Standard C++ IOStreams and Locales: Advanced Programmer's Guide and Reference

streambuf layering: give formatted output a different destination

Licensed excerpt streambuf: The Stream Buffer Classes, source marked pp. 84–109; The Stream Buffer Abstraction / Output / File Stream Buffers

This excerpt recasts streambuf, often treated as an internal detail, as an explicit extension interface: the stream decides how characters are represented, and the buffer layer decides how characters are delivered. The get-area and put-area pointers are a model for understanding the boundary; they do not imply that every file stream uses the same physical layout. The original exercise chooses a fixed-capacity character destination and implements only the sequential writes it actually promises; formatting remains the job of ostream, and insufficient capacity is reported through the standard stream-state mechanism. This lets you observe already-written content and whether the operation succeeded separately, and it also shows that “failure” does not automatically roll back. The unread locale chapters cannot be replaced by this material, and it especially cannot support a claim of having mastered the full design of internationalized parsing and custom facets.