class 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.
Member types
traits_type | Traits |
Member functions
| (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) |
std::basic_istream::sentry::sentry
explicit sentry(std::basic_istream<CharT,Traits>& is, bool noskipws = false); |
Prepares the stream for formatted input.
If is.good() is false, calls is.setstate(failbit)(since c++11) and returns. Otherwise, if is.tie() is not a null pointer, calls is.tie()->flush() to synchronize the output sequence with external streams. This call can be suppressed if the put area of is.tie() is empty. The implementation may defer the call to flush until a call of is.rdbuf()->underflow() occurs. If no such call occurs before the sentry object is destroyed, it may be eliminated entirely.
If noskipws is zero and is.flags() & ios_base::skipws is nonzero, the function extracts and discards all whitespace characters until the next available character is not a whitespace character (as determined by the currently imbued locale in is). If is.rdbuf()->sbumpc() or is.rdbuf()->sgetc() returns traits::eof(), the function calls setstate(failbit | eofbit) (which may throw std::ios_base::failure).
Additional implementation-defined preparation may take place, which may call setstate(failbit) (which may throw std::ios_base::failure).
If after preparation is completed, is.good() == true, then any subsequent calls to operator bool will return true.
Parameters
| is | - | input stream to prepare |
| noskipws | - | true if whitespace should not be skipped |
Exceptions
std::ios_base::failure if the end of file condition occurs when skipping whitespace.
std::basic_istream::sentry::~sentry
~sentry(); |
Does nothing.
std::basic_istream::sentry::operator bool
explicit operator bool() const; |
Checks whether the preparation of the input stream was successful.
Parameters
(none).
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';
}Output:
abcde
See also
| extracts formatted data (public member function) | |
| extracts characters and character arrays (function template) |
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