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

basic_ostream and character

basic_ostream and character array

basic_ostream rvalue

template< class Ostream, class T >
Ostream&& operator<<( Ostream&& os, const T& value );

deleted overloads for basic_ostream and UTF character/array

Inserts a character or a character string.

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,

is called to cancel the effects of

, if any.

Behaves as a

. After constructing and checking the sentry object, inserts successive characters from the character array whose first element is pointed to by

.

• 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

, 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,

is called to cancel the effects of

, if any.

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

Calls the appropriate insertion operator, given an rvalue reference to an output stream object (equivalent to

). This overload participates in overload resolution only if the expression

is well-formed and

is a class type publicly and unambiguously derived from

.

Overloads that accept

,

etc (or null terminated sequence thereof) are deleted:

is not allowed. Previously, these would print an integer or pointer value.

1,2) os

3) std::move(os)

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

#include <fstream>
#include <iostream>
 
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<char>(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:

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

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