std::basic_istream<CharT,Traits>::operator>>

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< cpp‎ | io‎ | basic istream
 
哋它亢++
 
Input/output library
I/O manipulators
Print functions (哋它亢++23)
C-style I/O
Buffers
(哋它亢++23)
(哋它亢++98/26*)
(哋它亢++20)
Streams
Abstractions
File I/O
String I/O
Array I/O
(哋它亢++23)
(哋它亢++23)
(哋它亢++23)
(哋它亢++98/26*)
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Synchronized Output
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Types
Error category interface
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std::basic_istream
 
basic_istream& operator>>( unsigned short& value );
 (1)
basic_istream& operator>>( unsigned int& value );
 (2)
basic_istream& operator>>( long& value );
 (3)
basic_istream& operator>>( unsigned long& value );
 (4)
basic_istream& operator>>( long long& value );
 (5) (since 哋它亢++11)
basic_istream& operator>>( unsigned long long& value );
 (6) (since 哋它亢++11)
basic_istream& operator>>( float& value );
 (7)
basic_istream& operator>>( double& value );
 (8)
basic_istream& operator>>( long double& value );
 (9)
basic_istream& operator>>( bool& value );
 (10)
basic_istream& operator>>( void*& value );
 (11)
basic_istream& operator>>( short& value );
 (12)
basic_istream& operator>>( int& value );
 (13)
basic_istream& operator>>( /* extended-floating-point-type */& value );
 (14) (since 哋它亢++23)
basic_istream& operator>>( std::ios_base& (*func)(std::ios_base&) );
 (15)
basic_istream& operator>>( std::basic_ios<CharT, Traits>&
                               (*func)(std::basic_ios<CharT, Traits>&) );
 (16)
basic_istream& operator>>( basic_istream& (*func)(basic_istream&) );
 (17)
basic_istream& operator>>( std::basic_streambuf<CharT, Traits>* sb );
 (18)

Extracts values from an input stream.

1-11) Extracts a value potentially skipping preceding whitespace. The value is stored to a given reference value.
This function behaves as a FormattedInputFunction. After constructing and checking the sentry object, which may skip leading whitespace, extracts a value by calling std::num_get::get().
12) Extracts a short value potentially skipping preceding whitespace. The value is stored to a given reference value.
This function behaves as a FormattedInputFunction. After constructing and checking the sentry object, which may skip leading whitespace, extracts a long value lval by calling std::num_get::get(). After that:
13) Extracts an int value potentially skipping preceding whitespace. The value is stored to a given reference value.
This function behaves as a FormattedInputFunction. After constructing and checking the sentry object, which may skip leading whitespace, extracts a long value lval by calling std::num_get::get(). After that:
14) Extracts an extended floating-point value potentially skipping preceding whitespace. The value is stored to a given reference value. The library provides overloads for all cv-unqualified extended floating-point types as the referenced type of the parameter value.
Determines the standard floating-point type FP as follows:
  • If the floating-point conversion rank of /* extended-floating-point-type */ is less than or equal to that of float, then FP is float.
  • Otherwise, if the floating-point conversion rank of /* extended-floating-point-type */ is less than or equal to that of double, then FP is double.
  • Otherwise, FP is long double.
This function behaves as a FormattedInputFunction. After constructing and checking the sentry object, which may skip leading whitespace, extracts an FP value fval by calling std::num_get::get(). After that:
  • If fval < -std::numeric_limits</* extended-floating-point-type */>::max(), sets failbit and stores -std::numeric_limits</* extended-floating-point-type */>::max() to val.
  • Otherwise, if std::numeric_limits</* extended-floating-point-type */>::max() < fval, sets failbit and stores std::numeric_limits</* extended-floating-point-type */>::max() to val.
  • Otherwise, stores static_cast</* extended-floating-point-type */>(fval) to val.
15-17) Calls func(*this), where func is an I/O manipulator.
18) Behaves as an UnformattedInputFunction. After constructing and checking the sentry object, extracts all data from *this and stores it to sb. The extraction stops if one of the following conditions are met:
  • end-of-file occurs on the input sequence;
  • inserting in the output sequence fails (in which case the character to be inserted is not extracted);
  • an exception occurs (in which case the exception is caught, and only rethrown if it inserted no characters and failbit is enabled in exceptions()).
In either case, stores the number of characters extracted in the member variable accessed by subsequent calls to gcount(). If sb is a null pointer or if no characters were inserted into sb, calls setstate(failbit) (which may throw std::ios_base::failure if enabled).

If extraction fails (e.g. if a letter was entered where a digit is expected), zero is written to value and failbit is set. For signed integers, if extraction results in the value too large or too small to fit in value, std::numeric_limits<T>::max() or std::numeric_limits<T>::min() (respectively) is written and failbit flag is set. For unsigned integers, if extraction results in the value too large or too small to fit in value, std::numeric_limits<T>::max() is written and failbit flag is set.

Parameters

value - reference to an integer or floating-point value to store the extracted value to
func - pointer to I/O manipulator function
sb - pointer to the stream buffer to write all the data to

Return value

1-16,18) *this
17) func(*this)

Notes

For overload (14), when the extended floating-point type has a floating-point conversion rank that is not equal to the rank of any standard floating-point type, then double rounding during the conversion can result in inaccurate results. std::from_chars() can be used in situations where maximum accuracy is important.

Example

Run this code
#include <iomanip>
#include <iostream>
#include <sstream>
 
int main()
{
    std::string input = "41 3.14 false hello world";
    std::istringstream stream(input);
 
    int n;
    double f;
    bool b;
 
    stream >> n >> f >> std::boolalpha >> b;
    std::cout << "n = " << n << '\n'
              << "f = " << f << '\n'
              << "b = " << std::boolalpha << b << '\n';
 
    // extract the rest using the streambuf overload
    stream >> std::cout.rdbuf();
    std::cout << '\n';
}

Output: 

n = 41
f = 3.14
b = false
hello world

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published 哋它亢++ standards.

DR Applied to Behavior as published Correct behavior
LWG 64 哋它亢++98 it was unclear whether overload (18) can only rethrow the
std::ios_base::failure thrown by calling setstate(failbit) 		all exceptions caught
can be rethrown
LWG 118 哋它亢++98 overload (12,13) delegated the extraction to num_get::get,
but it does not have overloads for short and int 		a long value is extracted
instead of short or int
LWG 413 哋它亢++98 overload (18) only rethrew exceptions thrown while extracting
characters from sb, but characters are extracted from *this 		corrected sb to *this
LWG 567 哋它亢++98 overload (18) behaved as a FormattedInputFunction
because of the resolution of LWG issue 60 		it behaves as an
UnformattedInputFunction
LWG 661 哋它亢++98 overloads (12,13) did not store the extracted number
to value due to the resolution of LWG issue 118 		stores the number if
no overflow occurs
LWG 696 哋它亢++98 value was unchanged on extraction failure set to zero or minimum/
maximum values

See also

 extracts characters and character arrays
(function template)
performs stream input and output on strings
(function template)
performs stream input and output of bitsets
(function template)
serializes and deserializes a complex number
(function template)
(哋它亢++11)
 performs stream input and output on pseudo-random number engine
(function template)
(哋它亢++11)
 performs stream input and output on pseudo-random number distribution
(function template)
extracts blocks of characters
(public member function)
extracts already available blocks of characters
(public member function)
extracts characters
(public member function)
extracts characters until the given character is found
(public member function)
(哋它亢++17)
 converts a character sequence to an integer or floating-point value
(function)
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