30 Input/output library [input.output]

30.8 String-based streams [string.streams]

30.8.1 Header <sstream> synopsis [sstream.syn]

namespace std {
  template <class charT, class traits = char_traits<charT>,
            class Allocator = allocator<charT>>
    class basic_stringbuf;

  using stringbuf  = basic_stringbuf<char>;
  using wstringbuf = basic_stringbuf<wchar_t>;

  template <class charT, class traits = char_traits<charT>,
            class Allocator = allocator<charT>>
    class basic_istringstream;

  using istringstream  = basic_istringstream<char>;
  using wistringstream = basic_istringstream<wchar_t>;

  template <class charT, class traits = char_traits<charT>,
            class Allocator = allocator<charT>>
    class basic_ostringstream;
  using ostringstream  = basic_ostringstream<char>;
  using wostringstream = basic_ostringstream<wchar_t>;

  template <class charT, class traits = char_traits<charT>,
            class Allocator = allocator<charT>>
    class basic_stringstream;
  using stringstream  = basic_stringstream<char>;
  using wstringstream = basic_stringstream<wchar_t>;
}
1
#
The header <sstream> defines four class templates and eight types that associate stream buffers with objects of class basic_­string, as described in [string.classes].

30.8.2 Class template basic_­stringbuf [stringbuf]

namespace std {
  template <class charT, class traits = char_traits<charT>,
            class Allocator = allocator<charT>>
  class basic_stringbuf : public basic_streambuf<charT, traits> {
  public:
    using char_type      = charT;
    using int_type       = typename traits::int_type;
    using pos_type       = typename traits::pos_type;
    using off_type       = typename traits::off_type;
    using traits_type    = traits;
    using allocator_type = Allocator;

    // [stringbuf.cons], constructors
    explicit basic_stringbuf(
      ios_base::openmode which = ios_base::in | ios_base::out);
    explicit basic_stringbuf(
      const basic_string<charT, traits, Allocator>& str,
      ios_base::openmode which = ios_base::in | ios_base::out);
    basic_stringbuf(const basic_stringbuf& rhs) = delete;
    basic_stringbuf(basic_stringbuf&& rhs);

    // [stringbuf.assign], assign and swap
    basic_stringbuf& operator=(const basic_stringbuf& rhs) = delete;
    basic_stringbuf& operator=(basic_stringbuf&& rhs);
    void swap(basic_stringbuf& rhs);

    // [stringbuf.members], get and set
    basic_string<charT, traits, Allocator> str() const;
    void str(const basic_string<charT, traits, Allocator>& s);

  protected:
    // [stringbuf.virtuals], overridden virtual functions
    int_type underflow() override;
    int_type pbackfail(int_type c = traits::eof()) override;
    int_type overflow (int_type c = traits::eof()) override;
    basic_streambuf<charT, traits>* setbuf(charT*, streamsize) override;

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

  private:
    ios_base::openmode mode;  // exposition only
  };

  template <class charT, class traits, class Allocator>
    void swap(basic_stringbuf<charT, traits, Allocator>& x,
              basic_stringbuf<charT, traits, Allocator>& y);
}
1
#
The class basic_­stringbuf is derived from basic_­streambuf to associate possibly the input sequence and possibly the output sequence with a sequence of arbitrary characters.
The sequence can be initialized from, or made available as, an object of class basic_­string.
2
#
For the sake of exposition, the maintained data is presented here as:
  • (2.1)
    ios_­base​::​openmode mode, has in set if the input sequence can be read, and out set if the output sequence can be written.

30.8.2.1 basic_­stringbuf constructors [stringbuf.cons]

🔗
explicit basic_stringbuf( ios_base::openmode which = ios_base::in | ios_base::out);
1
#
Effects: Constructs an object of class basic_­stringbuf, initializing the base class with basic_­streambuf() ([streambuf.cons]), and initializing mode with which.
2
#
Postconditions: str() == "".
🔗
explicit basic_stringbuf( const basic_string<charT, traits, Allocator>& s, ios_base::openmode which = ios_base::in | ios_base::out);
3
#
Effects: Constructs an object of class basic_­stringbuf, initializing the base class with basic_­streambuf() ([streambuf.cons]), and initializing mode with which.
Then calls str(s).
🔗
basic_stringbuf(basic_stringbuf&& rhs);
4
#
Effects: Move constructs from the rvalue rhs.
It is implementation-defined whether the sequence pointers in *this (eback(), gptr(), egptr(), pbase(), pptr(), epptr()) obtain the values which rhs had.
Whether they do or not, *this and rhs reference separate buffers (if any at all) after the construction.
The openmode, locale and any other state of rhs is also copied.
5
#
Postconditions: Let rhs_­p refer to the state of rhs just prior to this construction and let rhs_­a refer to the state of rhs just after this construction.
  • (5.1)
    str() == rhs_­p.str()
  • (5.2)
    gptr() - eback() == rhs_­p.gptr() - rhs_­p.eback()
  • (5.3)
    egptr() - eback() == rhs_­p.egptr() - rhs_­p.eback()
  • (5.4)
    pptr() - pbase() == rhs_­p.pptr() - rhs_­p.pbase()
  • (5.5)
    epptr() - pbase() == rhs_­p.epptr() - rhs_­p.pbase()
  • (5.6)
    if (eback()) eback() != rhs_­a.eback()
  • (5.7)
    if (gptr()) gptr() != rhs_­a.gptr()
  • (5.8)
    if (egptr()) egptr() != rhs_­a.egptr()
  • (5.9)
    if (pbase()) pbase() != rhs_­a.pbase()
  • (5.10)
    if (pptr()) pptr() != rhs_­a.pptr()
  • (5.11)
    if (epptr()) epptr() != rhs_­a.epptr()

30.8.2.2 Assign and swap [stringbuf.assign]

🔗
basic_stringbuf& operator=(basic_stringbuf&& rhs);
1
#
Effects: After the move assignment *this has the observable state it would have had if it had been move constructed from rhs (see [stringbuf.cons]).
2
#
Returns: *this.
🔗
void swap(basic_stringbuf& rhs);
3
#
Effects: Exchanges the state of *this and rhs.
🔗
template <class charT, class traits, class Allocator> void swap(basic_stringbuf<charT, traits, Allocator>& x, basic_stringbuf<charT, traits, Allocator>& y);
4
#
Effects: As if by x.swap(y).

30.8.2.3 Member functions [stringbuf.members]

🔗
basic_string<charT, traits, Allocator> str() const;
1
#
Returns: A basic_­string object whose content is equal to the basic_­stringbuf underlying character sequence.
If the basic_­stringbuf was created only in input mode, the resultant basic_­string contains the character sequence in the range [eback(), egptr()).
If the basic_­stringbuf was created with which & ios_­base​::​out being nonzero then the resultant basic_­string contains the character sequence in the range [pbase(), high_­mark), where high_­mark represents the position one past the highest initialized character in the buffer.
Characters can be initialized by writing to the stream, by constructing the basic_­stringbuf with a basic_­string, or by calling the str(basic_­string) member function.
In the case of calling the str(basic_­string) member function, all characters initialized prior to the call are now considered uninitialized (except for those characters re-initialized by the new basic_­string).
Otherwise the basic_­stringbuf has been created in neither input nor output mode and a zero length basic_­string is returned.
🔗
void str(const basic_string<charT, traits, Allocator>& s);
2
#
Effects: Copies the content of s into the basic_­stringbuf underlying character sequence and initializes the input and output sequences according to mode.
3
#
Postconditions: If mode & ios_­base​::​out is nonzero, pbase() points to the first underlying character and epptr() >= pbase() + s.size() holds; in addition, if mode & ios_­base​::​ate is nonzero, pptr() == pbase() + s.size() holds, otherwise pptr() == pbase() is true.
If mode & ios_­base​::​in is nonzero, eback() points to the first underlying character, and both gptr() == eback() and egptr() == eback() + s.size() hold.

30.8.2.4 Overridden virtual functions [stringbuf.virtuals]

🔗
int_type underflow() override;
1
#
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;
2
#
Effects: Puts back the character designated by c to the input sequence, if possible, in one of three ways:
  • (2.1)
    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.
  • (2.2)
    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.
  • (2.3)
    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).
3
#
Returns: traits​::​eof() to indicate failure.
4
#
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;
5
#
Effects: Appends the character designated by c to the output sequence, if possible, in one of two ways:
  • (5.1)
    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.
  • (5.2)
    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().
6
#
Remarks: The function can alter the number of write positions available as a result of any call.
7
#
Returns: traits​::​eof() to indicate failure.
8
#
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;
9
#
Effects: Alters the stream position within one of the controlled sequences, if possible, as indicated in Table 110.
Table 110 — seekoff positioning
Conditions
Result
(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.
10
#
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 111.
Table 111 — newoff values
Condition
newoff 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).
11
#
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.
12
#
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;
13
#
Effects: Equivalent to seekoff(off_­type(sp), ios_­base​::​beg, which).
14
#
Returns: sp to indicate success, or pos_­type(off_­type(-1)) to indicate failure.
🔗
basic_streambuf<charT, traits>* setbuf(charT* s, streamsize n);
15
#
Effects: implementation-defined, except that setbuf(0, 0) has no effect.
16
#
Returns: this.

30.8.3 Class template basic_­istringstream [istringstream]

namespace std {
  template <class charT, class traits = char_traits<charT>,
            class Allocator = allocator<charT>>
  class basic_istringstream : public basic_istream<charT, traits> {
  public:
    using char_type      = charT;
    using int_type       = typename traits::int_type;
    using pos_type       = typename traits::pos_type;
    using off_type       = typename traits::off_type;
    using traits_type    = traits;
    using allocator_type = Allocator;

    // [istringstream.cons], constructors
    explicit basic_istringstream(
      ios_base::openmode which = ios_base::in);
    explicit basic_istringstream(
      const basic_string<charT, traits, Allocator>& str,
      ios_base::openmode which = ios_base::in);
    basic_istringstream(const basic_istringstream& rhs) = delete;
    basic_istringstream(basic_istringstream&& rhs);

    // [istringstream.assign], assign and swap
    basic_istringstream& operator=(const basic_istringstream& rhs) = delete;
    basic_istringstream& operator=(basic_istringstream&& rhs);
    void swap(basic_istringstream& rhs);

    // [istringstream.members], members
    basic_stringbuf<charT, traits, Allocator>* rdbuf() const;

    basic_string<charT, traits, Allocator> str() const;
    void str(const basic_string<charT, traits, Allocator>& s);
  private:
    basic_stringbuf<charT, traits, Allocator> sb; // exposition only
  };

  template <class charT, class traits, class Allocator>
    void swap(basic_istringstream<charT, traits, Allocator>& x,
              basic_istringstream<charT, traits, Allocator>& y);
}
1
#
The class basic_­istringstream<charT, traits, Allocator> supports reading objects of class basic_­string<​charT, traits, Allocator>.
It uses a basic_­stringbuf<charT, traits, Allocator> object to control the associated storage.
For the sake of exposition, the maintained data is presented here as:
  • (1.1)
    sb, the stringbuf object.

30.8.3.1 basic_­istringstream constructors [istringstream.cons]

🔗
explicit basic_istringstream(ios_base::openmode which = ios_base::in);
1
#
Effects: Constructs an object of class basic_­istringstream<charT, traits>, initializing the base class with basic_­istream(&sb) and initializing sb with basic_­stringbuf<charT, traits, Allocator>(which | ios_­base​::​in)) ([stringbuf.cons]).
🔗
explicit basic_istringstream( const basic_string<charT, traits, Allocator>& str, ios_base::openmode which = ios_base::in);
2
#
Effects: Constructs an object of class basic_­istringstream<charT, traits>, initializing the base class with basic_­istream(&sb) and initializing sb with basic_­stringbuf<charT, traits, Allocator>(str, which | ios_­base​::​in)) ([stringbuf.cons]).
🔗
basic_istringstream(basic_istringstream&& rhs);
3
#
Effects: Move constructs from the rvalue rhs.
This is accomplished by move constructing the base class, and the contained basic_­stringbuf.
Next basic_­istream<charT, traits>​::​set_­rdbuf(&sb) is called to install the contained basic_­stringbuf.

30.8.3.2 Assign and swap [istringstream.assign]

🔗
basic_istringstream& operator=(basic_istringstream&& rhs);
1
#
Effects: Move assigns the base and members of *this from the base and corresponding members of rhs.
2
#
Returns: *this.
🔗
void swap(basic_istringstream& rhs);
3
#
Effects: Exchanges the state of *this and rhs by calling basic_­istream<charT, traits>​::​swap(rhs) and sb.swap(rhs.sb).
🔗
template <class charT, class traits, class Allocator> void swap(basic_istringstream<charT, traits, Allocator>& x, basic_istringstream<charT, traits, Allocator>& y);
4
#
Effects: As if by x.swap(y).

30.8.3.3 Member functions [istringstream.members]

🔗
basic_stringbuf<charT, traits, Allocator>* rdbuf() const;
1
#
Returns: const_­cast<basic_­stringbuf<charT, traits, Allocator>*>(&sb).
🔗
basic_string<charT, traits, Allocator> str() const;
2
#
Returns: rdbuf()->str().
🔗
void str(const basic_string<charT, traits, Allocator>& s);
3
#
Effects: Calls rdbuf()->str(s).

30.8.4 Class template basic_­ostringstream [ostringstream]

namespace std {
  template <class charT, class traits = char_traits<charT>,
            class Allocator = allocator<charT>>
  class basic_ostringstream : public basic_ostream<charT, traits> {
  public:
    using char_type      = charT;
    using int_type       = typename traits::int_type;
    using pos_type       = typename traits::pos_type;
    using off_type       = typename traits::off_type;
    using traits_type    = traits;
    using allocator_type = Allocator;

    // [ostringstream.cons], constructors
    explicit basic_ostringstream(
      ios_base::openmode which = ios_base::out);
    explicit basic_ostringstream(
      const basic_string<charT, traits, Allocator>& str,
      ios_base::openmode which = ios_base::out);
    basic_ostringstream(const basic_ostringstream& rhs) = delete;
    basic_ostringstream(basic_ostringstream&& rhs);

    // [ostringstream.assign], assign and swap
    basic_ostringstream& operator=(const basic_ostringstream& rhs) = delete;
    basic_ostringstream& operator=(basic_ostringstream&& rhs);
    void swap(basic_ostringstream& rhs);

    // [ostringstream.members], members
    basic_stringbuf<charT, traits, Allocator>* rdbuf() const;

    basic_string<charT, traits, Allocator> str() const;
    void str(const basic_string<charT, traits, Allocator>& s);
   private:
    basic_stringbuf<charT, traits, Allocator> sb; // exposition only
  };

  template <class charT, class traits, class Allocator>
    void swap(basic_ostringstream<charT, traits, Allocator>& x,
              basic_ostringstream<charT, traits, Allocator>& y);
}
1
#
The class basic_­ostringstream<charT, traits, Allocator> supports writing objects of class basic_­string<​charT, traits, Allocator>.
It uses a basic_­stringbuf object to control the associated storage.
For the sake of exposition, the maintained data is presented here as:
  • (1.1)
    sb, the stringbuf object.

30.8.4.1 basic_­ostringstream constructors [ostringstream.cons]

🔗
explicit basic_ostringstream( ios_base::openmode which = ios_base::out);
1
#
Effects: Constructs an object of class basic_­ostringstream, initializing the base class with basic_­ostream(​&sb) and initializing sb with basic_­stringbuf<charT, traits, Allocator>(which | ​ios_­base​::​out)) ([stringbuf.cons]).
🔗
explicit basic_ostringstream( const basic_string<charT, traits, Allocator>& str, ios_base::openmode which = ios_base::out);
2
#
Effects: Constructs an object of class basic_­ostringstream<charT, traits>, initializing the base class with basic_­ostream(&sb) and initializing sb with basic_­stringbuf<charT, traits, Allocator>(str, which | ios_­base​::​out)) ([stringbuf.cons]).
🔗
basic_ostringstream(basic_ostringstream&& rhs);
3
#
Effects: Move constructs from the rvalue rhs.
This is accomplished by move constructing the base class, and the contained basic_­stringbuf.
Next basic_­ostream<charT, traits>​::​set_­rdbuf(&sb) is called to install the contained basic_­stringbuf.

30.8.4.2 Assign and swap [ostringstream.assign]

🔗
basic_ostringstream& operator=(basic_ostringstream&& rhs);
1
#
Effects: Move assigns the base and members of *this from the base and corresponding members of rhs.
2
#
Returns: *this.
🔗
void swap(basic_ostringstream& rhs);
3
#
Effects: Exchanges the state of *this and rhs by calling basic_­ostream<charT, traits>​::​swap(rhs) and sb.swap(rhs.sb).
🔗
template <class charT, class traits, class Allocator> void swap(basic_ostringstream<charT, traits, Allocator>& x, basic_ostringstream<charT, traits, Allocator>& y);
4
#
Effects: As if by x.swap(y).

30.8.4.3 Member functions [ostringstream.members]

🔗
basic_stringbuf<charT, traits, Allocator>* rdbuf() const;
1
#
Returns: const_­cast<basic_­stringbuf<charT, traits, Allocator>*>(&sb).
🔗
basic_string<charT, traits, Allocator> str() const;
2
#
Returns: rdbuf()->str().
🔗
void str(const basic_string<charT, traits, Allocator>& s);
3
#
Effects: Calls rdbuf()->str(s).

30.8.5 Class template basic_­stringstream [stringstream]

namespace std {
  template <class charT, class traits = char_traits<charT>,
            class Allocator = allocator<charT>>
  class basic_stringstream : public basic_iostream<charT, traits> {
  public:
    using char_type      = charT;
    using int_type       = typename traits::int_type;
    using pos_type       = typename traits::pos_type;
    using off_type       = typename traits::off_type;
    using traits_type    = traits;
    using allocator_type = Allocator;

    // [stringstream.cons], constructors
    explicit basic_stringstream(
      ios_base::openmode which = ios_base::out | ios_base::in);
    explicit basic_stringstream(
      const basic_string<charT, traits, Allocator>& str,
      ios_base::openmode which = ios_base::out | ios_base::in);
    basic_stringstream(const basic_stringstream& rhs) = delete;
    basic_stringstream(basic_stringstream&& rhs);

    // [stringstream.assign], assign and swap
    basic_stringstream& operator=(const basic_stringstream& rhs) = delete;
    basic_stringstream& operator=(basic_stringstream&& rhs);
    void swap(basic_stringstream& rhs);

    // [stringstream.members], members
    basic_stringbuf<charT, traits, Allocator>* rdbuf() const;
    basic_string<charT, traits, Allocator> str() const;
    void str(const basic_string<charT, traits, Allocator>& str);

  private:
    basic_stringbuf<charT, traits> sb;  // exposition only
  };

  template <class charT, class traits, class Allocator>
    void swap(basic_stringstream<charT, traits, Allocator>& x,
              basic_stringstream<charT, traits, Allocator>& y);
}
1
#
The class template basic_­stringstream<charT, traits> supports reading and writing from objects of class basic_­string<charT, traits, Allocator>.
It uses a basic_­stringbuf<charT, traits, Allocator> object to control the associated sequence.
For the sake of exposition, the maintained data is presented here as
  • (1.1)
    sb, the stringbuf object.

30.8.5.1 basic_­stringstream constructors [stringstream.cons]

🔗
explicit basic_stringstream( ios_base::openmode which = ios_base::out | ios_base::in);
1
#
Effects: Constructs an object of class basic_­stringstream<charT, traits>, initializing the base class with basic_­iostream(&sb) and initializing sb with basic_­stringbuf<charT, traits, Allocator>(which).
🔗
explicit basic_stringstream( const basic_string<charT, traits, Allocator>& str, ios_base::openmode which = ios_base::out | ios_base::in);
2
#
Effects: Constructs an object of class basic_­stringstream<charT, traits>, initializing the base class with basic_­iostream(&sb) and initializing sb with basic_­stringbuf<charT, traits, Allocator>(str, which).
🔗
basic_stringstream(basic_stringstream&& rhs);
3
#
Effects: Move constructs from the rvalue rhs.
This is accomplished by move constructing the base class, and the contained basic_­stringbuf.
Next basic_­istream<charT, traits>​::​set_­rdbuf(&sb) is called to install the contained basic_­stringbuf.

30.8.5.2 Assign and swap [stringstream.assign]

🔗
basic_stringstream& operator=(basic_stringstream&& rhs);
1
#
Effects: Move assigns the base and members of *this from the base and corresponding members of rhs.
2
#
Returns: *this.
🔗
void swap(basic_stringstream& rhs);
3
#
Effects: Exchanges the state of *this and rhs by calling basic_­iostream<charT,traits>​::​swap(rhs) and sb.swap(rhs.sb).
🔗
template <class charT, class traits, class Allocator> void swap(basic_stringstream<charT, traits, Allocator>& x, basic_stringstream<charT, traits, Allocator>& y);
4
#
Effects: As if by x.swap(y).

30.8.5.3 Member functions [stringstream.members]

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basic_stringbuf<charT, traits, Allocator>* rdbuf() const;
1
#
Returns: const_­cast<basic_­stringbuf<charT, traits, Allocator>*>(&sb)
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basic_string<charT, traits, Allocator> str() const;
2
#
Returns: rdbuf()->str().
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void str(const basic_string<charT, traits, Allocator>& str);
3
#
Effects: Calls rdbuf()->str(str).