STL internals: deque implementation

I am using a std::deque for stori开发者_运维技巧ng a large collection of items.

I know that deques is implemented as a list of vectors. The size of those vectors cannot be set but I wander what is the algorithm for choosing that size.

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deque is implemented as a vector of vectors (a list of vectors would impede the constant time random access). The size of the secondary vectors is implementation dependent, a common algorithm is to use a constant size in bytes.

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My deque implementation, the one from GNU which is derived from the HP/SGI version, is not a list of vectors; at least, not an std::list of std::vectors. The comments state

*  In previous HP/SGI versions of deque, there was an extra template
*  parameter so users could control the node size.  This extension turned
*  out to violate the C++ standard (it can be detected using template
*  template parameters), and it was removed.
*
*  Here's how a deque<Tp> manages memory.  Each deque has 4 members:
*
*  - Tp**        _M_map
*  - size_t      _M_map_size
*  - iterator    _M_start, _M_finish
*
*  map_size is at least 8.  %map is an array of map_size
*  pointers-to-"nodes".  (The name %map has nothing to do with the
*  std::map class, and "nodes" should not be confused with
*  std::list's usage of "node".)