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std::basic_stringbuf::underflow(3) | C++ Standard Libary | std::basic_stringbuf::underflow(3) |
NAME¶
std::basic_stringbuf::underflow - std::basic_stringbuf::underflow
Synopsis¶
protected:
virtual int_type underflow()
Reads the next character from the get area of the buffer.
Specifically:
1) If the input sequence has a read position available (egptr() > gptr()),
returns
Traits::to_int_type(*gptr())
2) Otherwise, if pptr() > egptr() (some characters were inserted into the
stream
since the last time overflow() changed egptr()) then extends the end of the
get area
to include the most recently inserted characters, by changing egptr() to
equal
pptr(), and then returns Traits::to_int_type(*gptr())
3) Otherwise, returns Traits::eof().
Any character in the buffer which has been initialized, regardless of whether
it
originated from the string passed in the constructor or was appended by
overflow(),
is considered to be part of the input sequence.
Parameters¶
(none)
Return value¶
Traits::to_int_type(*gptr()) (the next character to read in the
get area) in case of
success, or Traits::eof() in case of failure.
Example¶
// Run this code
#include <sstream>
#include <iostream>
struct mybuf : std::stringbuf
{
mybuf(const std::string& new_str,
std::ios_base::openmode which = std::ios_base::in|std::ios_base::out)
: std::stringbuf(new_str, which) {}
int_type overflow(int_type c) {
std::cout << "Before overflow(): get area size is " <<
egptr()-eback() << ' '
<< " the put area size is " << epptr()-pbase() <<
'\n';
int_type rc = std::stringbuf::overflow(c);
std::cout << "After overflow(): get area size is " <<
egptr()-eback() << ' '
<< " put area size is " << epptr()-pbase() <<
'\n';
return rc;
}
int_type underflow() {
std::cout << "Before underflow(): get area size is " <<
egptr()-eback() << ' '
<< " put area size is " << epptr()-pbase() <<
'\n';
int_type ch = std::stringbuf::underflow();
std::cout << "After underflow(): get area size is " <<
egptr()-eback() << ' '
<< " put area size is " << epptr()-pbase() <<
'\n';
if(ch == EOF)
std::cout << "underflow() returns EOF\n";
else
std::cout << "underflow() returns '" << char(ch)
<< "'\n";
return ch;
}
};
int main()
{
mybuf sbuf("123"); // read-write stream
std::iostream stream(&sbuf);
int n;
stream >> n; // calls sgetc() four times
// three calls return the characters '1', '2', '3'
// the fourth call, gptr() == egptr() and underflow() is called
// underflow returns EOF
std::cout << n << '\n';
stream.clear(); // clear the eofbit
stream << "123456"; // sputc() is called 6 times
// first three calls store "123" in the existing buffer
// 4th call finds that pptr() == epptr() and calls overflow()
// overflow() grows the buffer and sets egptr() to 4
// 5th and 6th calls store '5' and '6', advancing pptr()
stream >> n; // calls sgetc() 4 times
// 1st call returns the '4' that was made available by overflow()
// on the 2nd call, egptr() == egptr() and underflow() is called
// underflow advances egptr() to equal pptr() (which is 6)
// 3rd sgetc() returns '6'
// 4th sgetc() finds gptr() == egptr(), calls underflow()
// underflow() returns EOF
std::cout << n << '\n';
}
Possible output:¶
Before underflow(): get area size is 3 put area size is 3
After underflow(): get area size is 3 put area size is 3
underflow() returns EOF
123
Before overflow(): get area size is 3 the put area size is 3
After overflow(): get area size is 4 put area size is 35
Before underflow(): get area size is 4 put area size is 35
After underflow(): get area size is 6 put area size is 35
underflow() returns '5'
Before underflow(): get area size is 6 put area size is 35
After underflow(): get area size is 6 put area size is 35
underflow() returns EOF
456
See also¶
underflow reads characters from the associated input sequence to
the get area
[virtual] (virtual protected member function of
std::basic_streambuf<CharT,Traits>)
underflow reads from the associated file
[virtual] (virtual protected member function of
std::basic_filebuf<CharT,Traits>)
underflow reads a character from the input sequence without advancing the
next
[virtual] pointer
(virtual protected member function of std::strstreambuf)
sgetc reads one character from the input sequence without advancing the
sequence
(public member function of
std::basic_streambuf<CharT,Traits>)
2022.07.31 | http://cppreference.com |