[Note: this document is formatted similarly to the SGI STL implementation documentation pages, and refers to concepts and classes defined there. However, neither this document nor the code it describes is associated with SGI, nor is it necessary to have SGI's STL implementation installed in order to use this class.]

sparsetable<T, GROUP_SIZE>

A sparsetable is a Random Access Container that supports constant time random access to elements, and constant time insertion and removal of elements. It implements the "array" or "table" abstract data type. The number of elements in a sparsetable is set at constructor time, though you can change it at any time by calling resize().

sparsetable is distinguished from other array implementations, including the default C implementation, in its stingy use of memory -- in particular, unused array elements require only 1 bit of disk space to store, rather than sizeof(T) bytes -- and by the ability to save and restore contents to disk. On the other hand, this array implementation, while still efficient, is slower than other array implementations.

A sparsetable distinguishes between table elements that have been assigned and those that are unassigned. Assigned table elements are those that have had a value set via set(), operator(), assignment via an iterator, and so forth. Unassigned table elements are those that have not had a value set in one of these ways, or that have been explicitly unassigned via a call to erase() or clear(). Lookup is valid on both assigned and unassigned table elements; for unassigned elements, lookup returns the default value T().

This class is appropriate for applications that need to store large arrays in memory, or for applications that need these arrays to be persistent.

Example

#include <google/sparsetable>

using google::sparsetable;      // namespace where class lives by default

sparsetable<int> t(100);
t[5] = 6;
cout << "t[5] = " << t[5];
cout << "Default value = " << t[99];

Definition

Defined in the header sparsetable. This class is not part of the C++ standard.

Template parameters

ParameterDescriptionDefault
T The sparsetable's value type: the type of object that is stored in the table.  
GROUP_SIZE The number of elements in each sparsetable group (see the implementation doc for more details on this value). This almost never need be specified; the default template parameter value works well in all situations.  

Model of

Random Access Container

Type requirements

None, except for those imposed by the requirements of Random Access Container

Public base classes

None.

Members

MemberWhere definedDescription
value_type Container The type of object, T, stored in the table.
pointer Container Pointer to T.
reference Container Reference to T.
const_reference Container Const reference to T.
size_type Container An unsigned integral type.
difference_type Container A signed integral type.
iterator Container Iterator used to iterate through a sparsetable.
const_iterator Container Const iterator used to iterate through a sparsetable.
reverse_iterator Reversible Container Iterator used to iterate backwards through a sparsetable.
const_reverse_iterator Reversible Container Const iterator used to iterate backwards through a sparsetable.
nonempty_iterator sparsetable Iterator used to iterate through the assigned elements of the sparsetable.
const_nonempty_iterator sparsetable Const iterator used to iterate through the assigned elements of the sparsetable.
reverse_nonempty_iterator sparsetable Iterator used to iterate backwards through the assigned elements of the sparsetable.
const_reverse_nonempty_iterator sparsetable Const iterator used to iterate backwards through the assigned elements of the sparsetable.
destructive_iterator sparsetable Iterator used to iterate through the assigned elements of the sparsetable, erasing elements as it iterates. [1]
iterator begin() Container Returns an iterator pointing to the beginning of the sparsetable.
iterator end() Container Returns an iterator pointing to the end of the sparsetable.
const_iterator begin() const Container Returns an const_iterator pointing to the beginning of the sparsetable.
const_iterator end() const Container Returns an const_iterator pointing to the end of the sparsetable.
reverse_iterator rbegin() Reversible Container Returns a reverse_iterator pointing to the beginning of the reversed sparsetable.
reverse_iterator rend() Reversible Container Returns a reverse_iterator pointing to the end of the reversed sparsetable.
const_reverse_iterator rbegin() const Reversible Container Returns a const_reverse_iterator pointing to the beginning of the reversed sparsetable.
const_reverse_iterator rend() const Reversible Container Returns a const_reverse_iterator pointing to the end of the reversed sparsetable.
nonempty_iterator nonempty_begin() sparsetable Returns a nonempty_iterator pointing to the first assigned element of the sparsetable.
nonempty_iterator nonempty_end() sparsetable Returns a nonempty_iterator pointing to the end of the sparsetable.
const_nonempty_iterator nonempty_begin() const sparsetable Returns a const_nonempty_iterator pointing to the first assigned element of the sparsetable.
const_nonempty_iterator nonempty_end() const sparsetable Returns a const_nonempty_iterator pointing to the end of the sparsetable.
reverse_nonempty_iterator nonempty_rbegin() sparsetable Returns a reverse_nonempty_iterator pointing to the first assigned element of the reversed sparsetable.
reverse_nonempty_iterator nonempty_rend() sparsetable Returns a reverse_nonempty_iterator pointing to the end of the reversed sparsetable.
const_reverse_nonempty_iterator nonempty_rbegin() const sparsetable Returns a const_reverse_nonempty_iterator pointing to the first assigned element of the reversed sparsetable.
const_reverse_nonempty_iterator nonempty_rend() const sparsetable Returns a const_reverse_nonempty_iterator pointing to the end of the reversed sparsetable.
destructive_iterator destructive_begin() sparsetable Returns a destructive_iterator pointing to the first assigned element of the sparsetable.
destructive_iterator destructive_end() sparsetable Returns a destructive_iterator pointing to the end of the sparsetable.
size_type size() const Container Returns the size of the sparsetable.
size_type max_size() const Container Returns the largest possible size of the sparsetable.
bool empty() const Container true if the sparsetable's size is 0.
size_type num_nonempty() const sparsetable Returns the number of sparsetable elements that are currently assigned.
sparsetable(size_type n) Container Creates a sparsetable with n elements.
sparsetable(const sparsetable&) Container The copy constructor.
~sparsetable() Container The destructor.
sparsetable& operator=(const sparsetable&) Container The assignment operator
void swap(sparsetable&) Container Swaps the contents of two sparsetables.
reference operator[](size_type n) Random Access Container Returns the n'th element. [2]
const_reference operator[](size_type n) const Random Access Container Returns the n'th element.
bool test(size_type i) const sparsetable true if the i'th element of the sparsetable is assigned.
bool test(iterator pos) const sparsetable true if the sparsetable element pointed to by pos is assigned.
bool test(const_iterator pos) const sparsetable true if the sparsetable element pointed to by pos is assigned.
const_reference get(size_type i) const sparsetable returns the i'th element of the sparsetable.
reference set(size_type i, const_reference val) sparsetable Sets the i'th element of the sparsetable to value val.
void erase(size_type i) sparsetable Erases the i'th element of the sparsetable.
void erase(iterator pos) sparsetable Erases the element of the sparsetable pointed to by pos.
void erase(iterator first, iterator last) sparsetable Erases the elements of the sparsetable in the range [first, last).
void clear() sparsetable Erases all of the elements.
void resize(size_type n) sparsetable Changes the size of sparsetable to n.
bool write_metadata(FILE *fp) sparsetable See below.
bool read_metadata(FILE *fp) sparsetable See below.
bool write_nopointer_data(FILE *fp) sparsetable See below.
bool read_nopointer_data(FILE *fp) sparsetable See below.
bool operator==(const sparsetable&, const sparsetable&)
Forward Container Tests two sparsetables for equality. This is a global function, not a member function.
bool operator<(const sparsetable&, const sparsetable&)
Forward Container Lexicographical comparison. This is a global function, not a member function.

New members

These members are not defined in the Random Access Container requirement, but are specific to sparsetable.
MemberDescription
nonempty_iterator Iterator used to iterate through the assigned elements of the sparsetable.
const_nonempty_iterator Const iterator used to iterate through the assigned elements of the sparsetable.
reverse_nonempty_iterator Iterator used to iterate backwards through the assigned elements of the sparsetable.
const_reverse_nonempty_iterator Const iterator used to iterate backwards through the assigned elements of the sparsetable.
destructive_iterator Iterator used to iterate through the assigned elements of the sparsetable, erasing elements as it iterates. [1]
nonempty_iterator nonempty_begin() Returns a nonempty_iterator pointing to the first assigned element of the sparsetable.
nonempty_iterator nonempty_end() Returns a nonempty_iterator pointing to the end of the sparsetable.
const_nonempty_iterator nonempty_begin() const Returns a const_nonempty_iterator pointing to the first assigned element of the sparsetable.
const_nonempty_iterator nonempty_end() const Returns a const_nonempty_iterator pointing to the end of the sparsetable.
reverse_nonempty_iterator nonempty_rbegin() Returns a reverse_nonempty_iterator pointing to the first assigned element of the reversed sparsetable.
reverse_nonempty_iterator nonempty_rend() Returns a reverse_nonempty_iterator pointing to the end of the reversed sparsetable.
const_reverse_nonempty_iterator nonempty_rbegin() const Returns a const_reverse_nonempty_iterator pointing to the first assigned element of the reversed sparsetable.
const_reverse_nonempty_iterator nonempty_rend() const Returns a const_reverse_nonempty_iterator pointing to the end of the reversed sparsetable.
destructive_iterator destructive_begin() Returns a destructive_iterator pointing to the first assigned element of the sparsetable.
destructive_iterator destructive_end() Returns a destructive_iterator pointing to the end of the sparsetable.
size_type num_nonempty() const Returns the number of sparsetable elements that are currently assigned.
bool test(size_type i) const true if the i'th element of the sparsetable is assigned.
bool test(iterator pos) const true if the sparsetable element pointed to by pos is assigned.
bool test(const_iterator pos) const true if the sparsetable element pointed to by pos is assigned.
const_reference get(size_type i) const returns the i'th element of the sparsetable. If the i'th element is assigned, the assigned value is returned, otherwise, the default value T() is returned.
reference set(size_type i, const_reference val) Sets the i'th element of the sparsetable to value val, and returns a reference to the i'th element of the table. This operation causes the i'th element to be assigned.
void erase(size_type i) Erases the i'th element of the sparsetable. This operation causes the i'th element to be unassigned.
void erase(iterator pos) Erases the element of the sparsetable pointed to by pos. This operation causes the i'th element to be unassigned.
void erase(iterator first, iterator last) Erases the elements of the sparsetable in the range [first, last). This operation causes these elements to be unassigned.
void clear() Erases all of the elements. This causes all elements to be unassigned.
void resize(size_type n) Changes the size of sparsetable to n. If n is greater than the old size, new, unassigned elements are appended. If n is less than the old size, all elements in position >n are deleted.
bool write_metadata(FILE *fp) Write hashtable metadata to fp. See below.
bool read_metadata(FILE *fp) Read hashtable metadata from fp. See below.
bool write_nopointer_data(FILE *fp) Write hashtable contents to fp. This is valid only if the hashtable key and value are "plain" data. See below.
bool read_nopointer_data(FILE *fp) Read hashtable contents to fp. This is valid only if the hashtable key and value are "plain" data. See below.

Notes

[1] sparsetable::destructive_iterator iterates through a sparsetable like a normal iterator, but ++it may delete the element being iterated past. Obviously, this iterator can only be used once on a given table! One application of this iterator is to copy data from a sparsetable to some other data structure without using extra memory to store the data in both places during the copy.

[2] Since operator[] might insert a new element into the sparsetable, it can't possibly be a const member function. In theory, since it might insert a new element, it should cause the element it refers to to become assigned. However, this is undesirable when operator[] is used to examine elements, rather than assign them. Thus, as an implementation trick, operator[] does not really return a reference. Instead it returns an object that behaves almost exactly like a reference. This object, however, delays setting the appropriate sparsetable element to assigned to when it is actually assigned to.

For a bit more detail: the object returned by operator[] is an opaque type which defines operator=, operator reference(), and operator&. The first operator controls assigning to the value. The second controls examining the value. The third controls pointing to the value.

All three operators perform exactly as an object of type reference would perform. The only problems that arise is when this object is accessed in situations where C++ cannot do the conversion by default. By far the most common situation is with variadic functions such as printf. In such situations, you may need to manually cast the object to the right type:

   printf("%d", static_cast<typename table::reference>(table[i]));

Input/Output

It is possible to save and restore sparsetable objects to disk. Storage takes place in two steps. The first writes the table metadata. The second writes the actual data.

To write a sparsetable to disk, first call write_metadata() on an open file pointer. This saves the sparsetable information in a byte-order-independent format.

After the metadata has been written to disk, you must write the actual data stored in the sparsetable to disk. If the value is "simple" enough, you can do this by calling write_nopointer_data(). "Simple" data is data that can be safely copied to disk via fwrite(). Native C data types fall into this category, as do structs of native C data types. Pointers and STL objects do not.

Note that write_nopointer_data() does not do any endian conversion. Thus, it is only appropriate when you intend to read the data on the same endian architecture as you write the data.

If you cannot use write_nopointer_data() for any reason, you can write the data yourself by iterating over the sparsetable with a const_nonempty_iterator and writing the key and data in any manner you wish.

To read the hashtable information from disk, first you must create a sparsetable object. Then open a file pointer to point to the saved sparsetable, and call read_metadata(). If you saved the data via write_nopointer_data(), you can follow the read_metadata() call with a call to read_nopointer_data(). This is all that is needed.

If you saved the data through a custom write routine, you must call a custom read routine to read in the data. To do this, iterate over the sparsetable with a nonempty_iterator; this operation is sensical because the metadata has already been set up. For each iterator item, you can read the key and value from disk, and set it appropriately. The code might look like this:

   for (sparsetable<int*>::nonempty_iterator it = t.nonempty_begin();
        it != t.nonempty_end(); ++it) {
       *it = new int;
       fread(*it, sizeof(int), 1, fp);
   }

Here's another example, where the item stored in the sparsetable is a C++ object with a non-trivial constructor. In this case, you must use "placement new" to construct the object at the correct memory location.

   for (sparsetable<ComplicatedCppClass>::nonempty_iterator it = t.nonempty_begin();
        it != t.nonempty_end(); ++it) {
       int constructor_arg;   // ComplicatedCppClass takes an int to construct
       fread(&constructor_arg, sizeof(int), 1, fp);
       new (&(*it)) ComplicatedCppClass(constructor_arg);     // placement new
   }

See also

The following are SGI STL concepts and classes related to sparsetable.

Container, Random Access Container, sparse_hash_set, sparse_hash_map