30 Input/output library [input.output]

30.8 String-based streams [string.streams]

30.8.2 Class template basic_­stringbuf [stringbuf]

30.8.2.4 Overridden virtual functions [stringbuf.virtuals]

int_type underflow() override;

Returns: If the input sequence has a read position available, returns traits​::​to_­int_­type(*gptr()). Otherwise, returns traits​::​eof(). Any character in the underlying buffer which has been initialized is considered to be part of the input sequence.

int_type pbackfail(int_type c = traits::eof()) override;

Effects: Puts back the character designated by c to the input sequence, if possible, in one of three ways:

  • If traits​::​eq_­int_­type(c, traits​::​eof()) returns false and if the input sequence has a putback position available, and if traits​::​eq(to_­char_­type(c), gptr()[-1]) returns true, assigns gptr() - 1 to gptr().

    Returns: c.

  • If traits​::​eq_­int_­type(c, traits​::​eof()) returns false and if the input sequence has a putback position available, and if mode & ios_­base​::​out is nonzero, assigns c to *--gptr().

    Returns: c.

  • If traits​::​eq_­int_­type(c, traits​::​eof()) returns true and if the input sequence has a putback position available, assigns gptr() - 1 to gptr().

    Returns: traits​::​not_­eof(c).

Returns: traits​::​eof() to indicate failure.

Remarks: If the function can succeed in more than one of these ways, it is unspecified which way is chosen.

int_type overflow(int_type c = traits::eof()) override;

Effects: Appends the character designated by c to the output sequence, if possible, in one of two ways:

  • If traits​::​eq_­int_­type(c, traits​::​eof()) returns false and if either the output sequence has a write position available or the function makes a write position available (as described below), the function calls sputc(c).

    Signals success by returning c.

  • If traits​::​eq_­int_­type(c, traits​::​eof()) returns true, there is no character to append.

    Signals success by returning a value other than traits​::​eof().

Remarks: The function can alter the number of write positions available as a result of any call.

Returns: traits​::​eof() to indicate failure.

The function can make a write position available only if (mode & ios_­base​::​out) != 0. To make a write position available, the function reallocates (or initially allocates) an array object with a sufficient number of elements to hold the current array object (if any), plus at least one additional write position. If (mode & ios_­base​::​in) != 0, the function alters the read end pointer egptr() to point just past the new write position.

pos_type seekoff(off_type off, ios_base::seekdir way, ios_base::openmode which = ios_base::in | ios_base::out) override;

Effects: Alters the stream position within one of the controlled sequences, if possible, as indicated in Table 115.

Table 115seekoff positioning
ConditionsResult
(which & ios_­base​::​in) == ios_­base​::​in positions the input sequence
(which & ios_­base​::​out) == ios_­base​::​out positions the output sequence
(which & (ios_­base​::​in |
ios_­base​::​out)) ==
(ios_­base​::​in) |
ios_­base​::​out))
and way == either
ios_­base​::​beg or
ios_­base​::​end
positions both the input and the output sequences
Otherwise the positioning operation fails.

For a sequence to be positioned, if its next pointer (either gptr() or pptr()) is a null pointer and the new offset newoff is nonzero, the positioning operation fails. Otherwise, the function determines newoff as indicated in Table 116.

Table 116newoff values
Conditionnewoff Value
way == ios_­base​::​beg 0
way == ios_­base​::​cur the next pointer minus the beginning pointer (xnext - xbeg).
way == ios_­base​::​end the high mark pointer minus the beginning pointer (high_­mark - xbeg).

If (newoff + off) < 0, or if newoff + off refers to an uninitialized character ([stringbuf.members]), the positioning operation fails. Otherwise, the function assigns xbeg + newoff + off to the next pointer xnext.

Returns: pos_­type(newoff), constructed from the resultant offset newoff (of type off_­type), that stores the resultant stream position, if possible. If the positioning operation fails, or if the constructed object cannot represent the resultant stream position, the return value is pos_­type(off_­type(-1)).

pos_type seekpos(pos_type sp, ios_base::openmode which = ios_base::in | ios_base::out) override;

Effects: Equivalent to seekoff(off_­type(sp), ios_­base​::​beg, which).

Returns: sp to indicate success, or pos_­type(off_­type(-1)) to indicate failure.

basic_streambuf<charT, traits>* setbuf(charT* s, streamsize n);

Effects: implementation-defined, except that setbuf(0, 0) has no effect.

Returns: this.