std::basic_istream::sentry

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Defined in header `<istream>`

template< class CharT, class Traits = std::char_traits<CharT>> class 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 if skipws 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.


(constructor)	constructs the sentry object. All the preparation tasks are done here (public member function)

(destructor)	finalizes the stream object after formatted input or after exception, if necessary (public member function)

operator= [deleted]	not copy assignable (public member function)

operator bool	checks if the preparation of the stream object was successful (public member function)

[edit] Example

run this code

#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';
}

Output:

abcde

[edit] See also


operator>>	extracts formatted data (public member function)

operator>>(std::basic_istream)	extracts characters and character arrays (function template)

Language
Standard library headers
Concepts
Utilities library
Strings library
Containers library
Algorithms library
Iterators library
Numerics library
Input/output library
Localizations library
Regular expressions library (C++11)
Atomic operations library (C++11)
Thread support library (C++11)

std::basic_istream::sentry

Contents

[edit] Member types

[edit] Member functions

[edit] Example

[edit] See also