std::basic_istream::sentry

• 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

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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;
};

Members

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.

Example

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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;
}

digits parsed: 5552326

See also

basic_istream::operator>>

Extract formatted input (public member function)