrix block for a Schur complement solver. (step-20 does this, for example, but instead just keeps a vector around permanently for temporary storage.)

In such situations, allocating and deallocating vectors anew in each call to the inner solver is expensive and leads to memory fragmentation. The present class allows to avoid this by offering an interface that other classes can use to allocate and deallocate vectors. Different derived classes then implement different strategies to provide temporary storage vectors to using classes.

For example, the PrimitiveVectorMemory class simply allocates and deallocates vectors via the operating system facilities (i.e., using new and delete) each time it is asked for a vector. It is an appropriate implementation to use for iterative solvers that are called only once, or very infrequently.

On the other hand, the GrowingVectorMemory class never returns memory space to the operating system memory management subsystem during its lifetime; it only marks them as unused and allows them to be reused next time a vector is requested.

Practical use

Classes derived from this base class return pointers to new vectors via the VectorMemory::alloc() function, and re-claim the vector when it is returned via VectorMemory::free(). These two functions therefore play a similar role as new and delete. This includes the usual drawbacks: It is simple to forget to call VectorMemory::free() at the end of a function that uses this facility, or to forget it in an if branch of the function where one has an early return from the function. In both cases, this results in a memory leak: a correct piece of code has to call VectorMemory::free() for all allocated vectors at all possible exit points. This includes places where a function is left because an exception is thrown further down in the call stack and not explicitly handled here.

In other words, vectors allocated via VectorMemory::alloc() have the same issue as raw pointers allocated via new: It is easy to write code that has memory leaks. In the case of raw pointers, the common solution is to use the std::unique_ptr class instead (see http://en.cppreference.com/w/cpp/memory/unique_ptr). In the case of the current class, the VectorMemory::Pointer class is the solution: it is a class that for all practical purposes looks like a pointer, but upon destruction also returns the vector back to the VectorMemory object from which it got it. Since destruction of the VectorMemory::Pointer class happens whenever it goes out of scope (whether because the function explicitly returns, or because control flow leaves it due to an exception), a memory leak cannot happen: the vector the VectroMemory::Pointer object points to is always returned.

Author
Guido Kanschat, 1998-2003; Wolfgang Bangerth, 2017.

Definition at line 26 of file pointer_matrix.h.

Constructor & Destructor Documentation

template<typename VectorType = ::Vector<double>>
virtual VectorMemory< VectorType >::~VectorMemory ( )
virtualdefault

Virtual destructor. This destructor is declared virtual to allow destroying objects of derived type through pointers to this base class.

Member Function Documentation

template<typename VectorType = ::Vector<double>>
virtual VectorType* VectorMemory< VectorType >::alloc ( )
pure virtual

Return a pointer to a new vector. The number of elements or their subdivision into blocks (if applicable) is unspecified and users of this function should reset vectors to their proper size. The same holds for the contents of vectors: they are unspecified. In other words, the place that calls this function will need to resize or reinitialize it appropriately.

Warning
Just like using new and delete explicitly in code invites bugs where memory is leaked (either because the corresponding delete is forgotten altogether, or because of exception safety issues), using the alloc() and free() functions explicitly invites writing code that accidentally leaks memory. You should consider using the VectorMemory::Pointer class instead, which provides the same kind of service that std::unique provides for arbitrary memory allocated on the heap.

Implemented in GrowingVectorMemory< VectorType >, GrowingVectorMemory< BlockVector< number > >, and PrimitiveVectorMemory< VectorType >.

template<typename VectorType = ::Vector<double>>
virtual void VectorMemory< VectorType >::free ( const VectorType *  const)
pure virtual

Return a vector and indicate that it is not going to be used any further by the place that called alloc() to get a pointer to it.

Warning
Just like using new and delete explicitly in code invites bugs where memory is leaked (either because the corresponding delete is forgotten altogether, or because of exception safety issues), using the alloc() and free() functions explicitly invites writing code that accidentally leaks memory. You should consider using the VectorMemory::Pointer class instead, which provides the same kind of service that std::unique provides for arbitrary memory allocated on the heap.

Implemented in GrowingVectorMemory< VectorType >, GrowingVectorMemory< BlockVector< number > >, and PrimitiveVectorMemory< VectorType >.


The documentation for this class was generated from the following files: