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

An object of class basic_istream::sentry is constructed in local scope at the beginning of each member function of std::basic_istream that performs input (both formatted and unformatted). Its constructor prepares the input stream: checks if the stream is already in a failed state, flushes the tie()’d output streams, skips leading whitespace unless noskipws flag is set, and performs other implementation-defined tasks if necessary. All cleanup, if necessary, is performed in the destructor, so that it is guaranteed to happen if exceptions are thrown during input.

# Declarations

class sentry;

# Parameters

• is: input stream to prepare
• noskipws: true if whitespace should not be skipped

# Return value

true if the initialization of the input stream was successful, false otherwise.

# Example

#include <iostream>
#include <sstream>
 
struct Foo
{
    char n[5];
};
 
std::istream& operator>>(std::istream& is, Foo& f)
{
    std::istream::sentry s(is);
    if (s)
        is.read(f.n, 5);
    return is;
}
 
int main()
{
    std::string input = "   abcde";
    std::istringstream stream(input);
    Foo f;
    stream >> f;
    std::cout.write(f.n, 5);
    std::cout << '\n';
}

# Defect reports

# See also

• operator»
• operator»(std::basic_istream)