• C 2b 2b Ostream Dev Null Command
• C 2b 2b Ostream Dev Null Key
• C%2b%2b Ostream Dev Null
• C 2b 2b Ostream Dev Null Test
• C 2b 2b Ostream Dev Null Code

这个问题已经在这里有了答案: 9年前关闭。 Possible Duplicate: Implementing a no-op std::ostream 在c中是否有与NULL等价的流？我想编写一个函数，如果用户希望将内部输出输出到某个地方，则将其带入一个流中，但是如果没有，则输出进入某个伪造的地方.

C++

Language

Standard Library Headers

Freestanding and hosted implementations

Named requirements

Language support library

Concepts library(C++20)

Diagnostics library

Utilities library

Strings library

Containers library

Iterators library

Ranges library(C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library(C++17)

Regular expressions library(C++11)

Atomic operations library(C++11)

Thread support library(C++11)

Technical Specifications

971 // with rawnullostream, but it's better to have rawnullostream follow 972 // the rules than to change the rules just for rawnullostream. 我有一个C应用程序，其中包含大量的std::cout。它运行在linux 2.6.x上。 它运行在linux 2.6.x上。 我需要测试应用程序的性能，因此我正在考虑将 std::cout 重定向到 /dev/null 。.

1. Lo standard garantisce che questo 27.6.2.2 lib.ostream.cons p1, a partire da 27.6.2.2 lib.ostream.cons p1 che descrive il costruttore di ostream che accetta un puntatore a uno streambuf: Effetti: basicostream un object di class basicostream, assegnando i valori iniziali alla class base chiamando basicios::init(sb).
2. They are two different code std::ostream/code objects that map to two different file descriptors of the OS (namely: 1 and 2). By default they both map to the console output, but they can be redirected in different devices or files.

Input/output library

I/O manipulators

C-style I/O

Buffers

(deprecated in C++98)

(C++20)

Streams

Abstractions

File I/O

String I/O

Array I/O

(deprecated in C++98)

(deprecated in C++98)

(deprecated in C++98)

Synchronized Output

(C++20)

Types

Error category interface

(C++11)

(C++11)

std::basic_ostream

Global objects

Member functions

(C++11)

Formatted output

Unformatted output

Positioning

Miscellaneous

(C++11)

Member classes

Non-member functions

basic_ostream& operator<<(short value ); basic_ostream& operator<<(unsignedshort value );	(1)
basic_ostream& operator<<(int value ); basic_ostream& operator<<(unsignedint value );	(2)
basic_ostream& operator<<(long value ); basic_ostream& operator<<(unsignedlong value );	(3)
basic_ostream& operator<<(longlong value ); basic_ostream& operator<<(unsignedlonglong value );	(4)	(since C++11)
basic_ostream& operator<<(float value ); basic_ostream& operator<<(double value ); basic_ostream& operator<<(longdouble value );	(5)
(6)
basic_ostream& operator<<(constvoid* value );	(7)
(8)	(since C++17)
basic_ostream& operator<<(std::basic_streambuf<CharT, Traits>* sb);	(9)
basic_ostream& operator<<( std::ios_base&(*func)(std::ios_base&));	(10)
basic_ostream& operator<<( std::basic_ios<CharT,Traits>&(*func)(std::basic_ios<CharT,Traits>&));	(11)
basic_ostream& operator<<( std::basic_ostream<CharT,Traits>&(*func)(std::basic_ostream<CharT,Traits>&));	(12)

Inserts data into the stream.

C 2b 2b Ostream Dev Null Command

9) Behaves as an UnformattedOutputFunction. After constructing and checking the sentry object, checks if sb is a null pointer. If it is, executes setstate(badbit) and exits. Otherwise, extracts characters from the input sequence controlled by sb and inserts them into *this until 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).

If no characters were inserted, executes setstate(failbit). If an exception was thrown while extracting, sets failbit and, if failbit is set in exceptions(), rethrows the exception.

[edit]Parameters

value	-	integer, floating-point, boolean, or pointer value to insert
func	-	function to call
sb	-	pointer to the streambuffer to read the data from

[edit]Return value

[edit]Notes

There are no overload for pointers to non-static member, pointers to volatile, or function pointers (other than the ones with signatures accepted by the (10-12) overloads). Attempting to output such objects invokes implicit conversion to bool, and, for any non-null pointer value, the value 1 is printed (unless boolalpha was set, in which case true is printed).

Character and character string arguments (e.g., of type char or constchar*) are handled by the non-member overloads of operator<<. Attempting to output a character using the member function call syntax (e.g., std::cout.operator<<('c');) will call one of overloads (2-4) and output the numerical value. Attempting to output a character string using the member function call syntax will call overload (7) and print the pointer value instead.

[edit]Example

Output:

[edit]See also

inserts character data (function template)[edit]
performs stream I/O of strings (function template)
performs stream input and output of bitsets (function)
serializes and deserializes a complex number (function template)
performs stream input and output on pseudo-random number engine (function template)[edit]
performs stream input and output on pseudo-random number distribution (function template)[edit]
inserts a character (public member function)[edit]
inserts blocks of characters (public member function)[edit]
(C++17)	converts an integer or floating-point value to a character sequence (function)[edit]

C++

Language

Standard Library Headers

Freestanding and hosted implementations

Named requirements

Language support library

Concepts library(C++20)

Diagnostics library

Utilities library

Strings library

Containers library

Iterators library

Ranges library(C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library(C++17)

Regular expressions library(C++11)

Atomic operations library(C++11)

Thread support library(C++11)

Technical Specifications

C 2b 2b Ostream Dev Null Key

Input/output library

I/O manipulators

C-style I/O

Buffers

(deprecated in C++98)

(C++20)

Streams

Abstractions

File I/O

String I/O

Array I/O

(deprecated in C++98)

(deprecated in C++98)

(deprecated in C++98)

Synchronized Output

(C++20)

Types

Error category interface

(C++11)

(C++11)

std::basic_ostream

Global objects

Member functions

(C++11)

Formatted output

Unformatted output

Positioning

Miscellaneous

(C++11)

Member classes

Non-member functions

Defined in header <ostream>
(1)
	template<class CharT, class Traits> basic_ostream<CharT,Traits>& operator<<( basic_ostream<CharT,Traits>& os, CharT ch );
	template<class CharT, class Traits> basic_ostream<CharT,Traits>& operator<<( basic_ostream<CharT,Traits>& os, char ch );
	template<class Traits > basic_ostream<char,Traits>& operator<<( basic_ostream<char,Traits>& os, char ch );
	template<class Traits > basic_ostream<char,Traits>& operator<<( basic_ostream<char,Traits>& os, signedchar ch );
	template<class Traits > basic_ostream<char,Traits>& operator<<( basic_ostream<char,Traits>& os, unsignedchar ch );
	(2)
		template<class CharT, class Traits > basic_ostream<CharT,Traits>& operator<<( basic_ostream<CharT,Traits>& os, const CharT* s );
		template<class CharT, class Traits > basic_ostream<CharT,Traits>& operator<<( basic_ostream<CharT,Traits>& os, constchar* s );
		template<class Traits > basic_ostream<char,Traits>& operator<<( basic_ostream<char,Traits>& os, constchar* s );
		template<class Traits > basic_ostream<char,Traits>& operator<<( basic_ostream<char,Traits>& os, constsignedchar* s );
		template<class Traits > basic_ostream<char,Traits>& operator<<( basic_ostream<char,Traits>& os, constunsignedchar* s );
		template<class Ostream, class T > Ostream&& operator<<( Ostream&& os, const T& value );	(3)	(since C++11)
		deleted overloads for basic_ostream and UTF character/array
		(4)	(since C++20)
				template<class Traits > basic_ostream<char,Traits>& operator<<( basic_ostream<char,Traits>& os, wchar_t ch )= delete;
				template<class Traits > basic_ostream<char,Traits>& operator<<( basic_ostream<char,Traits>& os, char8_t ch )= delete;
				template<class Traits > basic_ostream<char,Traits>& operator<<( basic_ostream<char,Traits>& os, char16_t ch )= delete;
				template<class Traits > basic_ostream<char,Traits>& operator<<( basic_ostream<char,Traits>& os, char32_t ch )= delete;
				template<class Traits > basic_ostream<wchar_t,Traits>& operator<<( basic_ostream<wchar_t,Traits>& os, char8_t ch )= delete;
				template<class Traits > basic_ostream<wchar_t,Traits>& operator<<( basic_ostream<wchar_t,Traits>& os, char16_t ch )= delete;
				template<class Traits > basic_ostream<wchar_t,Traits>& operator<<( basic_ostream<wchar_t,Traits>& os, char32_t ch )= delete;
				template<class Traits > basic_ostream<char,Traits>& operator<<( basic_ostream<char,Traits>& os, constwchar_t* ch )= delete;
				template<class Traits > basic_ostream<char,Traits>& operator<<( basic_ostream<char,Traits>& os, const char8_t* ch )= delete;
				template<class Traits > basic_ostream<char,Traits>& operator<<( basic_ostream<char,Traits>& os, constchar16_t* ch )= delete;
				template<class Traits > basic_ostream<char,Traits>& operator<<( basic_ostream<char,Traits>& os, constchar32_t* ch )= delete;
				template<class Traits > basic_ostream<wchar_t,Traits>& operator<<( basic_ostream<wchar_t,Traits>& os, const char8_t* ch )= delete;
				template<class Traits > basic_ostream<wchar_t,Traits>& operator<<( basic_ostream<wchar_t,Traits>& os, constchar16_t* ch )= delete;
				template<class Traits > basic_ostream<wchar_t,Traits>& operator<<( basic_ostream<wchar_t,Traits>& os, constchar32_t* ch )= delete;

Inserts a character or a character string.

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

C%2b%2b Ostream Dev Null

• 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<constchar*>(s)) are inserted.

C 2b 2b Ostream Dev Null Test

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

The behavior is undefined if s is a null pointer.

[edit]Parameters

os	-	output stream to insert data to
ch	-	reference to a character to insert
s	-	pointer to a character string to insert

[edit]Return value

[edit]Notes

C 2b 2b Ostream Dev Null Code

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

[edit]Example

Output:

[edit]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 1203	C++11	overload for rvalue stream returned lvalue reference to the base class	returns rvalue reference to the derived class
LWG 2534	C++11	overload for rvalue stream was not constrained	constrained

[edit]See also

inserts formatted data (public member function)[edit]

widens characters (public member function of std::basic_ios<CharT,Traits>)[edit]