std::basic_istream::read

basic_istream& read( char_type* s, std::streamsize count );

Extracts characters from stream.

Behaves as UnformattedInputFunction. After constructing and checking the sentry object, extracts characters and stores them into successive locations of the character array whose first element is pointed to by s. Characters are extracted and stored until any of the following conditions occurs:

count characters were extracted and stored

end of file condition occurs on the input sequence (in which case, setstate(failbit|eofbit) is called). The number of successfully extracted characters can be queried using gcount().

Parameters

s	-	pointer to the character array to store the characters to
count	-	number of characters to read

Return value

*this

Exceptions

failure if an error occurred (the error state flag is not goodbit) and exceptions() is set to throw for that state.

If an internal operation throws an exception, it is caught and badbit is set. If exceptions() is set for badbit, the exception is rethrown.

Notes

When using a non-converting locale (the default locale is non-converting), the overrider of this function in std::basic_ifstream may be optimized for zero-copy bulk I/O (by means of overriding std::streambuf::xsgetn)

Example

Run this code

#include <iostream>
#include <fstream>
#include <sstream>
#include <string>
#include <cstdint>

int main()
{
    // read() is often used for binary I/O
    std::string bin = {'\x12', '\x12', '\x12', '\x12'};
    std::istringstream raw(bin);
    std::uint32_t n;
    if(raw.read(reinterpret_cast<char*>(&n), sizeof n))
        std::cout << std::hex << std::showbase << n << '\n';

    // prepare file for next snippet
    std::ofstream("test.txt", std::ios::binary) << "abcd1\nabcd2\nabcd3";

    // read entire file into string
    if(std::ifstream is{"test.txt", std::ios::binary | std::ios::ate}) {
        auto size = is.tellg();
        std::string str(size, '\0'); // construct string to stream size
        is.seekg(0);
        if(is.read(&str[0], size))
            std::cout << str << '\n';
    }
}

Output:

0x12121212
abcd1
abcd2
abcd3

Language
Headers
Freestanding and hosted implementations
Named requirements
Language support library
Concepts library (C++20)
Diagnostics library
Utilities library
Strings library
Containers library
Algorithms library
Iterators library
Numerics library
Input/output library
Localizations library
Regular expressions library (C++11)
Atomic operations library (C++11)
Thread support library (C++11)
Filesystem library (C++17)
Technical Specifications

I/O manipulators
C-style I/O
Buffers
basic_streambuf
basic_filebuf
basic_stringbuf
strstreambuf (deprecated)
basic_syncbuf (C++20)
Streams
Abstractions
ios_base
basic_ios
basic_istream
basic_ostream
basic_iostream
File I/O
basic_ifstream
basic_ofstream
basic_fstream
String I/O
basic_istringstream
basic_ostringstream
basic_stringstream
Array I/O
istrstream (deprecated)
ostrstream (deprecated)
strstream (deprecated)
Synchronized Output
basic_osyncstream (C++20)
Types
streamoff
streamsize
fpos
Error category interface
iostream_category (C++11)
io_errc (C++11)

Global objects
cinwcin
Member functions
basic_istream::basic_istream
basic_istream::~basic_istream
basic_istream::operator= (C++11)
Formatted input
basic_istream::operator>>
Unformatted input
basic_istream::get
basic_istream::peek
basic_istream::unget
basic_istream::putback
basic_istream::getline
basic_istream::ignore
basic_istream::read
basic_istream::readsome
basic_istream::gcount
Positioning
basic_istream::tellg
basic_istream::seekg
Miscellaneous
basic_istream::sync
basic_istream::swap (C++11)
Member classes
basic_istream::sentry
Non-member functions
operator>>(std::basic_istream)