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std::basic_streambuf<CharT,Traits>:: sputn, std::basic_streambuf<CharT,Traits>:: xsputn

From cppreference.net
C++
Input/output library
std::basic_streambuf
std:: streamsize sputn ( const char_type * s, std:: streamsize count ) ;
(1)
protected :
virtual std:: streamsize xsputn ( const char_type * s, std:: streamsize count ) ;
(2)
1) Calls xsputn ( s, count ) of the most derived class.
2) Writes count characters to the output sequence from the character array whose first element is pointed to by s . The characters are written as if by repeated calls to sputc() . Writing stops when either count characters are written or a call to sputc() would have returned Traits :: eof ( ) .

If the put area becomes full ( pptr ( ) == epptr ( ) ), it is unspecified whether overflow() is actually called or its effect is achieved by other means.

Contents

Parameters

(none)

Return value

The number of characters successfully written.

Notes

"achieved by other means" permits bulk I/O without intermediate buffering: that is how std::ofstream::write() simply passes the pointer to the suitable system call in some implementations.

Example

Run this code 
#include <iostream>
#include <sstream>
int main()
{
    std::ostringstream s1;
    std::streamsize sz = s1.rdbuf()->sputn("This is a test", 14);
    s1 << '\n';
    std::cout << "The call to sputn() returned " << sz << '\n'
              << "The output sequence contains " << s1.str();
    std::istringstream s2;
    sz = s2.rdbuf()->sputn("This is a test", 14);
    std::cout << "The call to sputn() on an input stream returned " << sz << '\n';
}

Output: 

The call to sputn() returned 14
The output sequence contains This is a test
The call to sputn() on an input stream returned 0

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 565 C++98 xsputn() always called overflow() if pptr ( ) == epptr ( ) it does not actually need to be called

See also

invokes xsgetn ( )
(public member function)