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strstreambuf Class

  • Artikkel

Describes a stream buffer that controls the transmission of elements to and from a sequence of elements stored in a char array object.

Syntax

class strstreambuf : public streambuf

Remarks

Depending on how the object is constructed, it can be allocated, extended, and freed as necessary to accommodate changes in the sequence.

An object of class strstreambuf stores several bits of mode information as its strstreambuf mode. These bits indicate whether the controlled sequence:

  • Has been allocated and needs to be freed eventually.

  • Is modifiable.

  • Is extendable by reallocating storage.

  • Has been frozen and hence needs to be unfrozen before the object is destroyed, or freed (if allocated) by an agency other than the object.

A controlled sequence that is frozen cannot be modified or extended, regardless of the state of these separate mode bits.

The object also stores pointers to two functions that control strstreambuf allocation. If these are null pointers, the object devises its own method of allocating and freeing storage for the controlled sequence.

Note

This class is deprecated. Consider using stringbuf or wstringbuf instead.

Constructors

Constructor Description
strstreambuf Constructs an object of type strstreambuf.

Member functions

Member function Description
freeze Causes a stream buffer to be unavailable through stream buffer operations.
overflow A protected virtual function that can be called when a new character is inserted into a full buffer.
pbackfail A protected virtual member function that tries to put back an element into the input stream, and then make it the current element (pointed to by the next pointer).
pcount Returns a count of the number of elements written to the controlled sequence.
seekoff A protected virtual member function that tries to alter the current positions for the controlled streams.
seekpos A protected virtual member function that tries to alter the current positions for the controlled streams.
str Calls freeze, and then returns a pointer to the beginning of the controlled sequence.
underflow A protected virtual function to extract the current element from the input stream.

Requirements

Header: <strstream>

Namespace: std

strstreambuf::freeze

Causes a stream buffer to be unavailable through stream buffer operations.

void freeze(bool _Freezeit = true);

Parameters

_Freezeit
A bool indicating whether you want the stream to be frozen.

Remarks

If _Freezeit is true, the function alters the stored strstreambuf mode to make the controlled sequence frozen. Otherwise, it makes the controlled sequence not frozen.

str implies freeze.

Note

A frozen buffer will not be freed during strstreambuf destruction. You must unfreeze the buffer before it is freed to avoid a memory leak.

Example

// strstreambuf_freeze.cpp
// compile with: /EHsc

#include <iostream>
#include <strstream>

using namespace std;

void report(strstream &x)
{
    if (!x.good())
        cout << "stream bad" << endl;
    else
        cout << "stream good" << endl;
}

int main()
{
    strstream x;

    x << "test1";
    cout << "before freeze: ";
    report(x);

    // Calling str freezes stream.
    cout.write(x.rdbuf()->str(), 5) << endl;
    cout << "after freeze: ";
    report(x);

    // Stream is bad now, wrote on frozen stream
    x << "test1.5";
    cout << "after write to frozen stream: ";
    report(x);

    // Unfreeze stream, but it is still bad
    x.rdbuf()->freeze(false);
    cout << "after unfreezing stream: ";
    report(x);

    // Clear stream
    x.clear();
    cout << "after clearing stream: ";
    report(x);

    x << "test3";
    cout.write(x.rdbuf()->str(), 10) << endl;

    // Clean up.  Failure to unfreeze stream will cause a
    // memory leak.
    x.rdbuf()->freeze(false);
}

before freeze: stream good
test1
after freeze: stream good
after write to frozen stream: stream bad
after unfreezing stream: stream bad
after clearing stream: stream good
test1test3

strstreambuf::overflow

A protected virtual function that can be called when a new character is inserted into a full buffer.

virtual int overflow(int _Meta = EOF);

Parameters

_Meta
The character to insert into the buffer, or EOF.

Return Value

If the function cannot succeed, it returns EOF. Otherwise, if _Meta == EOF, it returns some value other than EOF. Otherwise, it returns _Meta.

Remarks

If _Meta != EOF, the protected virtual member function tries to insert the element (char)_Meta into the output buffer. It can do so in various ways:

  • If a write position is available, it can store the element into the write position and increment the next pointer for the output buffer.

  • If the stored strstreambuf mode says the controlled sequence is modifiable, extendable, and not frozen, the function can make a write position available by allocating new for the output buffer. Extending the output buffer this way also extends any associated input buffer.

strstreambuf::pbackfail

A protected virtual member function that tries to put back an element into the input stream, and then makes it the current element (pointed to by the next pointer).

virtual int pbackfail(int _Meta = EOF);

Parameters

_Meta
The character to insert into the buffer, or EOF.

Return Value

If the function cannot succeed, it returns EOF. Otherwise, if _Meta == EOF, it returns some value other than EOF. Otherwise, it returns _Meta.

Remarks

The protected virtual member function tries to put back an element into the input buffer, and then make it the current element (pointed to by the next pointer).

If _Meta == EOF, the element to push back is effectively the one already in the stream before the current element. Otherwise, that element is replaced by ch = (char)_Meta. The function can put back an element in various ways:

  • If a putback position is available, and the element stored there compares equal to ch, it can decrement the next pointer for the input buffer.

  • If a putback position is available, and if the strstreambuf mode says the controlled sequence is modifiable, the function can store ch into the putback position and decrement the next pointer for the input buffer.

strstreambuf::pcount

Returns a count of the number of elements written to the controlled sequence.

streamsize pcount() const;

Return Value

A count of the number of elements written to the controlled sequence.

Remarks

Specifically, if pptr is a null pointer, the function returns zero. Otherwise, it returns pptr - pbase.

Example

// strstreambuf_pcount.cpp
// compile with: /EHsc
#include <iostream>
#include <strstream>
using namespace std;

int main( )
{
   strstream x;
   x << "test1";
   cout << x.rdbuf( )->pcount( ) << endl;
   x << "test2";
   cout << x.rdbuf( )->pcount( ) << endl;
}

strstreambuf::seekoff

A protected virtual member function that tries to alter the current positions for the controlled streams.

virtual streampos seekoff(streamoff _Off,
    ios_base::seekdir _Way,
    ios_base::openmode _Which = ios_base::in | ios_base::out);

Parameters

_Off
The position to seek for relative to _Way.

_Way
The starting point for offset operations. See seekdir for possible values.

_Which
Specifies the mode for the pointer position. The default is to allow you to modify the read and write positions.

Return Value

If the function succeeds in altering either or both stream positions, it returns the resultant stream position. Otherwise, it fails and returns an invalid stream position.

Remarks

The protected virtual member function endeavors to alter the current positions for the controlled streams. For an object of class strstreambuf, a stream position consists purely of a stream offset. Offset zero designates the first element of the controlled sequence.

The new position is determined as follows:

  • If _Way == ios_base::beg, the new position is the beginning of the stream plus _Off.

  • If _Way == ios_base::cur, the new position is the current stream position plus _Off.

  • If _Way == ios_base::end, the new position is the end of the stream plus _Off.

If _Which & ios_base::in is nonzero and the input buffer exist, the function alters the next position to read in the input buffer. If _Which & ios_base::out is also nonzero, _Way != ios_base::cur, and the output buffer exists, the function also sets the next position to write to match the next position to read.

Otherwise, if _Which & ios_base::out is nonzero and the output buffer exists, the function alters the next position to write in the output buffer. Otherwise, the positioning operation fails. For a positioning operation to succeed, the resulting stream position must lie within the controlled sequence.

strstreambuf::seekpos

A protected virtual member function that tries to alter the current positions for the controlled streams.

virtual streampos seekpos(streampos _Sp, ios_base::openmode _Which = ios_base::in | ios_base::out);

Parameters

_Sp
The position to seek for.

_Which
Specifies the mode for the pointer position. The default is to allow you to modify the read and write positions.

Return Value

If the function succeeds in altering either or both stream positions, it returns the resultant stream position. Otherwise, it fails and returns an invalid stream position. To determine if the stream position is invalid, compare the return value with pos_type(off_type(-1)).

Remarks

The protected virtual member function endeavors to alter the current positions for the controlled streams. For an object of class strstreambuf, a stream position consists purely of a stream offset. Offset zero designates the first element of the controlled sequence. The new position is determined by _Sp.

If _Which & ios_base::in is nonzero and the input buffer exists, the function alters the next position to read in the input buffer. If _Which & ios_base::out is nonzero and the output buffer exists, the function also sets the next position to write to match the next position to read. Otherwise, if _Which & ios_base::out is nonzero and the output buffer exists, the function alters the next position to write in the output buffer. Otherwise, the positioning operation fails. For a positioning operation to succeed, the resulting stream position must lie within the controlled sequence.

strstreambuf::str

Calls freeze, and then returns a pointer to the beginning of the controlled sequence.

char *str();

Return Value

A pointer to the beginning of the controlled sequence.

Remarks

No terminating null element exists, unless you explicitly insert one.

Example

See strstreambuf::freeze for a sample that uses str.

strstreambuf::strstreambuf

Constructs an object of type strstreambuf.

explicit strstreambuf(streamsize count = 0);

strstreambuf(void (* alloc_func)(size_t),
    void (* free_func)(void*));

strstreambuf(char* getptr,
    streamsize count,
    char* putptr = 0);

strstreambuf(signed char* getptr,
    streamsize count,
    signed char* putptr = 0);

strstreambuf(unsigned char* getptr,
    streamsize count,
    unsigned char* putptr = 0);

strstreambuf(const char* getptr,
    streamsize count);

strstreambuf(const signed char* getptr,
    streamsize count);

strstreambuf(const unsigned char* getptr,
    streamsize count);

Parameters

alloc_func
The function used to allocate buffer memory.

count
Determines the length of the buffer pointed to by getptr. If getptr is not an argument (first constructor form), a suggested allocation size for the buffers.

_Freefunc
The function used to free buffer memory.

getptr
A buffer used for input.

putptr
A buffer used for output.

Remarks

The first constructor stores a null pointer in all the pointers controlling the input buffer, the output buffer, and strstreambuf allocation. It sets the stored strstreambuf mode to make the controlled sequence modifiable and extendable. It also accepts count as a suggested initial allocation size.

The second constructor behaves like the first, except that it stores alloc_func as the pointer to the function to call to allocate storage and free_func as the pointer to the function to call to free that storage.

The three constructors:

strstreambuf(char *getptr,
    streamsize count,
    char *putptr = 0);

strstreambuf(signed char *getptr,
    streamsize count,
    signed char *putptr = 0);

strstreambuf(unsigned char *getptr,
    streamsize count,
    unsigned char *putptr = 0);

also behave like the first, except that getptr designates the array object used to hold the controlled sequence. (Hence, it must not be a null pointer.) The number of elements N in the array is determined as follows:

  • If (count > 0), then N is count.

  • If (count == 0), then N is strlen((const char *) getptr ).

  • If (count < 0), then N is INT_MAX.

If putptr is a null pointer, the function establishes just an input buffer by executing:

setg(getptr,
    getptr,
    getptr + N);

Otherwise, it establishes both input and output buffers by executing:

setg(getptr,
    getptr,
    putptr);

setp(putptr,
    getptr + N);

In this case, putptr must be in the interval [ getptr, getptr + N].

Finally, the three constructors:

strstreambuf(const char *getptr,
    streamsize count);

strstreambuf(const signed char *getptr,
    streamsize count);

strstreambuf(const unsigned char *getptr,
    streamsize count);

all behave the same as:

streambuf((char *)getptr, count);

except that the stored mode makes the controlled sequence neither modifiable nor extendable.

strstreambuf::underflow

A protected virtual function to extract the current element from the input stream.

virtual int underflow();

Return Value

If the function cannot succeed, it returns EOF. Otherwise, it returns the current element in the input stream, converted as described above.

Remarks

The protected virtual member function endeavors to extract the current element ch from the input buffer, then advance the current stream position, and return the element as (int)(unsigned char)ch. It can do so in only one way: if a read position is available, it takes ch as the element stored in the read position and advances the next pointer for the input buffer.

See also

streambuf
Thread Safety in the C++ Standard Library
iostream Programming
iostreams Conventions