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Showing content from https://cplusplus.com/reference/istream/basic_istream/sentry/ below:

public member class

<istream> <iostream>

Prepare stream for input

Its constructor performs the following operations on the stream object passed as its argument (in the same order):

• If any of its internal error flags is set, the function sets its failbit flag and returns.
• If it is a tied stream, the function flushes the stream it is tied to (if its output buffer is not empty). The class may implement ways for library functions to defer this flush until the next call to overflow by its associated stream buffer.
• If its skipws format flag is set, and the constructor is not passed true as second argument (noskipws), all leading whitespace characters (locale-specific) are extracted and discarded. If this operation exhausts the source of characters, the function sets both the failbit and eofbit internal state flags

There are no required operations to be performed by its destructor. But implementations may use the construction and destruction of sentry objects to perform additional initialization or cleanup operations on the stream common to all input operations.

All member functions that perform an input operation automatically construct an object of this class and then evaluate it (which returns true if no state flag was set). Only if this object evaluates to true, the function attempts the input operation (otherwise, it returns without performing it). Before returning, the function destroys the sentry object.

The operator>> formatted input operations construct the sentry object by passing false as second argument (which skips leading whitespaces). All other member functions that construct a sentry object pass true as second argument (which does not skip leading whitespaces).

The structure of this class is:

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class sentry {
public:
  explicit sentry (basic_istream& is, bool noskipws = false);
  ~sentry();
  operator bool() const;
private:
  sentry (const sentry&);             // not defined
  sentry& operator= (const sentry&);  // not defined
};

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class sentry {
public:
  explicit sentry (basic_istream& is, bool noskipws = false);
  ~sentry();
  explicit operator bool() const;
  sentry (const sentry&) = delete;
  sentry& operator= (const sentry&) = delete;
};

explicit sentry (basic_istream& is, bool noskipws = false);

Prepares the output stream for an output operation, performing the actions described above.

~sentry();

Performs no operations (implementation-defined).

explicit operator bool() const;

When the object is evaluated, it returns a bool value indicating whether the sentry constructor successfully performed all its tasks: If at some point of the construction process, an internal error flags was set, this function always returns false for that object.

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// istream::sentry example
#include <iostream>     // std::istream, std::cout
#include <string>       // std::string
#include <sstream>      // std::stringstream
#include <locale>       // std::isspace, std::isdigit

struct Phone {
  std::string digits;
};

// custom extractor for objects of type Phone
std::istream& operator>>(std::istream& is, Phone& tel)
{
    std::istream::sentry s(is);
    if (s) while (is.good()) {
      char c = is.get();
      if (std::isspace(c,is.getloc())) break;
      if (std::isdigit(c,is.getloc())) tel.digits+=c;
    }
    return is;
}

int main () {
  std::stringstream parseme ("   (555)2326");
  Phone myphone;
  parseme >> myphone;
  std::cout << "digits parsed: " << myphone.digits << '\n';
  return 0;
}

basic_istream::operator>>

Extract formatted input (public member function)

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