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cppdraft/streambuf/reqts.md

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+[streambuf.reqts]
+
+# 31 Input/output library [[input.output]](./#input.output)
+
+## 31.6 Stream buffers [[stream.buffers]](stream.buffers#streambuf.reqts)
+
+### 31.6.2 Stream buffer requirements [streambuf.reqts]
+
+[1](#1)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L2891)
+
+Stream buffers can impose various constraints on the sequences they control[.](#1.sentence-1)
+
+Some constraints are:
+
+- [(1.1)](#1.1)
+
+  The controlled input sequence can be not readable[.](#1.1.sentence-1)
+
+- [(1.2)](#1.2)
+
+  The controlled output sequence can be not writable[.](#1.2.sentence-1)
+
+- [(1.3)](#1.3)
+
+  The controlled sequences can be associated with the contents of other
+representations for character sequences, such as external files[.](#1.3.sentence-1)
+
+- [(1.4)](#1.4)
+
+  The controlled sequences can support operations*directly* to or from associated sequences[.](#1.4.sentence-1)
+
+- [(1.5)](#1.5)
+
+  The controlled sequences can impose limitations on how the program can
+read characters from a sequence,
+write characters to a sequence,
+put characters back into an input sequence, or
+alter the stream position[.](#1.5.sentence-1)
+
+[2](#2)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L2914)
+
+Each sequence is characterized by three pointers which, if non-null,
+all point into the samecharT array object[.](#2.sentence-1)
+
+The array object represents, at any moment,
+a (sub)sequence of characters from the sequence[.](#2.sentence-2)
+
+Operations performed on a sequence alter the values stored in these pointers,
+perform reads and writes directly to or from associated sequences, and
+alter “the stream position” and conversion state as needed to
+maintain this subsequence relationship[.](#2.sentence-3)
+
+The three pointers are:
+
+- [(2.1)](#2.1)
+
+the[*beginning pointer*](#def:beginning_pointer),
+or lowest element address in the array
+(called xbeg here);
+
+- [(2.2)](#2.2)
+
+the[*next pointer*](#def:next_pointer),
+or next element address that is a current
+candidate for reading or writing (called xnext here);
+
+- [(2.3)](#2.3)
+
+the[*end pointer*](#def:end_pointer),
+or first element address beyond the end of
+the array (called xend here)[.](#2.sentence-4)
+
+[3](#3)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L2944)
+
+The following semantic constraints shall always apply for any set of
+three pointers for a sequence, using the pointer names given immediately above:
+
+- [(3.1)](#3.1)
+
+  If xnext is not a null pointer, thenxbeg and xend shall also be non-null pointers
+into the samecharT array, as described above; otherwise, xbeg andxend shall also be null[.](#3.1.sentence-1)
+
+- [(3.2)](#3.2)
+
+  If xnext is not a null pointer andxnext < xend for an output sequence, then a[*write position*](#def:write_position) is available[.](#3.2.sentence-1)
+  In this case,*xnext shall be assignable as the
+next element to write
+(to put, or to store a character value, into the sequence)[.](#3.2.sentence-2)
+
+- [(3.3)](#3.3)
+
+  If xnext is not a null pointer andxbeg < xnext for an input sequence,
+then a[*putback position*](#def:putback_position) is available[.](#3.3.sentence-1)
+  In this case,xnext[-1] shall have a defined value and is the next (preceding) element
+to store a character that is put back into the input sequence[.](#3.3.sentence-2)
+
+- [(3.4)](#3.4)
+
+  If xnext is not a null pointer andxnext < xend for an input sequence,
+then a[*read position*](#def:read_position) is available[.](#3.4.sentence-1)
+  In this case,*xnext shall have a defined value
+and is the next element to read
+(to get, or to obtain a character value, from the sequence)[.](#3.4.sentence-2)