std::basic_istream<CharT,Traits>::read (3) - Linux Manuals

std::basic_istream<CharT,Traits>::read: std::basic_istream<CharT,Traits>::read

NAME

std::basic_istream<CharT,Traits>::read - std::basic_istream<CharT,Traits>::read

Synopsis

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