C%2b%2b Ostream Dev Null

6 min readJun 1, 2021

‘Thomas Tutone’ <Th***********@yahoo.com> wrote in message
news:11**********************@g47g2000cwa.googlegr oups.com…

Angel Tsankov wrote:
How do I define a null ostream that inherits publicly std::ostream
and
ignores anything that would otherwise be output?
I cribbed this from c.l.c++ years ago — don’t remember the original
author (but it’s not me):
#include <streambuf>
#include <ostream>
template <class cT, class traits = std::char_traits<cT> >
class basic_nullbuf: public std::basic_streambuf<cT, traits> {
typename traits::int_type overflow(typename traits::int_type c)
{
return traits::not_eof(c); // indicate success
}
};
template <class cT, class traits = std::char_traits<cT> >
class basic_onullstream: public std::basic_ostream<cT, traits> {
public:
basic_onullstream():
std::basic_ios<cT, traits>(&m_sbuf),
std::basic_ostream<cT, traits>(&m_sbuf)
{
init(&m_sbuf);
}
private:
basic_nullbuf<cT, traits> m_sbuf;
};
typedef basic_onullstream<char> onullstream;
typedef basic_onullstream<wchar_t> wonullstream;
Best regards,
Tom

Thanks, Tom! This seems to be exactly what I need:)

C 2b 2b Ostream Dev Null Test
C 2b 2b Ostream Dev Null Command

< cpp‎ | io‎ | basic ostream

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)

Synchronized Output

(C++20)

Types Error category interface

(C++11)

std::basic_ostream

This benchmark formats a simple message and prints it to the output stream redirected to / dev / null. It uses the Google Benchmark library GOOGLE-BENCH to measure timings: #include #include #include #include ostream.h void printf ( benchmark:: State & s ) while ( s. I combined three different proposals above and one writing directly to /dev/null (so it involves kernel.) Surprisingly the NullStream that got the most votes performed the worst. Here are results for 100,000,000 writes: a) /dev/null: 30 seconds b) NullStream: 50 seconds c) badbit: 16 seconds (the winner in speed, but cannot test for errors!).

Global objects Member functions

(C++11)

Formatted output Unformatted output Positioning Miscellaneous

(C++11)

Member classes Non-member functions

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.
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.
An Ostream is an abstract base class for all output systems (streams, files, token lists.

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.

1) Behaves as an FormattedOutputFunction. After constructing and checking the sentry object, inserts the character ch. If the type of the character is not CharT, it is first converted with os.widen(ch). 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.

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.

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.

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.

3) 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.

4) 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

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

1–2)os

[edit]Notes

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 behaviorLWG 1203 C++11 overload for rvalue stream returned lvalue reference to the base class returns rvalue reference to the derived classLWG 2534 C++11 overload for rvalue stream was not constrained constrained

[edit]See also