# Data-Structure Genericity

This section describes genericity over different underlying data-structures. It is organized as follows.

1. The Basic Problem
2. Container Hierarchy
3. Data-Structure Tags and Traits
4. Find-Type and Range-Type Methods and Iterators

## The Basic Problem

The design attempts to address the following problem. When writing a function manipulating a generic container object, what is the behaviour of the object? E.g., suppose one writes

    template<
        class Cntnr>
    void some_op_sequence
        (Cntnr &r_cntnr)
    {
        ...
    }

then one needs to address the following questions in the body of `some_op_sequence`:

1. Which types and methods does `Cntnr` support? Containers based on hash tables can be queries for the hash-functor type and object; this is meaningless for tree-based containers. Containers based on trees can be split, joined, or can erase iterators and return the following iterator; this cannot be done by hash-based containers.
2. What are the guarantees of `Cntnr`? A container based on a probing hash-table invalidates all iterators when it is modified; this is not the case for containers based on node-based trees. Containers based on a node-based tree can be split or joined without exceptions; this is not the case for containers based on vector-based trees.
3. How does the container maintain its elements? containers based on splay trees or lists with update policies "cache" "frequently accessed" elements; containers based on most other underlying data-structures do not.

## Container Hierarchy

Figure Class hierarchy shows the container hierarchy.

1. `basic_assoc_cntnr` contains types and methods shared by all associative containers, e.g., the type `allocator` and the method `find`.
2. `basic_hash_assoc_cntnr` subclasses `basic_assoc_cntnr`, and contains types and methods shared by all hash-based containers, e.g., the type `hash_fn`.

1. `cc_hash_assoc_cntnr` and `gp_hash_assoc_cntnr` each subclass `basic_hash_assoc_cntnr`, and encapsulate collision-chaining and (general) probing hash tables, respectively. These two types of hash tables have somewhat different policies and methods (i.e., constructors and policy-access methods).

3. `tree_assoc_cntnr` subclasses one of `basic_tree_assoc_cntnr` which subclasses `basic_assoc_cntnr`. `tree_assoc_cntnr` encapsulates a tree-based container, and is parameterized by which underlying data-structure to use (e.g., a red-black tree); `basic_assoc_cntnr`. is specialized to the capabilities of the underlying structure. `tree_assoc_cntnr` contains some additional methods over `basic_assoc_cntnr`, e.g., split and join methods.
4. `lu_assoc_cntnr` subclasses `basic_assoc_cntnr`, and encapsulates a list with update policies.

The hierarchy is composed naturally, such that each container inherits all types and methods from its base. Data-Structure Tags and Traits discusses how to query which types and methods each container supports.

## Data-Structure Tags and Traits

`pb_assoc` contains a tag and traits mechanism similar to that of the STL's iterators.

`pb_assoc` contains a tag hierarchy corresponding to the hierarchy in Figure Class hierarchy. The tag hierarchy is shown in Figure Data-structure tag class hierarchy.

######

###### Data-structure tag class hierarchy.

`basic_assoc_cntnr` publicly defines `ds_category` as one of the classes in Figure . Given any container `Cntnr`, the tag of the underlying data-structure can be found via `typename Cntnr::ds_category`.

Additionally, a traits mechanism can be used to query a container type for its attributes. Given any container `Cntnr`, then `ds_traits<Cntnr>` is a traits class identifying the properties of the container.

To find if a container can throw when a key is erased (which is true for vector-based trees, for example), one can use

`ds_traits` `<Cntnr>::erase_can_throw`, for example.

Some of the definitions in `ds_traits` are dependent on other definitions. E.g., if `ds_traits` `<Cntnr>::split_join` is `true` (which is the case for containers based on trees), then `ds_traits` `<Cntnr>::split_join_can_throw` indicates whether splits or joins can throw exceptions (which is true for vector-based trees); otherwise `ds_traits` `<Cntnr>::split_join_can_throw` will yield a compilation error. (This is somewhat similar to a compile-time version of the COM model [mscom]).

## Find-Type and Range-Type Methods and Iterators

`pb_assoc` differentiates between two types of methods: find-type methods, and range-type methods. For example, `find` is a find-type method, since a container object searches for an element with a given key; `insert` is a find-type method, since, by STL convention, a container object returns an iterator corresponding to an element with a given key; `begin` and `end` are range-type methods, since they are not used to find a specific element, but rather to go over all elements in a container object.

Correspondingly, containers in `pb_assoc` define two families of iterators. `const_find_iterator` and `find_iterator` are the iterator types returned by find-type methods; `const_iterator` and `iterator` are the iterator types returned by range-type methods.

The relationship between these iterator types varies between container types. In a tree-based container, for example, `const_find_iterator` and `const_iterator` are synonymous, and `find_iterator` and `iterator` are synonymous; in a hash-based container, for example, this is not the case. Futhermore, find-type iterators in a hash-based container lack movement operators, such as `operator++`. All containers, however, maintain the invariants shown in Figure .

This distinction between find-type and range-type iterators and methods, while complicating the interface, has several advantages:

### Iterators in unordered container types

Given an unordered container type, e.g., a hash-based container, it makes no sense to move an iterator returned by a find-type method. Let `cntnr` be an associative-container object, and consider:

    std::for_each(m.find(1), m.find(5), foo);

which applies `foo` to all elements in `m` between `1` and `5`.

If `cntnr` is a tree-based container object, then an in-order walk will apply `foo` to the relevant elements, e.g., as in Figure Range iteration in different data-structures -A. If `m` is a hash-based container, then the order of elements between any two elements is undefined (and probably time-varying); there is no guarantee that the elements traversed will coincide with the logical elements between 1 and 5, e.g., as in Figure Range iteration in different data-structures-B.

The application of a range function e.g., `for_each`, to a pair of hash-based container's iterators is possibly sensical only if the iterators are those returned by `begin` and `end`, respectively. Therefore, the iterator returned by `m`'s `find` method should be immovable.

Another point also indicates that hash-based containers' find-type iterators and range-type iterators should be distinct. Consider Figure Find-type iterators in hash tables-A. An (immovable) find-type iterator, designed only to access an element, requires at most a single pointer to the element's link. Conversely, an iterator designed for range operations requires some more information e.g., the bucket number), since a cross-list traversal might be necessary. Alternatively, the lists might be linked, forming a monolithic total-element list, as in Figure Find-type iterators in hash tables-B (this seems similar to the Dinkumware design [dinkumware_stl]). This, however, complicates the hash-table's operations.

######

###### Range iteration in different data-structures.

######

###### Find-type iterators in hash tables.

As a consequence of this design,

    std::for_each(m.find(1), m.find(5), foo);

will compile for tree-based containers, but will not compile for hash-tables or other types. The returned type of `find` is a find-type iterator. For tree-based containers, this is synonymous with a range-type iterator, and therefore supports `operator++`; for other types of containers, a find-type iterator lacks `operator++`.

### Invalidation Guarantees

Consider the following snippet:

    it = c.find(3);

    c.erase(5);

Following the call to `erase`, what is the validity of `it`: can it be dereferenced? can it be incremented?

The answer depends on the underlying data-structure of the container. Figure Effect of erase in different underlying data-structures shows three cases: A1 and A2 show a red-black tree; B1 and B2 show an ordered-vector tree; C1 and C2 show a collision-chaining hash table.

######

###### Effect of erase in different underlying data-structures.

1. `Erasing 5 from A1 yields A2. Clearly, an iterator to 3 can be dereferenced and incremented.
2. Erasing 5 from B1 yields B2. Clearly, an iterator to 3 is not valid at all.
3. Erasing 5 from C1 yields C2. Here the situation is more complicated. On the one hand, incrementing `it` can be undefined. On the other hand, there is no problem in dereferencing `it`. In classic STL, it is not possible to express whether `it` is valid or not.

Thus again, the iterator concept seems overloaded. Distinguishing between find and range types allows fine-grained invalidation guarantees. Invalidation guarantees class hierarchy shows tags corresponding to different types of invalidation guarantees.

######

###### Invalidation guarantees class hierarchy.

1. `basic_invalidation_guarantee` corresponds to a basic guarantee that a find-type iterator, a found pointer, or a found reference, remains valid as long as the container object is not modified.
2. `find_invalidation_guarantee` corresponds to a guarantee that a find-type iterator, a found pointer, or a found reference, remains valid even if the containter object is modified.
3. `range_invalidation_guarantee` corresponds to a guarantee that a range-type iterator remains valid even if the containter object is modified.

To find the invalidation guarantee of a container, one can use

    typename ds_traits<Cntnr>::invalidation_guarantee

which is one of the classes in Figure Invalidation guarantees class hierarchy.