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A.18 Containers

{AI95-00302-03} This clause presents the specifications of the package Containers and several child packages, which provide facilities for storing collections of elements.
Glossary entry: A container is an object that contain other objects all of the same type, which could be class-wide. Several predefined container types are provided by the children of package Ada.Containers (see A.18.1).
{AI95-00302-03} A variety of sequence and associative containers are provided. Each container includes a cursor type. A cursor is a reference to an element within a container. Many operations on cursors are common to all of the containers. A cursor referencing an element in a container is considered to be overlapping with the container object itself.
Reason: The last sentence is intended to clarify that operations that just use a cursor are on the same footing as operations that use a container in terms of the reentrancy rules of Annex A. 
{AI95-00302-03} Within this clause we provide Implementation Advice for the desired average or worst case time complexity of certain operations on a container. This advice is expressed using the Landau symbol O(X). Presuming f is some function of a length parameter N and t(N) is the time the operation takes (on average or worst case, as specified) for the length N, a complexity of O(f(N)) means that there exists a finite A such that for any N, t(N)/f(N) < A.
Discussion: Of course, an implementation can do better than a specified O(f(N)): for example, O(1) meets the requirements for O(log N).
This concept seems to have as many names as there are authors. We used “Landau symbol” because that's what our reference does. But we'd also seen this referred as big-O notation (sometimes written as big-oh), and as Bachmann notation. Whatever the name, it always has the above definition. 
If the advice suggests that the complexity should be less than O(f(N)), then for any arbitrarily small positive real D, there should exist a positive integer M such that for all N > M, t(N)/f(N) < D.
{AI05-0001-1} {AI05-0044-1} When a formal function is used to provide an ordering for a container, it is generally required to define a strict weak ordering. A function "<" defines a strict weak ordering if it is irreflexive, asymmetric, transitive, and in addition, if x < y for any values x and y, then for all other values z, (x < z) or (z < y).

Language Design Principles

{AI95-00302-03} {AI05-0299-1} This subclause provides a number of useful containers for Ada. Only the most useful containers are provided. Ones that are relatively easy to code, redundant, or rarely used are omitted from this set, even if they are generally included in containers libraries.
The containers packages are modeled on the Standard Template Library (STL), an algorithms and data structure library popularized by Alexander Stepanov, and included in the C++ standard library. The structure and terminology differ from the STL where that better maps to common Ada usage. For instance, what the STL calls “iterators” are called “cursors” here.
The following major nonlimited containers are provided:
(Expandable) Vectors of any nonlimited type;
Doubly-linked Lists of any nonlimited type;
Hashed Maps keyed by any nonlimited hashable type, and containing any nonlimited type;
Ordered Maps keyed by any nonlimited ordered type, and containing any nonlimited type;
{AI05-0136-1} Hashed Sets of any nonlimited hashable type;
{AI05-0136-1} Ordered Sets of any nonlimited ordered type;
{AI05-0136-1} Multiway Trees of any nonlimited type;
{AI05-0069-1} Holders of any (indefinite) nonlimited type;
{AI05-0159-1} Synchronized queues of any definite nonlimited type; and
{AI05-0159-1} Priority queues of any definite nonlimited type. 
{AI05-0001-1} Separate versions for definite and indefinite element types are provided, as those for definite types can be implemented more efficiently. Similarly, a separate bounded version is provided in order to give more predictable memory usage.
Each container includes a cursor, which is a reference to an element within a container. Cursors generally remain valid as long as the container exists and the element referenced is not deleted. Many operations on cursors are common to all of the containers. This makes it possible to write generic algorithms that work on any kind of container.
The containers packages are structured so that additional packages can be added in the future. Indeed, we hope that these packages provide the basis for a more extensive secondary standard for containers.
If containers with similar functionality (but different performance characteristics) are provided (by the implementation or by a secondary standard), we suggest that a prefix be used to identify the class of the functionality: "Ada.Containers.Bounded_Sets" (for a set with a maximum number of elements); "Ada.Containers.Protected_Maps" (for a map which can be accessed by multiple tasks at one time); "Ada.Containers.Persistent_Vectors" (for a persistent vector which continues to exist between executions of a program) and so on.
Note that the language already includes several requirements that are important to the use of containers. These include:
Library packages must be reentrant – multiple tasks can use the packages as long as they operate on separate containers. Thus, it is only necessary for a user to protect a container if a single container needs to be used by multiple tasks.
Language-defined types must stream "properly". That means that the stream attributes can be used to implement persistence of containers when necessary, and containers can be passed between partitions of a program.
Equality of language-defined types must compose “properly”. This means that the version of "=" directly used by users is the same one that will be used in generics and in predefined equality operators of types with components of the containers and/or cursors. This prevents the abstraction from breaking unexpectedly.
{AI05-0048-1} Redispatching is not allowed (unless it is required). That means that overriding a container operation will not change the behavior of any other predefined container operation. This provides a stable base for extensions. 
If a container's element type is controlled, the point at which the element is finalized will depend on the implementation of the container. We do not specify precisely where this will happen (it will happen no later than the finalization of the container, of course) in order to give implementation's flexibility to cache, block, or split the nodes of the container. In particular, Delete does not necessarily finalize the element; the implementation may (or may not) hold the space for reuse.
This is not likely to be a hardship, as the element type has to be nonlimited. Types used to manage scarce resources generally need to be limited. Otherwise, the amount of resources needed is hard to control, as the language allows a lot of variation in the number or order of adjusts/finalizations. For common uses of nonlimited controlled types such as managing storage, the types already have to manage arbitrary copies.
The use of controlled types also brings up the possibility of failure of finalization (and thus deallocation) of an element. This is a “serious bug”, as AI95-179 puts it, so we don't try to specify what happens in that case. The implementation should propagate the exception. 
Implementation Note: It is expected that exceptions propagated from these operations do not damage containers. That is, if Storage_Error is propagated because of an allocation failure, or Constraint_Error is propagated by the assignment of elements, the container can continue to be used without further exceptions. The intent is that it should be possible to recover from errors without losing data. We don't try to state this formally in most cases, because it is hard to define precisely what is and is not allowed behavior. 
Implementation Note: When this clause says that the behavior of something is unspecified, we really mean that any result of executing Ada code short of erroneous execution is allowed. We do not mean that memory not belonging to the parameters of the operation can be trashed. When we mean to allow erroneous behavior, we specifically say that execution is erroneous. All this means if the containers are written in Ada is that checks should not be suppressed or removed assuming some behavior of other code, and that the implementation should take care to avoid creating internal dangling accesses by assuming behavior from generic formals that can't be guaranteed. We don't try to say this normatively because it would be fairly complex, and implementers are unlikely to increase their support costs by fielding implementations that are unstable if given buggy hash functions, et al. 

Static Semantics

{AI12-0035-1} Certain subprograms declared within instances of some of the generic packages presented in this clause are said to perform indefinite insertion. These subprograms are those corresponding (in the sense of the copying described in subclause 12.3) to subprograms that have formal parameters of a generic formal indefinite type and that are identified as performing indefinite insertion in the subclause defining the generic package.
{AI12-0035-1} If a subprogram performs indefinite insertion, then certain run-time checks are performed as part of a call to the subprogram; if any of these checks fail, then the resulting exception is propagated to the caller and the container is not modified by the call. These checks are performed for each parameter corresponding (in the sense of the copying described in 12.3) to a parameter in the corresponding generic whose type is a generic formal indefinite type. The checks performed for a given parameter are those checks explicitly specified in subclause 4.8 that would be performed as part of the evaluation of an initialized allocator whose access type is declared immediately within the instance, where:
the value of the qualified_expression is that of the parameter; and
the designated subtype of the access type is the subtype of the parameter; and
finalization of the collection of the access type has started if and only if the finalization of the instance has started. 
Discussion: The phrase "explicitly specified" means those checks for which subclause 4.8 includes the phrase "<some exception> is raised if ...". It does not refer, for example, to any checks performed as part of any subtype conversion. In particular, this wording includes the checks described in subclause 4.8 to be performed in the case of a class-wide designated type, and of a designated subtype that has access discriminant parts. These checks are needed to prevent containers from outliving their contained (Element_Type or Key_Type) values. 
Implementation Note: These rules have a dual purpose. Mainly, we are requiring checks needed to prevent dangling references. As a side effect, we are also allowing checks needed to permit an implementation of a container generic to make use of access types in a straightforward way. As an example of the second purpose, suppose that an implementation does declare such an access type and suppose further that the finalization of the collection of the access type has started. These rules allow Program_Error to be propagated in this case (as specified in 4.8); this is necessary to allow an all-Ada implementation of these packages.

Extensions to Ada 95

{AI95-00302-03} {AI05-0299-1} This subclause is new. It just provides an introduction to the following subclauses. 

Wording Changes from Ada 2005

{AI05-0044-1} Correction: Added a definition of strict weak ordering. 

Wording Changes from Ada 2012

{AI05-0035-1} Corrigendum: Added a definition of “performs indefinite insertion”. This is used in other subclauses and any resulting inconsistencies are documented there. 

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