basic_filebuf Class

บทความ
06/09/2022

Describes a stream buffer that controls the transmission of elements of type Char_T, whose character traits are determined by the class Tr, to and from a sequence of elements stored in an external file.

Syntax

template <class Char_T, class Tr = char_traits<Char_T>>
class basic_filebuf : public basic_streambuf<Char_T, Tr>

Parameters

Char_T
The basic element of the file buffer.

Tr
The traits of the basic element of the file buffer (usually char_traits<Char_T>).

Remarks

The class template describes a stream buffer that controls the transmission of elements of type Char_T, whose character traits are determined by the class Tr, to and from a sequence of elements stored in an external file.

Note

Objects of type basic_filebuf are created with an internal buffer of type char* regardless of the char_type specified by the type parameter Char_T. This means that a Unicode string (containing wchar_t characters) will be converted to an ANSI string (containing char characters) before it is written to the internal buffer. To store Unicode strings in the buffer, create a new buffer of type wchar_t and set it using the basic_streambuf::pubsetbuf() method. To see an example that demonstrates this behavior, see below.

An object of class basic_filebuf<Char_T, Tr> stores a file pointer, which designates the FILE object that controls the stream associated with an open file. It also stores pointers to two file conversion facets for use by the protected member functions overflow and underflow. For more information, see basic_filebuf::open.

Example

The following example demonstrates how to force an object of type basic_filebuf<wchar_t> to store Unicode characters in its internal buffer by calling the pubsetbuf() method.

// unicode_basic_filebuf.cpp
// compile with: /EHsc

#include <iostream>
#include <string>
#include <fstream>
#include <iomanip>
#include <memory.h>
#include <string.h>

#define IBUFSIZE 16

using namespace std;

void hexdump(const string& filename);

int main()
{
    wchar_t* wszHello = L"Hello World";
    wchar_t wBuffer[128];

    basic_filebuf<wchar_t> wOutFile;

    // Open a file, wcHello.txt, then write to it, then dump the
    // file's contents in hex
    wOutFile.open("wcHello.txt",
        ios_base::out | ios_base::trunc | ios_base::binary);
    if(!wOutFile.is_open())
    {
        cout << "Error Opening wcHello.txt\n";
        return -1;
    }
    wOutFile.sputn(wszHello, (streamsize)wcslen(wszHello));
    wOutFile.close();
    cout << "Hex Dump of wcHello.txt - note that output is ANSI chars:\n";
    hexdump(string("wcHello.txt"));

    // Open a file, wwHello.txt, then set the internal buffer of
    // the basic_filebuf object to be of type wchar_t, then write
    // to the file and dump the file's contents in hex
    wOutFile.open("wwHello.txt",
        ios_base::out | ios_base::trunc | ios_base::binary);
    if(!wOutFile.is_open())
    {
        cout << "Error Opening wwHello.txt\n";
        return -1;
    }
    wOutFile.pubsetbuf(wBuffer, (streamsize)128);
    wOutFile.sputn(wszHello, (streamsize)wcslen(wszHello));
    wOutFile.close();
    cout << "\nHex Dump of wwHello.txt - note that output is wchar_t chars:\n";
    hexdump(string("wwHello.txt"));

    return 0;
}

// dump contents of filename to stdout in hex
void hexdump(const string& filename)
{
    fstream ifile(filename.c_str(),
        ios_base::in | ios_base::binary);
    char *ibuff = new char[IBUFSIZE];
    char *obuff = new char[(IBUFSIZE*2)+1];
    int i;

    if(!ifile.is_open())
    {
        cout << "Cannot Open " << filename.c_str()
             << " for reading\n";
        return;
    }
    if(!ibuff || !obuff)
    {
        cout << "Cannot Allocate buffers\n";
        ifile.close();
        return;
    }

    while(!ifile.eof())
    {
        memset(obuff,0,(IBUFSIZE*2)+1);
        memset(ibuff,0,IBUFSIZE);
        ifile.read(ibuff,IBUFSIZE);

        // corner case where file is exactly a multiple of
        // 16 bytes in length
        if(ibuff[0] == 0 && ifile.eof())
            break;

        for(i = 0; i < IBUFSIZE; i++)
        {
            if(ibuff[i] >= ' ')
                obuff[i] = ibuff[i];
            else
                obuff[i] = '.';

            cout << setfill('0') << setw(2) << hex
                 << (int)ibuff[i] << ' ';
        }
        cout << "  " << obuff << endl;
    }
    ifile.close();
}

Hex Dump of wcHello.txt - note that output is ANSI chars:
48 65 6c 6c 6f 20 57 6f 72 6c 64 00 00 00 00 00   Hello World.....

Hex Dump of wwHello.txt - note that output is wchar_t chars:
48 00 65 00 6c 00 6c 00 6f 00 20 00 57 00 6f 00   H.e.l.l.o. .W.o.
72 00 6c 00 64 00 00 00 00 00 00 00 00 00 00 00   r.l.d...........

Constructors

Constructor	Description
basic_filebuf	Constructs an object of type `basic_filebuf`.

Typedefs

Type name	Description
char_type	Associates a type name with the `Char_T` template parameter.
int_type	Makes this type within `basic_filebuf`'s scope equivalent to the type of the same name in the `Tr` scope.
off_type	Makes this type within `basic_filebuf`'s scope equivalent to the type of the same name in the `Tr` scope.
pos_type	Makes this type within `basic_filebuf`'s scope equivalent to the type of the same name in the `Tr` scope.
traits_type	Associates a type name with the `Tr` template parameter.

Member functions

Member function	Description
close	Closes a file.
is_open	Indicates whether a file is open.
open	Opens a file.
overflow	A protected virtual function that can be called when a new character is inserted into a full buffer.
pbackfail	The protected virtual member function tries to put back an element into the input stream, then make it the current element (pointed to by the next pointer).
seekoff	The protected virtual member function tries to alter the current positions for the controlled streams.
seekpos	The protected virtual member function tries to alter the current positions for the controlled streams.
setbuf	The protected virtual member function performs an operation particular to each derived stream buffer.
Swap	Exchanges the content of this `basic_filebuf` for the content of the provided `basic_filebuf` parameter.
sync	Protected, virtual function tries to synchronize the controlled streams with any associated external streams.
uflow	Protected, virtual function to extract the current element from the input stream.
underflow	Protected, virtual function to extract the current element from the input stream.

Requirements

Header: <fstream>

Namespace: std

basic_filebuf::basic_filebuf

Constructs an object of type basic_filebuf.

basic_filebuf();

basic_filebuf(basic_filebuf&& right);

Remarks

The first constructor stores a null pointer in all the pointers controlling the input buffer and the output buffer. It also stores a null pointer in the file pointer.

The second constructor initializes the object with the contents of right, treated as an rvalue reference.

basic_filebuf::char_type

Associates a type name with the Char_T template parameter.

typedef Char_T char_type;

basic_filebuf::close

Closes a file.

basic_filebuf<Char_T, Tr> *close();

Return Value

The member function returns a null pointer if the file pointer is a null pointer.

Remarks

close calls fclose(fp). If that function returns a nonzero value, the function returns a null pointer. Otherwise, it returns this to indicate that the file was successfully closed.

For a wide stream, if any insertions have occurred since the stream was opened, or since the last call to streampos, the function calls overflow. It also inserts any sequence needed to restore the initial conversion state, by using the file conversion facet fac to call fac.unshift as needed. Each produced element byte of type char is written to the associated stream designated by the file pointer fp as if by successive calls of the form fputc(byte, fp). If the call to fac.unshift or any write fails, the function doesn't succeed.

Example

The following sample assumes two files in the current directory: basic_filebuf_close.txt (contents is "testing") and iotest.txt (contents is "ssss").

// basic_filebuf_close.cpp
// compile with: /EHsc
#include <fstream>
#include <iostream>

int main() {
   using namespace std;
   ifstream file;
   basic_ifstream <wchar_t> wfile;
   char c;
   // Open and close with a basic_filebuf
   file.rdbuf()->open( "basic_filebuf_close.txt", ios::in );
   file >> c;
   cout << c << endl;
   file.rdbuf( )->close( );

   // Open/close directly
   file.open( "iotest.txt" );
   file >> c;
   cout << c << endl;
   file.close( );

   // open a file with a wide character name
   wfile.open( L"iotest.txt" );

   // Open and close a nonexistent with a basic_filebuf
   file.rdbuf()->open( "ziotest.txt", ios::in );
   cout << file.fail() << endl;
   file.rdbuf( )->close( );

   // Open/close directly
   file.open( "ziotest.txt" );
   cout << file.fail() << endl;
   file.close( );
}

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basic_filebuf::int_type

Makes this type within basic_filebuf scope equivalent to the type of the same name in the Tr scope.

typedef typename traits_type::int_type int_type;

basic_filebuf::is_open

Indicates whether a file is open.

bool is_open() const;

Return Value

true if the file pointer isn't null.

Example

// basic_filebuf_is_open.cpp
// compile with: /EHsc
#include <fstream>
#include <iostream>

int main( )
{
   using namespace std;
   ifstream file;
   cout << boolalpha << file.rdbuf( )->is_open( ) << endl;

   file.open( "basic_filebuf_is_open.cpp" );
   cout << file.rdbuf( )->is_open( ) << endl;
}

false
true

basic_filebuf::off_type

Makes this type within basic_filebuf scope equivalent to the type of the same name in the Tr scope.

typedef typename traits_type::off_type off_type;

basic_filebuf::open

Opens a file.

basic_filebuf<Char_T, Tr> *open(
    const char* filename,
    ios_base::openmode mode,
    int protection = (int)ios_base::_Openprot);

basic_filebuf<Char_T, Tr> *open(
    const char* filename,
    ios_base::openmode mode);

basic_filebuf<Char_T, Tr> *open(
    const wchar_t* filename,
    ios_base::openmode mode,
    int protection = (int)ios_base::_Openprot);

basic_filebuf<Char_T, Tr> *open(
    const wchar_t* filename,
    ios_base::openmode mode);

Parameters

filename
The name of the file to open.

mode
One of the enumerations in ios_base::openmode.

protection
The default file opening protection, equivalent to the shflag parameter in _fsopen, _wfsopen.

Return Value

If the buffer is already open, or if the file pointer is a null pointer, the function returns a null pointer. Otherwise, it returns this.

Remarks

This function uses a FILE * to back the basic_filebuf as though you had called fopen/wfopen(filename, strmode). strmode is determined from mode & ~(ate | binary):

ios_base::in becomes "r" (open existing file for reading).
ios_base::out or ios_base::out | ios_base::trunc becomes "w" (truncate existing file or create for writing).
ios_base::out | app becomes "a" (open existing file for appending all writes).
ios_base::in | ios_base::out becomes "r+" (open existing file for reading and writing).
ios_base::in | ios_base::out | ios_base::trunc becomes "w+" (truncate existing file or create for reading and writing).
ios_base::in | ios_base::out | ios_base::app becomes "a+" (open existing file for reading and for appending all writes).

If mode & ios_base::binary is nonzero, the function appends b to strmode to open a binary stream instead of a text stream. If mode & ios_base::ate is nonzero and the file was successfully opened, the current location in the stream is positioned at the end of file. If that fails, the file is closed.

If the above operations completed successfully, the file conversion facet is determined: use_facet<codecvt<Char_T, char, traits_type::state_type> >(getloc), for use by underflow and overflow.

If the file couldn't be successfully opened, nullptr is returned.

Example

See basic_filebuf::close for an example that uses open.

basic_filebuf::operator=

Assign the content of this stream buffer object. This is a move assignment involving an rvalue that doesn't leave a copy behind.

basic_filebuf& operator=(basic_filebuf&& right);

Parameters

right
An rvalue reference to a basic_filebuf object.

Return Value

Returns *this.

Remarks

The member operator replaces the contents of the object by using the contents of right, treated as an rvalue reference. For more information, see Rvalue reference declarator: &&.

basic_filebuf::overflow

Called when a new character is inserted into a full buffer.

virtual int_type overflow(int_type _Meta = traits_type::eof);

Parameters

_Meta
The character to insert into the buffer or traits_type::eof.

Return Value

If the function can't succeed, it returns traits_type::eof. Otherwise, it returns traits_type::not_eof(_Meta).

Remarks

If _Meta != traits_type::eof, the protected virtual member function attempts to insert the element ch = traits_type::to_char_type(_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.
It can make a write position available by allocating new or additional storage for the output buffer.
It can convert any pending output in the output buffer, followed by ch, by using the file conversion facet fac to call fac.out as needed. Each produced element ch of type char is written to the associated stream designated by the file pointer fp as if by successive calls of the form fputc(ch, fp). If any conversion or write fails, the function doesn't succeed.

basic_filebuf::pbackfail

Tries to put back an element into the input stream, then make it the current element (pointed to by the next pointer).

virtual int_type pbackfail(int_type _Meta = traits_type::eof);

Parameters

_Meta
The character to insert into the buffer, or traits_type::eof.

Return Value

If the function can't succeed, it returns traits_type::eof. Otherwise, it returns traits_type::not_eof(_Meta).

Remarks

The protected virtual member function puts back an element into the input buffer and then makes it the current element (pointed to by the next pointer). If _Meta == traits_type::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 = traits_type::to_char_type(_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 the function can make a putback position available, it can do so, set the next pointer to point at that position, and store ch in that position.
If the function can push back an element onto the input stream, it can do so, such as by calling ungetc for an element of type char.

basic_filebuf::pos_type

Makes this type within basic_filebuf scope equivalent to the type of the same name in the Tr scope.

typedef typename traits_type::pos_type pos_type;

basic_filebuf::seekoff

Tries to alter the current positions for the controlled streams.

virtual pos_type seekoff(
    off_type _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

Returns the new position or an invalid stream position.

Remarks

The protected virtual member function attempts to alter the current positions for the controlled streams. For an object of class basic_filebuf<Char_T, Tr>, a stream position can be represented by an object of type fpos_t, which stores an offset and any state information needed to parse a wide stream. Offset zero refers to the first element of the stream. (An object of type pos_type stores at least an fpos_t object.)

For a file opened for both reading and writing, both the input and output streams are positioned in tandem. To switch between inserting and extracting, you must call either pubseekoff or pubseekpos. Calls to pubseekoff (and hence to seekoff) have various limitations for text streams, binary streams, and wide streams.

If the file pointer fp is a null pointer, the function fails. Otherwise, it attempts to alter the stream position by calling fseek(fp, _Off, _Way). If that function succeeds and the resulting position fposn can be determined by calling fgetpos(fp, &fposn), the function succeeds. If the function succeeds, it returns a value of type pos_type containing fposn. Otherwise, it returns an invalid stream position.

basic_filebuf::seekpos

Tries to alter the current positions for the controlled streams.

virtual pos_type seekpos(
    pos_type _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 file pointer fp is a null pointer, the function fails. Otherwise, it attempts to alter the stream position by calling fsetpos(fp, &fposn), where fposn is the fpos_t object stored in pos. If that function succeeds, the function returns pos. Otherwise, it returns an invalid stream position. To determine if the stream position is invalid, compare the return value with pos_type(off_type(-1)).

Remarks

For a file opened for both reading and writing, both the input and output streams are positioned in tandem. To switch between inserting and extracting, you must call either pubseekoff or pubseekpos. Calls to pubseekoff (and to seekoff) have various limitations for text streams, binary streams, and wide streams.

basic_filebuf::setbuf

Performs an operation particular to each derived stream buffer.

virtual basic_streambuf<Char_T, Tr> *setbuf(
    char_type* _Buffer,
    streamsize count);

Parameters

_Buffer
Pointer to a buffer.

count
Size of the buffer.

Return Value

The protected member function returns zero if the file pointer fp is a null pointer.

Remarks

setbuf calls setvbuf( fp, (char*) _Buffer, _IOFBF, count * sizeof( Char_T)) to offer the array of count elements beginning at _Buffer as a buffer for the stream. If that function returns a nonzero value, the function returns a null pointer. Otherwise, it returns this to signal success.

basic_filebuf::swap

Exchanges the contents of this basic_filebuf for the contents of the provided basic_filebuf.

void swap(basic_filebuf& right);

Parameters

right
An lvalue reference to another basic_filebuf.

basic_filebuf::sync

Tries to synchronize the controlled streams with any associated external streams.

virtual int sync();

Return Value

Returns zero if the file pointer fp is a null pointer. Otherwise, it returns zero only if calls to both overflow and fflush(fp) succeed in flushing any pending output to the stream.

basic_filebuf::traits_type

Associates a type name with the Tr template parameter.

typedef Tr traits_type;

basic_filebuf::underflow

Extracts the current element from the input stream.

virtual int_type underflow();

Return Value

If the function can't succeed, it returns traits_type::eof. Otherwise, it returns ch, converted as described in the Remarks section.

Remarks

The protected virtual member function attempts to extract the current element ch from the input stream, and return the element as traits_type::to_int_type(ch). It can do so in various ways:

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.
It can read one or more elements of type char, as if by successive calls of the form fgetc(fp), and convert them to an element ch of type Char_T by using the file conversion facet fac to call fac.in as needed. If any read or conversion fails, the function doesn't succeed.

แชร์ผ่าน

basic_filebuf Class

Syntax

Parameters

Remarks

Example

Constructors

Typedefs

Member functions

Requirements

basic_filebuf::basic_filebuf

Remarks

basic_filebuf::char_type

basic_filebuf::close

Return Value

Remarks

Example

basic_filebuf::int_type

basic_filebuf::is_open

Return Value

Example

basic_filebuf::off_type

basic_filebuf::open

Parameters

Return Value

Remarks

Example

basic_filebuf::operator=

Parameters

Return Value

Remarks

basic_filebuf::overflow

Parameters

Return Value

Remarks

basic_filebuf::pbackfail

Parameters

Return Value

Remarks

basic_filebuf::pos_type

basic_filebuf::seekoff

Parameters

Return Value

Remarks

basic_filebuf::seekpos

Parameters

Return Value

Remarks

basic_filebuf::setbuf

Parameters

Return Value

Remarks

basic_filebuf::swap

Parameters

basic_filebuf::sync

Return Value

basic_filebuf::traits_type

basic_filebuf::underflow

Return Value

Remarks

See also

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