# operator<<,>>(std::basic_string)

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< cpp‎ | string‎ | basic string

Defined in header `<string>` |  |
template< class CharT, class Traits, class Allocator > std::basic_ostream <CharT, Traits>& operator<<( std::basic_ostream <CharT, Traits>& os, const std::basic_string <CharT, Traits, Allocator>& str ); | (1) |
template< class CharT, class Traits, class Allocator > std::basic_istream <CharT, Traits>& operator>>( std::basic_istream <CharT, Traits>& is, std::basic_string <CharT, Traits, Allocator>& str ); | (2) |

1) Behaves as a FormattedOutputFunction. After constructing and checking the sentry object, determines the output format padding.

Then inserts each character from the resulting sequence seq (the contents of str plus padding) to the output stream os as if by calling os.rdbuf()->sputn(seq, n), where n is std::max (os.width(), str.size()) Finally, calls os.width(0) to cancel the effects of std::setw, if any.

Equivalent to return os << std::basic_string_view <CharT, Traits>(str);. | (since C++17)

2) Behaves as a FormattedInputFunction. After constructing and checking the sentry object, which may skip leading whitespace, first clears str with str.erase(), then reads characters from is and appends them to str as if by str.append(1, c), until one of the following conditions becomes true:

- `N` characters are read, where `N` is is.width() if is.width() > 0, otherwise `N` is str.max_size(),
- the end-of-file condition occurs in the stream is, or
- std::isspace (c, is.getloc()) is true for the next character c in is (this whitespace character remains in the input stream).

If no characters are extracted then std::ios::failbit is set on is, which may throw std::ios_base::failure.

Finally, calls is.width(0) to cancel the effects of std::setw, if any.

### Exceptions

1) May throw std::ios_base::failure if an exception is thrown during output.
2) May throw std::ios_base::failure if no characters are extracted from is (e.g. the stream is at end of file, or consists of whitespace only), or if an exception is thrown during input.

### Parameters

os | - | a character output stream
is | - | a character input stream
str | - | the string to be inserted or extracted

### Return value

1) os
2) is

### Example

Run this code

    #include <iostream>
    #include <sstream>
    #include <string>

    int main()
    {
        std::string greeting = "Hello, whirled!";
        std::istringstream iss(greeting);

        std::string hello_comma, whirled, word;

        iss >> hello_comma;
        iss >> whirled;

        std::cout << greeting << '\n'
                  << hello_comma << '\n' << whirled << '\n';

        // Reset the stream
        iss.clear();
        iss.seekg(0);

        while (iss >> word)
            std::cout << '+' << word << '\n';
    }

Output:

    Hello, whirled!
    Hello,
    whirled!
    +Hello,
    +whirled!

### 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 25 | C++98 | n was the smaller of os.width() and str.size() | n is the larger of them
LWG 90 | C++98 | std::isspace (c, getloc()) was used to check for spaces, but `getloc` is not declared in `<string>` | replaced getloc() with is.getloc()
LWG 91 | C++98 | `operator>>` did not behave as a FormattedInputFunction | behaves as a FormattedInputFunction
LWG 211 | C++98 | `operator>>` did not set `failbit` if no character is extracted | sets `failbit`
LWG 435 | C++98 | characters were inserted by os.rdbuf()->sputn(str.data(), n), and the resolution of LWG issue 25 made the behavior undefined if os.width() is larger than str.size() | determines the padding first and inserts the padded character sequence instead
LWG 586 | C++98 | `operator<<` did not behave as a FormattedOutputFunction | behaves as a FormattedOutputFunction

### See also

operator<< (C++17) | performs stream output on string views (function template)

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