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Showing content from https://en.cppreference.com/w/cpp/language/../named_req/../io/basic_ostream/sentry.html below:

std::basic_ostream<CharT,Traits>::sentry - cppreference.com

An object of class basic_ostream::sentry is constructed in local scope at the beginning of each member function of std::basic_ostream that performs output (both formatted and unformatted). Its constructor prepares the output stream: checks if the stream is already in a failed state, flushes the tie()'d output streams, and performs other implementation-defined tasks if necessary. Implementation-defined cleanup, as well as flushing of the output stream if necessary, is performed in the destructor, so that it is guaranteed to happen if exceptions are thrown during output.

[edit] Member functions constructs the sentry object. All the preparation tasks are done here
(public member function) [edit] finalizes the stream object after formatted output or after exception, if necessary
(public member function) the assignment operator is deleted
(public member function) checks if the preparation of the stream object was successful
(public member function) [edit] std::basic_ostream::sentry::sentry

Prepares the stream for formatted output.

If os.good() is false, returns. Otherwise, if os.tie() is not a null pointer, calls os.tie()->flush() to synchronize the output sequence with external streams. During preparation, the constructor may call setstate(failbit) (which may throw std::ios_base::failure).

If after preparation is completed, os.good() == true, then any subsequent calls to operator bool will return true.

Parameters os - output stream to prepare Exceptions

std::ios_base::failure if the end of file condition occurs.

std::basic_ostream::sentry::~sentry

If (os.flags() & std::ios_base::unitbuf) && !std::uncaught_exception() && os.good()) is true, calls os.rdbuf()->pubsync(). If that function returns -1, sets badbit in os.rdstate() without propagating an exception.

std::basic_ostream::sentry::operator bool

explicit operator bool() const;

Checks whether the preparation of the output stream was successful.

Parameters

(none)

Return value

true if the preparation of the output stream was successful, false otherwise.

[edit] Example

#include <iostream>
#include <sstream>
 
struct Foo
{
    char n[6];
};
 
std::ostream& operator<<(std::ostream& os, Foo& f)
{
    std::ostream::sentry s(os);
    if (s)
        os.write(f.n, 5);
    return os;
}
 
int main()
{
    Foo f = {"abcde"};
    std::cout << f << '\n';
}

Output:

[edit] 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 397 C++98 the destructor might call os.flush(), which may throw exceptions the exception is not propagated LWG 442 C++98 operator bool was not declared const (it is const in the synopsis) added const LWG 835 C++98 if os sets unitbuf, the destructor would call os.flush(), which
is an UnformattedOutputFunction and creates another sentry object
(whose destructor then creates another sentry object and so on) calls
os.rdbuf()->pubsync()
in this case instead [edit] See also

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