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

To define the ostream class in C sources, the ostream header file must be included. To use the predefined ostream objects (std::cerr, std::cout etc.) the header file must be included. 6.4.1.1: Writing to `ostream' objects The class ostream supports both formatted and binary output.

这个问题已经在这里有了答案:
9年前关闭。

• Returns a quiet NaN (Not-A-Number) value of type double. The NaN values are used to identify undefined or non-representable values for floating-point elements, such as the square root of negative numbers or the result of 0/0.
• You can then use this buffer in any ostream class. NullBuffer nullbuffer; std::ostream nullstream(&nullbuffer); nullstream.

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
我希望能够选择执行data()或data(std::cout)

编辑:取自@Johannes Schaub-litb的邮件here，稍作修改:
使用这些:
现在，这看起来很酷，但是下面的代码更短并且更有效，因为如果将空指针提供给ostream的构造函数，它将自动设置Badbit并静默忽略任何写操作:
该标准从27.6.2.2 [lib.ostream.cons] p1开始，保证了它的工作，ostream描述了streambuf的构造函数，该构造函数采用指向basic_ios的指针:

Effects: Constructs an object of class basic_ostream, assigning initial values to the base class by calling basic_ios<charT,traits>::init(sb).

来自27.4.4.1 [lib.basic.ios.cons] p3，badbit的相关功能:

void init(basic_streambuf<charT,traits>* sb);
Postconditions: The postconditions of this function are indicated in Table 89:

表89中的重要行:

rdstate() -- goodbit if sb is not a null pointer, otherwise badbit.

如果设置了27.6.2.6 [lib.ostream.unformatted]，将在sentry下进行描述:

Each unformatted output function begins execution by constructing an object of class sentry. If this object returns true, while converting to a value of type bool, the function endeavors to generate the requested output.

这意味着，如果sentry为假，则不是。这是bool转换为27.6.2.3 [lib.ostream::sentry] p3 & p5的方式，取自ok_:

3) If, after any preparation is completed, os.good() is true, ok_ true otherwise, ok_ false.

5) operator bool();
Effects: Returns ok_.

(ostream::sentry

C%2b%2b Ostream Dev Null

是bool类型的27.6.2.2 [lib.ostream.cons] p1的成员。)
请注意，这些引号在C++ 11中仍然存在，只是在不同的位置。为了在这个答案中出现:

27.7.3.2 [ostream.cons] p1 =>27.4.4.1 [lib.basic.ios.cons] p3

27.5.5.2 [basic.ios.cons] =>27.6.2.6 [lib.ostream.unformatted]

表89 =>表128

27.7.3.7 [ostream.unformatted] p1 =>27.6.2.3 [lib.ostream::sentry] p3 & p5

27.7.3.4 [ostream::sentry] p4 & p5 => ojit_code

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

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.

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).

C 2b 2b Ostream Dev Null Test

If no characters were inserted, executes setstate(failbit). If an exception was thrown while extracting, sets failbit

C 2b 2b Ostream Dev Null Command

and, if failbit is set in exceptions()

C 2b 2b Ostream Dev Null Code

, 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:

C 2b 2b Ostream Dev Null Key

[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]