operator<<,>>(std::basic_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 );

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 );

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

• If str.size() is not less than os.width(), uses the range [str.begin(), str.end()) as-is.
• Otherwise, if (os.flags() & ios_base::adjustfield) == ios_base::left, places os.width() - str.size() copies of the os.fill() character after the character sequence.
• Otherwise, places os.width() - str.size() copies of the os.fill() character before the character sequence.

Then stores each character from the resulting sequence (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.

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

Return value

Example

#include <iostream>
#include <string>
#include <sstream>
 
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';
}

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

Defect reports

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

See also