# std::basic_ifstream

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< cpp‎ | io

Defined in header `<fstream>` |  |
template< class CharT, class Traits = std::char_traits<CharT> > class basic_ifstream : public std::basic_istream<CharT, Traits> |  |

The class template `basic_ifstream` implements high-level input operations on file-based streams. It interfaces a file-based streambuffer (std::basic_filebuf) with the high-level interface of (std::basic_istream).

A typical implementation of `std::basic_ifstream` holds only one non-derived data member: an instance of std::basic_filebuf<CharT, Traits>.

Inheritance diagram

Several typedefs for common character types are provided:

Defined in header `<fstream>`
Type | Definition
`std::ifstream` | std::basic_ifstream<char>
`std::wifstream` | std::basic_ifstream<wchar_t>

### Member types

Member type | Definition
`char_type` | `CharT`
`traits_type` | `Traits`; the program is ill-formed if `Traits::char_type` is not `CharT`.
`int_type` | `Traits::int_type`
`pos_type` | `Traits::pos_type`
`off_type` | `Traits::off_type`
`native_handle_type`(C++26) | implementation-defined type that is TriviallyCopyable and `semiregular`

### Member functions

(constructor) | constructs the file stream (public member function)
(destructor) [virtual] (implicitly declared) | destructs the `basic_ifstream` and the associated buffer, closes the file (virtual public member function)
operator= (C++11) | moves the file stream (public member function)
swap (C++11) | swaps two file streams (public member function)
rdbuf | returns the underlying raw file device object (public member function)
native_handle (C++26) | returns the underlying implementation-defined handle (public member function)
##### File operations
is_open | checks if the stream has an associated file (public member function)
open | opens a file and associates it with the stream (public member function)
close | closes the associated file (public member function)

### Non-member functions

std::swap(std::basic_ifstream) (C++11) | specializes the std::swap algorithm (function template)

## Inherited from std::basic_istream

### Member functions

##### Formatted input
operator>> | extracts formatted data (public member function of `std::basic_istream<CharT,Traits>`)
##### Unformatted input
get | extracts characters (public member function of `std::basic_istream<CharT,Traits>`)
peek | reads the next character without extracting it (public member function of `std::basic_istream<CharT,Traits>`)
unget | unextracts a character (public member function of `std::basic_istream<CharT,Traits>`)
putback | puts a character into input stream (public member function of `std::basic_istream<CharT,Traits>`)
getline | extracts characters until the given character is found (public member function of `std::basic_istream<CharT,Traits>`)
ignore | extracts and discards characters until the given character is found (public member function of `std::basic_istream<CharT,Traits>`)
read | extracts blocks of characters (public member function of `std::basic_istream<CharT,Traits>`)
readsome | extracts already available blocks of characters (public member function of `std::basic_istream<CharT,Traits>`)
gcount | returns number of characters extracted by last unformatted input operation (public member function of `std::basic_istream<CharT,Traits>`)
##### Positioning
tellg | returns the input position indicator (public member function of `std::basic_istream<CharT,Traits>`)
seekg | sets the input position indicator (public member function of `std::basic_istream<CharT,Traits>`)
##### Miscellaneous
sync | synchronizes with the underlying storage device (public member function of `std::basic_istream<CharT,Traits>`)

### Member classes

sentry | implements basic logic for preparation of the stream for input operations (public member class of `std::basic_istream<CharT,Traits>`)

## Inherited from std::basic_ios

### Member types

Member type | Definition
`char_type` | `CharT`
`traits_type` | `Traits`
`int_type` | `Traits::int_type`
`pos_type` | `Traits::pos_type`
`off_type` | `Traits::off_type`

### Member functions

##### State functions
good | checks if no error has occurred i.e. I/O operations are available (public member function of `std::basic_ios<CharT,Traits>`)
eof | checks if end-of-file has been reached (public member function of `std::basic_ios<CharT,Traits>`)
fail | checks if an error has occurred (public member function of `std::basic_ios<CharT,Traits>`)
bad | checks if a non-recoverable error has occurred (public member function of `std::basic_ios<CharT,Traits>`)
operator! | checks if an error has occurred (synonym of fail()) (public member function of `std::basic_ios<CharT,Traits>`)
operator bool | checks if no error has occurred (synonym of `!`fail()) (public member function of `std::basic_ios<CharT,Traits>`)
rdstate | returns state flags (public member function of `std::basic_ios<CharT,Traits>`)
setstate | sets state flags (public member function of `std::basic_ios<CharT,Traits>`)
clear | modifies state flags (public member function of `std::basic_ios<CharT,Traits>`)
##### Formatting
copyfmt | copies formatting information (public member function of `std::basic_ios<CharT,Traits>`)
fill | manages the fill character (public member function of `std::basic_ios<CharT,Traits>`)
##### Miscellaneous
exceptions | manages exception mask (public member function of `std::basic_ios<CharT,Traits>`)
imbue | sets the locale (public member function of `std::basic_ios<CharT,Traits>`)
rdbuf | manages associated stream buffer (public member function of `std::basic_ios<CharT,Traits>`)
tie | manages tied stream (public member function of `std::basic_ios<CharT,Traits>`)
narrow | narrows characters (public member function of `std::basic_ios<CharT,Traits>`)
widen | widens characters (public member function of `std::basic_ios<CharT,Traits>`)

## Inherited from std::ios_base

### Member functions

##### Formatting
flags | manages format flags (public member function of `std::ios_base`)
setf | sets specific format flag (public member function of `std::ios_base`)
unsetf | clears specific format flag (public member function of `std::ios_base`)
precision | manages decimal precision of floating point operations (public member function of `std::ios_base`)
width | manages field width (public member function of `std::ios_base`)
##### Locales
imbue | sets locale (public member function of `std::ios_base`)
getloc | returns current locale (public member function of `std::ios_base`)
##### Internal extensible array
xalloc [static] | returns a program-wide unique integer that is safe to use as index to `pword()` and `iword()` (public static member function of `std::ios_base`)
iword | resizes the private storage if necessary and access to the long element at the given index (public member function of `std::ios_base`)
pword | resizes the private storage if necessary and access to the void* element at the given index (public member function of `std::ios_base`)
##### Miscellaneous
register_callback | registers event callback function (public member function of `std::ios_base`)
sync_with_stdio [static] | sets whether C++ and C I/O libraries are interoperable (public static member function of `std::ios_base`)
##### Member classes
failure | stream exception (public member class of `std::ios_base`)
Init | initializes standard stream objects (public member class of `std::ios_base`)

### Member types and constants
Type | Explanation
openmode | stream open mode type The following constants are also defined:
Constant | Explanation
`app` | seek to the end of stream before each write
`binary` | open in binary mode
`in` | open for reading
`out` | open for writing
`trunc` | discard the contents of the stream when opening
`ate` | seek to the end of stream immediately after open
`noreplace` (C++23) | open in exclusive mode
(typedef)
fmtflags | formatting flags type The following constants are also defined:
Constant | Explanation
`dec` | use decimal base for integer I/O: see std::dec
`oct` | use octal base for integer I/O: see std::oct
`hex` | use hexadecimal base for integer I/O: see std::hex
`basefield` | dec | oct | hex. Useful for masking operations
`left` | left adjustment (adds fill characters to the right): see std::left
`right` | right adjustment (adds fill characters to the left): see std::right
`internal` | internal adjustment (adds fill characters to the internal designated point): see std::internal
`adjustfield` | left | right | internal. Useful for masking operations
`scientific` | generate floating point types using scientific notation, or hex notation if combined with fixed: see std::scientific
`fixed` | generate floating point types using fixed notation, or hex notation if combined with scientific: see std::fixed
`floatfield` | scientific | fixed. Useful for masking operations
`boolalpha` | insert and extract bool type in alphanumeric format: see std::boolalpha
`showbase` | generate a prefix indicating the numeric base for integer output, require the currency indicator in monetary I/O: see std::showbase
`showpoint` | generate a decimal-point character unconditionally for floating-point number output: see std::showpoint
`showpos` | generate a + character for non-negative numeric output: see std::showpos
`skipws` | skip leading whitespace before certain input operations: see std::skipws
`unitbuf` | flush the output after each output operation: see std::unitbuf
`uppercase` | replace certain lowercase letters with their uppercase equivalents in certain output operations: see std::uppercase
(typedef)
iostate | state of the stream type The following constants are also defined:
Constant | Explanation
`goodbit` | no error
`badbit` | irrecoverable stream error
`failbit` | input/output operation failed (formatting or extraction error)
`eofbit` | associated input sequence has reached end-of-file
(typedef)
seekdir | seeking direction type The following constants are also defined:
Constant | Explanation
`beg` | the beginning of a stream
`end` | the ending of a stream
`cur` | the current position of stream position indicator
(typedef)
event | specifies event type (enum)
event_callback | callback function type (typedef)

### Notes

Feature-test macro | Value | Std | Comment
--- | --- | --- | ---
`__cpp_lib_fstream_native_handle` | 202306L | (C++26) | native handles support

### Example

Run this code

    #include <iostream>
    #include <fstream>
    #include <string>

    int main()
    {
        std::string filename = "Test.b";

        // prepare a file to read
        double d = 3.14;
        std::ofstream(filename, std::ios::binary)
            .write(reinterpret_cast<char*>(&d), sizeof d) << 123 << "abc";

        // open file for reading
        std::ifstream istrm(filename, std::ios::binary);
        if (!istrm.is_open())
            std::cout << "failed to open " << filename << '\n';
        else
        {
            double d;
            istrm.read(reinterpret_cast<char*>(&d), sizeof d); // binary input
            int n;
            std::string s;
            if (istrm >> n >> s)                               // text input
                std::cout << "read back from file: " << d << ' ' << n << ' ' << s << '\n';
        }
    }

Output:

    read back from file: 3.14 123 abc

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