NAME¶

std::ws - std::ws

Synopsis¶

Defined in header <istream>
template< class CharT, class Traits >
std::basic_istream<CharT, Traits>& ws( std::basic_istream<CharT, Traits>& is );

Discards leading whitespace from an input stream.

Behaves as an UnformattedInputFunction, except that is.gcount() is not modified.
After constructing and checking the sentry object, extracts characters from the
stream and discards them until any one of the following conditions occurs:

* end of file condition occurs in the input sequence (in which case the function
calls setstate(eofbit) but does not set failbit; this does not apply if the
eofbit is already set on is prior to the call to ws, in which case the
construction of the sentry object would set failbit).
* the next available character c in the input sequence is not whitespace as
determined by std::isspace(c, is.getloc()). The non-whitespace character is not
extracted.

This is an input-only I/O manipulator, it may be called with an expression such as
in >> std::ws for any in of type std::basic_istream.

Parameters¶

is - reference to input stream

Return value¶

is (reference to the stream after extraction of consecutive whitespace).

Notes¶

If eofbit is set on the stream prior to the call, the construction of the sentry
object will set failbit.

Example¶

// Run this code

#include <iomanip>
#include <iostream>
#include <istream>
#include <sstream>
#include <string>

int main()
{
for (const char* str : {" #1 test", "\t #2 test", "#3 test"})
{
std::string line;
std::getline(std::istringstream{str}, line);
std::cout << "getline returns:\t" << std::quoted(line) << '\n';

std::istringstream iss{str};
std::getline(iss >> std::ws, line);
std::cout << "ws + getline returns:\t" << std::quoted(line) << '\n';
}
}

Output:¶

getline returns: " #1 test"
ws + getline returns: "#1 test"
getline returns: " #2 test"
ws + getline returns: "#2 test"
getline returns: "#3 test"
ws + getline returns: "#3 test"

Defect reports

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

DR Applied to Behavior as published Correct behavior
calling std::ws might not construct the
LWG 415 C++98 sentry required to construct
object (insonsistent with other input the sentry object
functions)

See also¶

ignore extracts and discards characters until the given character is found
(public member function of std::basic_istream<CharT,Traits>)