[filebuf.members]

# 31 Input/output library [[input.output]](./#input.output)

## 31.10 File-based streams [[file.streams]](file.streams#filebuf.members)

### 31.10.3 Class template basic_filebuf [[filebuf]](filebuf#members)

#### 31.10.3.4 Member functions [filebuf.members]

[🔗](#lib:is_open,basic_filebuf)

`bool is_open() const;
`

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L11209)

*Returns*: true if a previous call toopen succeeded (returned a non-null value) and there has been no intervening
call to close[.](#1.sentence-1)

[🔗](#lib:open,basic_filebuf)

`basic_filebuf* open(const char* s, ios_base::openmode mode);
basic_filebuf* open(const filesystem::path::value_type* s,
                    ios_base::openmode mode);  // wide systems only; see [[fstream.syn]](fstream.syn "31.10.1 Header <fstream> synopsis")
`

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L11226)

*Preconditions*: s points to an NTCTS ([[defns.ntcts]](defns.ntcts "3.36 NTCTS"))[.](#2.sentence-1)

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L11230)

*Effects*: Ifis_open() != false,
returns a null pointer[.](#3.sentence-1)

Otherwise,
initializes thefilebuf as required[.](#3.sentence-2)

It then opens
the file to which s resolves, if possible,
as if by a call to fopenwith the second argument determined frommode & ~ios_base​::​ate as indicated in Table [146](#tab:filebuf.open.modes "Table 146: File open modes")[.](#3.sentence-3)

If mode is not some combination of flags shown in the table then
the open fails[.](#3.sentence-4)

Table [146](#tab:filebuf.open.modes) — File open modes [[tab:filebuf.open.modes]](./tab:filebuf.open.modes)  

| [🔗](#tab:filebuf.open.modes-row-1)<br>ios_base flag combination | | | | | | stdio equivalent |
| --- | --- | --- | --- | --- | --- | --- |
| [🔗](#tab:filebuf.open.modes-row-2)<br>binary | in | out | trunc | app | noreplace |
| [🔗](#tab:filebuf.open.modes-row-3) |  | + |  |  |  | "w" |
| [🔗](#tab:filebuf.open.modes-row-4) |  | + |  |  | + | "wx" |
| [🔗](#tab:filebuf.open.modes-row-5) |  | + | + |  |  | "w" |
| [🔗](#tab:filebuf.open.modes-row-6) |  | + | + |  | + | "wx" |
| [🔗](#tab:filebuf.open.modes-row-7) |  | + |  | + |  | "a" |
| [🔗](#tab:filebuf.open.modes-row-8) |  |  |  | + |  | "a" |
| [🔗](#tab:filebuf.open.modes-row-9) | + |  |  |  |  | "r" |
| [🔗](#tab:filebuf.open.modes-row-10) | + | + |  |  |  | "r+" |
| [🔗](#tab:filebuf.open.modes-row-11) | + | + | + |  |  | "w+" |
| [🔗](#tab:filebuf.open.modes-row-12) | + | + | + |  | + | "w+x" |
| [🔗](#tab:filebuf.open.modes-row-13) | + | + |  | + |  | "a+" |
| [🔗](#tab:filebuf.open.modes-row-14) | + |  |  | + |  | "a+" |
| [🔗](#tab:filebuf.open.modes-row-15)<br>+ |  | + |  |  |  | "wb" |
| [🔗](#tab:filebuf.open.modes-row-16)<br>+ |  | + |  |  | + | "wbx" |
| [🔗](#tab:filebuf.open.modes-row-17)<br>+ |  | + | + |  |  | "wb" |
| [🔗](#tab:filebuf.open.modes-row-18)<br>+ |  | + | + |  | + | "wbx" |
| [🔗](#tab:filebuf.open.modes-row-19)<br>+ |  | + |  | + |  | "ab" |
| [🔗](#tab:filebuf.open.modes-row-20)<br>+ |  |  |  | + |  | "ab" |
| [🔗](#tab:filebuf.open.modes-row-21)<br>+ | + |  |  |  |  | "rb" |
| [🔗](#tab:filebuf.open.modes-row-22)<br>+ | + | + |  |  |  | "r+b" |
| [🔗](#tab:filebuf.open.modes-row-23)<br>+ | + | + | + |  |  | "w+b" |
| [🔗](#tab:filebuf.open.modes-row-24)<br>+ | + | + | + |  | + | "w+bx" |
| [🔗](#tab:filebuf.open.modes-row-25)<br>+ | + | + |  | + |  | "a+b" |
| [🔗](#tab:filebuf.open.modes-row-26)<br>+ | + |  |  | + |  | "a+b" |

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L11280)

If the open operation succeeds andios_base​::​ate is set in mode,
positions the file to the end
(as if by calling fseek(file, 0, SEEK_END), wherefile is the pointer returned by calling fopen)[.](#4.sentence-1)[292](#footnote-292 "The macro SEEK_­END is defined, and the function signatures fopen(const char*, const char*) and fseek(FILE*, long, int) are declared, in <cstdio>.")

[5](#5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L11297)

If the repositioning operation fails, callsclose() and returns a null pointer to indicate failure[.](#5.sentence-1)

[6](#6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L11302)

*Returns*: this if successful, a null pointer otherwise[.](#6.sentence-1)

[🔗](#lib:open,basic_filebuf_)

`basic_filebuf* open(const string& s, ios_base::openmode mode);
basic_filebuf* open(const filesystem::path& s, ios_base::openmode mode);
`

[7](#7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L11315)

*Returns*: open(s.c_str(), mode);

[🔗](#lib:close,basic_filebuf)

`basic_filebuf* close();
`

[8](#8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L11326)

*Effects*: Ifis_open() == false,
returns a null pointer[.](#8.sentence-1)

If a put area exists, callsoverflow(traits​::​​eof()) to flush characters[.](#8.sentence-2)

If the last virtual member function called on*this (betweenunderflow,overflow,seekoff,
andseekpos)
wasoverflow then callsa_codecvt.unshift (possibly several times) to determine a termination sequence, inserts those
characters and callsoverflow(traits​::​​eof()) again[.](#8.sentence-3)

Finally, regardless of whether any of the preceding calls fails or throws an
exception, the function closes the file
(as if by callingfclose(file))[.](#8.sentence-4)

If any of the calls made by the function, including fclose, fails,close fails by returning a null pointer[.](#8.sentence-5)

If one of these calls throws an
exception, the exception is caught and rethrown after closing the file[.](#8.sentence-6)

[9](#9)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L11359)

*Postconditions*: is_open() == false[.](#9.sentence-1)

[10](#10)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L11363)

*Returns*: this on success, a null pointer otherwise[.](#10.sentence-1)

[🔗](#lib:native_handle,basic_filebuf)

`native_handle_type native_handle() const noexcept;
`

[11](#11)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L11375)

*Preconditions*: is_open() is true[.](#11.sentence-1)

[12](#12)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L11379)

*Returns*: The native handle associated with *this[.](#12.sentence-1)

[292)](#footnote-292)[292)](#footnoteref-292)

The macro SEEK_END is defined, and the function signaturesfopen(const char*, const char*) andfseek(FILE*, long, int)are declared, in [<cstdio>](cstdio.syn#header:%3ccstdio%3e "31.13.1 Header <cstdio> synopsis [cstdio.syn]")[.](#footnote-292.sentence-1)