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| Category: containers | Component type: concept |
There is no guarantee that the elements of a Container are stored in any definite order; the order might, in fact, be different upon each iteration through the Container. Nor is there a guarantee that more than one iterator into a Container may be active at any one time. (Specific types of Containers, such as Forward Container, do provide such guarantees.)
A Container "owns" its elements: the lifetime of an element stored in a container cannot exceed that of the Container itself. [1]
| Value type | X::value_type | The type of the object stored in a container. The value type must be Assignable, but need not be DefaultConstructible. [2] |
| Iterator type | X::iterator | The type of iterator used to iterate through a container's elements. The iterator's value type is expected to be the container's value type. A conversion from the iterator type to the const iterator type must exist. The iterator type must be an input iterator. [3] |
| Const iterator type | X::const_iterator | A type of iterator that may be used to examine, but not to modify, a container's elements. [3] [4] |
| Reference type | X::reference | A type that behaves as a reference to the container's value type. [5] |
| Const reference type | X::const_reference | A type that behaves as a const reference to the container's value type. [5] |
| Pointer type | X::pointer | A type that behaves as a pointer to the container's value type. [6] |
| Distance type | X::difference_type | A signed integral type used to represent the distance between two of the container's iterators. This type must be the same as the iterator's distance type. [2] |
| Size type | X::size_type | An unsigned integral type that can represent any nonnegative value of the container's distance type. [2] |
| X | A type that is a model of Container |
| a, b | Object of type X |
| T | The value type of X |
The area of a container is the total number of bytes that it occupies. More specifically, it is the sum of the elements' areas plus whatever overhead is associated with the container itself. If a container's value type T is a simple type (as opposed to a container type), then the container's area is bounded above by a constant times the container's size times sizeof(T). That is, if a is a container with a simple value type, then a's area is O(a.size()).
A variable sized container is one that provides methods for inserting and/or removing elements; its size may vary during a container's lifetime. A fixed size container is one where the size is constant throughout the container's lifetime. In some fixed-size container types, the size is determined at compile time.
| Name | Expression | Type requirements | Return type |
|---|---|---|---|
| Beginning of range | a.begin() | iterator if a is mutable, const_iterator otherwise [4] [7] | |
| End of range | a.end() | iterator if a is mutable, const_iterator otherwise [4] | |
| Size | a.size() | size_type | |
| Maximum size | a.max_size() | size_type | |
| Empty container | a.empty() | Convertible to bool | |
| Swap | a.swap(b) | void |
| Name | Expression | Precondition | Semantics | Postcondition |
|---|---|---|---|---|
| Copy constructor | X(a) | X().size() == a.size(). X() contains a copy of each of a's elements. | ||
| Copy constructor | X b(a); | b.size() == a.size(). b contains a copy of each of a's elements. | ||
| Assignment operator | b = a | b.size() == a.size(). b contains a copy of each of a's elements. | ||
| Destructor | a.~X() | Each of a's elements is destroyed, and memory allocated for them (if any) is deallocated. | ||
| Beginning of range | a.begin() | Returns an iterator pointing to the first element in the container. [7] | a.begin() is either dereferenceable or past-the-end. It is past-the-end if and only if a.size() == 0. | |
| End of range | a.end() | Returns an iterator pointing one past the last element in the container. | a.end() is past-the-end. | |
| Size | a.size() | Returns the size of the container, that is, its number of elements. [8] | a.size() >= 0 && a.size() <= max_size() | |
| Maximum size | a.max_size() | Returns the largest size that this container can ever have. [8] | a.max_size() >= 0 && a.max_size() >= a.size() | |
| Empty container | a.empty() | Equivalent to a.size() == 0. (But possibly faster.) | ||
| Swap | a.swap(b) | Equivalent to swap(a,b) [9] |
begin() and end() are amortized constant time.
size() is linear in the container's size. [10] max_size() and empty() are amortized constant time. If you are testing whether a container is empty, you should always write c.empty() instead of c.size() == 0. The two expressions are equivalent, but the former may be much faster.
swap() is amortized constant time. [9]
| Valid range | For any container a, [a.begin(), a.end()) is a valid range. [11] |
| Range size | a.size() is equal to the distance from a.begin() to a.end(). |
| Completeness | An algorithm that iterates through the range [a.begin(), a.end()) will pass through every element of a. [11] |
[1] The fact that the lifetime of elements cannot exceed that of of their container may seem like a severe restriction. In fact, though, it is not. Note that pointers and iterators are objects; like any other objects, they may be stored in a container. The container, in that case, "owns" the pointers themselves, but not the objects that they point to.
[2] This expression must be a typedef, that is, a synonym for a type that already has some other name.
[3] This may either be a typedef for some other type, or else a unique type that is defined as a nested class within the class X.
[4] A container's iterator type and const iterator type may be the same: there is no guarantee that every container must have an associated mutable iterator type. For example, set and hash_set define iterator and const_iterator to be the same type.
[5] It is required that the reference type has the same semantics as an ordinary C++ reference, but it need not actually be an ordinary C++ reference. Some implementations, for example, might provide additional reference types to support non-standard memory models. Note, however, that "smart references" (user-defined reference types that provide additional functionality) are not a viable option. It is impossible for a user-defined type to have the same semantics as C++ references, because the C++ language does not support redefining the member access operator (operator.).
[6] As in the case of references [5], the pointer type must have the same semantics as C++ pointers but need not actually be a C++ pointer. "Smart pointers," however, unlike "smart references", are possible. This is because it is possible for user-defined types to define the dereference operator and the pointer member access operator, operator* and operator->.
[7] The iterator type need only be an input iterator, which provides a very weak set of guarantees; in particular, all algorithms on input iterators must be "single pass". It follows that only a single iterator into a container may be active at any one time. This restriction is removed in Forward Container.
[8] In the case of a fixed-size container, size() == max_size().
[9] For any Assignable type, swap can be defined in terms of assignment. This requires three assignments, each of which, for a container type, is linear in the container's size. In a sense, then, a.swap(b) is redundant. It exists solely for the sake of efficiency: for many containers, such as vector and list, it is possible to implement swap such that its run-time complexity is constant rather than linear. If this is possible for some container type X, then the template specialization swap(X&, X&) can simply be written in terms of X::swap(X&). The implication of this is that X::swap(X&) should only be defined if there exists such a constant-time implementation. Not every container class X need have such a member function, but if the member function exists at all then it is guaranteed to be amortized constant time.
[10] For many containers, such as vector and deque, size is O(1). This satisfies the requirement that it be O(N).
[11] Although [a.begin(), a.end()) must be a valid range, and must include every element in the container, the order in which the elements appear in that range is unspecified. If you iterate through a container twice, it is not guaranteed that the order will be the same both times. This restriction is removed in Forward Container.
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