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sequence_buffer<Container, buf_sz>

Categories: iterators, adaptors Component type: type


Sequence_buffer is similar to back_insert_iterator: it is an output iterator adaptor that appends elements to the end of a container.

The main difference between sequence_buffer and back_insert_iterator is that back_insert_iterator inserts elements into a sequence one element at a time; sequence_buffer, however, as the "buffer" part of the name suggests, accumulates elements into a buffer and appends the entire buffer in a single operation.

Specifically, the expression *it = v adds v to the end of it's internal buffer. The buffer is automatically flushed when it gets full, or when it is destroyed; flushing the buffer means emptying it and appending its contents to it's underlying container. (It is also possible to flush the buffer manually, by invoking the flush() member function.)

This difference has two implications. First, sequence_buffer is only useful if appending an array of N elements is much more efficient than inserting a single element N times. Second, sequence_buffer assumes that it can insert elements at the end of a container using an append member function. This member function is not part of the Container or Sequence requirements. The sequence_buffer adaptor can be used with rope, but not with any of the other containers in the library. (This is the reason why sequence_buffer is defined in the file rope.h, instead of in iterator.h.)

If you want to build up a string one character at a time, it is much more efficient to use sequence_buffer than to repeatedly add single characters to the end of a rope.


int main()
  const char* const s = "this is a test";
  const int N = strlen(s);

  crope r;
  transform(s, s + N,
  cout << "r = " << r << endl;


Defined in the header rope, and in the backward-compatibility header rope.h. The sequence_buffer class, and the rope header, are SGI extensions; they are not part of the C++ standard.

Template parameters

Parameter Description Default
Container The type of the underlying container that elements are being written to. [1]  
buf_sz Number of elements in the buffer. This is a number, not a type. buf_sz has type size_t. 100

Model of

Output Iterator.

Type requirements

Public base classes



Member Where defined Description
value_type sequence_buffer The underlying container's value type.
sequence_buffer(Container& C) sequence_buffer Create a sequence_buffer whose underlying container is C.
sequence_buffer() Default Constructible The default constructor. The resulting iterator is singular.
sequence_buffer(const sequence_buffer&) Assignable Copy constructor.
sequence_buffer& operator=(const sequence_buffer& s) Assignable Assignment operator.
sequence_buffer& operator=(sequence_buffer& s) Assignable Faster version of assignment operator.
sequence_buffer& operator=(const value_type&) Output Iterator Used to implement the Output Iterator requirement *i = t. [2]
sequence_buffer& operator*() Output Iterator Used to implement the Output Iterator requirement *i = t. [2]
sequence_buffer& operator++() Output Iterator Preincrement
sequence_buffer& operator++(int) Output Iterator Postincrement
void flush() sequence_buffer Flush the buffer.
void push_back(value_type) sequence_buffer i.push_back(x) is equivalent to *i = x.
void append(value_type* s, size_t len) sequence_buffer Append multiple values.

New members

These members are not defined in the Output Iterator requirements, but are specific to sequence_buffer.
Function Description
value_type The underlying container's value type. That is, typename Container::value_type.
sequence_buffer(Container& C) Create a sequence_buffer whose underlying container is C. Elements appended to the sequence_buffer will be appended to C whenever the sequence_buffer is flushed.
void flush() Append all elements in the buffer to the underlying container, and empty the buffer. That is, make the underlying container consistent with the sequence_buffer. Note that flush is called automatically whenever the buffer is full, and also by sequence_buffer's destructor. Sometimes, however, it is useful to be sure that the buffer is flushed at a particular time.
void push_back(value_type x) Append x to the sequence_buffer. Note that this member function is strictly unnecessary: i.push_back(x) is just alternate syntax for *i = x.
void append(value_type* s, size_t len) Append the range [s, s + len) to the sequence_buffer. Note that i.append(s, n) is just the same as copy(s, s + n, i). The append member function, however, is faster.


[1] Despite the name "sequence_buffer", this adaptor cannot actually be used with arbitrary sequences: it requires that the template argument Container have an append member function that can insert multiple elements at the end of the container. This member function is not part of the Sequence requirements. This means that sequence_buffer can be used with rope, but not with any of the other predefined container classes.

[2] Note how assignment through a sequence_buffer is implemented. In general, unary operator* must be defined so that it returns a proxy object, where the proxy object defines operator= to perform the output operation. In this case, for the sake of simplicity, the proxy object is the sequence_buffer itself. That is, *i simply returns i, and *i = t is equivalent to i = t. You should not, however, rely on this behavior. It is an implementation detail, and it is not guaranteed to remain the same in future versions.

See also

Output Iterator, rope, front_insert_iterator, back_insert_iterator, insert_iterator

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