[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 <sparsehash/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

Parameter | Description | Default
--- | --- | ---
`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

Member | Where defined | Description
--- | --- | ---
`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`.

Member | Description
--- | ---
`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`