operator<<(std::basic_ostream) - cppreference.com (original) (raw)

Inserts a character or a character string.

1. Behaves as a FormattedOutputFunction. After constructing and checking the sentry object, inserts the character ch. If ch has type char and the character container type of os is not char, os.widen(ch) will be inserted instead.

Padding is determined as follows:

• If os.width() > 1, then os.width() - 1 copies of os.fill() are added to the output character to form the output character sequence.
• If (out.flags() & std::ios_base::adjustfield) == std::ios_base::left, the fill characters are placed after the output character, otherwise before.

After insertion, os.width(0) is called to cancel the effects of std::setw, if any.

1. Behaves as a FormattedOutputFunction. After constructing and checking the sentry object, inserts successive characters from the character array whose first element is pointed to by s.

• For the first and third overloads (where CharT matches the type of ch), exactly traits::length(s) characters are inserted.
• For the second overload, exactly std::char_traits<char>::length(s) characters are inserted.
• For the last two overloads, exactly traits::length(reinterpret_cast<const char*>(s)) are inserted.

Before insertion, first, all characters are widened using os.widen(), then padding is determined as follows:

• If the number of characters to insert is less than os.width(), then enough copies of os.fill() are added to the character sequence to make its length equal os.width().
• If (out.flags() & std::ios_base::adjustfield) == std::ios_base::left, the fill characters are added at the end of the output sequence, otherwise they are added before the output sequence.

After insertion, os.width(0) is called to cancel the effects of std::setw, if any.

If s is a null pointer, the behavior is undefined.

1. Calls the appropriate insertion operator, given an rvalue reference to an output stream object (equivalent to os << value). This overload participates in overload resolution only if the expression os << value is well-formed and Ostream is a class type publicly and unambiguously derived from std::ios_base.

2. Overloads that accept char16_t, char32_t etc (or null terminated sequence thereof) are deleted: std::cout << u'X' is not allowed. Previously, these would print an integer or pointer value.

[edit] Parameters

[edit] Return value

1,2) os

1. std::move(os)

[edit] Notes

Before LWG issue 1203, code such as (std::ostringstream() << 1.2).str() does not compile.

[edit] Example

#include #include   void foo() { // error: operator<< (basic_ostream<char, _Traits>&, char8_t) is deleted // std::cout << u8'z' << '\n'; }   std::ostream& operator<<(std::ostream& os, char8_t const& ch) { return os << static_cast(ch); }   int main() { std::cout << "Hello, world" // uses const char* overload << '\n'; // uses char overload std::ofstream{"test.txt"} << 1.2; // uses rvalue overload   std::cout << u8'!' << '\n'; // uses program-defined operator<<(os, char8_t const&) }

Output:

[edit] Defect reports

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

[edit] See also