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VectorView< Number > Singleton Reference

#include <vector_view.h>

Inheritance diagram for VectorView< Number >:
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Public Types

typedef types::global_dof_index size_type
 

Public Member Functions

 VectorView (const size_type new_size, Number *ptr)
 
 VectorView (const size_type new_size, const Number *ptr)
 
 ~VectorView ()
 
virtual void reinit (const size_type N, const bool fast=false)
 
void reinit (const size_type N, Number *ptr)
 
void reinit (const size_type N, const Number *ptr)
 
virtual void swap (Vector< Number > &v)
 

Detailed Description

template<typename Number>
singleton VectorView< Number >

View of a numerical vector of data. This class provides an interface compatible with the Vector<double> class (from which it is inherited), that allows fast access to locations of memory already allocated with arrays of type Number.

This is in the same style of the vector view in the Trilinos library.

You should consider using the VectorView object ONLY when ALL of the following requirements are met:

  1. Your application requires a Vector<Number> object.
  2. All you have at your disposal is a Number* pointer.
  3. You are ABSOLUTELY SURE that the above pointer points to a valid area of memory of the correct size.
  4. You really believe that making a copy of that memory would be too expensive.
  5. You really know what you are doing.

Notice that NO CHECKS are performed on the actual memory, and if you try to access illegal areas of memory, your computer will suffer from it. Once again, use this class ONLY if you know exactly what you are doing.

Two constructors are provided. One for read-write access, and one for read only access, and you are allowed to use this class on objects of type const Number*. However you should be aware of the fact that the constness of the array pointed to is ignored, which means that you should only use the const constructor when the actual object you are constructing is itself a constant object. As a corollary, you will be allowed to call even functions of the base class that change data of the array; this being a violation of the C++ type model, you should make sure that this only happens if it is really valid and, in general, if you know what you are doing.

Since this class does not own the memory that you are accessing, you have to make sure that the lifetime of the section of memory you are viewing is longer than this object. No attempt is made to ensure that this is the case.

An example usage of this class is the following:

// Create an array of length 5;
double * array = new double[5];
// Now create a view of the above array that is compatible with the
// Vector<double> class
VectorView<double> view(5, array);
view(1) = 4;
// The following line should output 4.
cout << array[1] << endl;
// If debug mode is on, then the following triggers an execption:
view(6) = 4;
// But notice that no checks are performed, so this is legal but WILL
// NOT work
VectorView<double> wrong_view(10, array);
// Now no assert will be thrown if you type wrong_view(6), but most
// likely a seg fault will occur.
view(6) = 4;
// Notice that this construction is legal. It will create a copy of
// the array.
const Vector<double> const_copy(view);
// Now this is the correct way to instantiate a constant view of the
// above vector:
const VectorView<double> correct_const_copy_view(5, const_copy.begin());
// While this will compile, BUT WILL NOT COMPLAIN if you try to write
// on it!
VectorView<double> wrong_const_copy_view(5, const_copy.begin());
// Now writing to elements of wrong_const_copy_view is allowed, and
// will change the same memory as the const_copy object.
wrong_const_copy_view(1) = 5;
if(copy_view(1) == wrong_const_copy_view(1)) cout << "Tautology";
Note
Instantiations for this template are provided for <float>, <double>, <long double>, <std::complex<float>>, <std::complex<double>>, <std::complex<long double>>; others can be generated in application programs (see the section on Template instantiations in the manual).
Author
Luca Heltai, 2009

Definition at line 137 of file vector_view.h.

Member Typedef Documentation

template<typename Number>
typedef types::global_dof_index VectorView< Number >::size_type

Declare type for container size.

Definition at line 144 of file vector_view.h.

Constructor & Destructor Documentation

template<typename Number>
VectorView< Number >::VectorView ( const size_type  new_size,
Number *  ptr 
)

Read write constructor. Takes the size of the vector, just like the standard one, but the data is picked starting from the location of the pointer ptr.

template<typename Number>
VectorView< Number >::VectorView ( const size_type  new_size,
const Number *  ptr 
)

The constant constructor is the same as above, however you will not be able to access the data for write access.

You should only use this class by constructing it as a const VectorView<double>(size, ptr) object.

Undefined behavior will occur if you construct it as a non const object or attempt to write on it.

template<typename Number>
VectorView< Number >::~VectorView ( )

This desctructor will only reset the internal sizes and the internal pointers, but it will NOT clear the memory.

Member Function Documentation

template<typename Number>
virtual void VectorView< Number >::reinit ( const size_type  N,
const bool  fast = false 
)
virtual

The reinit function of this object has a behavior which is different from the one of the base class. VectorView does not handle memory, and you should not attempt to resize the memory that is pointed to by this object using the reinit function. You can, however, resize the view that you have of the original object. Notice that it is your own responsibility to ensure that the memory you are pointing to is big enough.

Similarly to what happens in the base class, if fast is false, then the entire content of the vector is set to 0, otherwise the content of the memory is left unchanged.

Notice that the following snippet of code may not produce what you expect:

// Create a vector of length 1.
Vector<double> long_vector(1);
// Make a view of it
VectorView<double> view_of_long_vector(1, long_vector.begin());
// Resize the original vector to a bigger size
long_vector.reinit(100);
// And the view, leaving the memory untouched
view_of_long_vector.reinit(100, true);

In the above case, the Vector<double>::reinit method is called, and a NEW area of memory is reserved, possibly not starting at the same place as before. Hoever, the VectorView<double> object keeps pointing to the same old area. After the two reinits, any call to view_of_long_vector(i), with i>0 might cause an attempt to access invalid areas of memory, or might function properly, depending on whether or not the system was able to allocate some memory consecutively after the original allocation.

In any case, you should not rely on this behavior, and you should only call this reinit function if you really know what you are doing.

template<typename Number>
void VectorView< Number >::reinit ( const size_type  N,
Number *  ptr 
)

This reinit function is equivalent to constructing a new object with the given size, starting from the pointer ptr.

template<typename Number>
void VectorView< Number >::reinit ( const size_type  N,
const Number *  ptr 
)

This reinit function is equivalent to constructing a new object with the given size, starting from the pointer ptr. The same considerations made for the constructor apply here.

template<typename Number>
virtual void VectorView< Number >::swap ( Vector< Number > &  v)
virtual

This function is here to prevent memory corruption. It should never be called, and will throw an exception if you try to do so.


The documentation for this singleton was generated from the following file: