Difference between revisions of "cpp/io/cerr"

Latest revision as of 11:29, 11 September 2023

Defined in header `<iostream>`
extern std::ostream cerr;	(1)
extern std::wostream wcerr;	(2)

The global objects std::cerr and std::wcerr control output to a stream buffer of implementation-defined type (derived from std::streambuf and std::wstreambuf, respectively), associated with the standard C error output stream stderr.

These objects are guaranteed to be initialized during or before the first time an object of type std::ios_base::Init is constructed and are available for use in the constructors and destructors of static objects with ordered initialization (as long as <iostream> is included before the object is defined).

Unless std::ios_base::sync_with_stdio(false) has been issued, it is safe to concurrently access these objects from multiple threads for both formatted and unformatted output.

Once initialized, (std::cerr.flags() & unitbuf) != 0 (same for std::wcerr) meaning that any output sent to these stream objects is immediately flushed to the OS (via std::basic_ostream::sentry's destructor).

In addition, std::cerr.tie() returns &std::cout (same for std::wcerr and std::wcout), meaning that any output operation on std::cerr first executes std::cout.flush() (via std::basic_ostream::sentry's constructor).

#include <chrono>
#include <iostream>
#include <thread>
using namespace std::chrono_literals;
 
void f()
{
    std::cout << "Output from thread...";
    std::this_thread::sleep_for(2s);
    std::cout << "...thread calls flush()" << std::endl;
}
 
int main()
{
    std::jthread t1{f};
    std::this_thread::sleep_for(1000ms);
    std::clog << "This output from main is not tie()'d to cout\n";
    std::cerr << "This output is tie()'d to cout\n";
}

Possible output:

This output from main is not tie()'d to cout
Output from thread...This output is tie()'d to cout
...thread calls flush()

[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 455	C++98	std::cerr.tie() and std::wcerr.tie() returned null pointers	they return &std::cout and &std::wcout respectively

[edit] See also

Init	initializes standard stream objects (public member class of `std::ios_base`) [edit]
clogwclog	writes to the standard C error stream stderr (global object)[edit]
coutwcout	writes to the standard C output stream stdout (global object)[edit]
stdinstdoutstderr	expression of type `FILE` associated with the input stream expression of type `FILE` associated with the output stream expression of type `FILE*` associated with the error output stream (macro constant) [edit]

@@ Line 1: / Line 1: @@
 {{cpp/title|cerr|wcerr}}
-{{cpp/io/basic_ostream/sidebar}}
+{{cpp/io/basic_ostream/navbar}}
-{{ddcl list begin}}
+{{dcl begin}}
-{{ddcl list header | iostream }}
+{{dcl header|iostream}}
-{{ddcl list item | num=1 | 1=
+{{dcl|num=1|1=
 extern std::ostream cerr;
 }}
-{{ddcl list item | num=2| 1=
+{{dcl|num=2|1=
 extern std::wostream wcerr;
 }}
-{{ddcl list end}}
+{{dcl end}}
-The global objects {{cpp|std::cerr}} and {{cpp|std::wcerr}} control output to a stream buffer of implementation-defined type (derived from {{cpp|std::streambuf}} and {{cpp|std::wstreambuf}}, respectively), associated with the standard C error output stream {{cpp|stderr}}.
+The global objects {{tt|std::cerr}} and {{tt|std::wcerr}} control output to a stream buffer of implementation-defined type (derived from {{lc|std::streambuf}} and {{lc|std::wstreambuf}}, respectively), associated with the standard C error output stream {{lc|stderr}}.
-These objects are guaranteed to be constructed before the first constructor of a static object is called and they are guaranteed to outlive the last destructor of a static object, so that it is always possible to write to {{cpp|std::cerr}} in user code.
+These objects are guaranteed to be initialized during or before the first time an object of type {{lc|std::ios_base::Init}} is constructed and are available for use in the constructors and destructors of static objects with [[cpp/language/initialization#Non-local_variables|ordered initialization]] (as long as {{header|iostream}} is included before the object is defined).
-Unless {{cpp|sync_with_stdio(false)}} has been issued, it is safe to concurrently access these objects from multiple threads for both formatted and unformatted output.
+Unless {{c|std::ios_base::sync_with_stdio(false)}} has been issued, it is safe to concurrently access these objects from multiple threads for both formatted and unformatted output.
-Once initialized, {{cpp|1=std::cerr.flags() & unitbuf != 0}} (same for {{tt|wcerr}}) meaning that any output sent to these stream objects is immediately flushed to the OS.
+Once initialized, {{c|1=(std::cerr.flags() & unitbuf) != 0}} (same for {{tt|std::wcerr}}) meaning that any output sent to these stream objects is immediately flushed to the OS (via {{lc|std::basic_ostream::sentry}}'s destructor).
-In addition, {{cpp|std::cerr.tie()}} returns {{cpp|&std::cout}} (same for {{tt|wcerr}} and {{tt|wcout}}), meaning that any output operation on {{cpp|std::cerr}} first executes {{cpp|std::cout.flush()}} (via {{cpp|std::basic_ostream::sentry}}'s constructor) {{mark since c++11}}
+In addition, {{c|std::cerr.tie()}} returns {{c|&std::cout}} (same for {{tt|std::wcerr}} and {{lc|std::wcout}}), meaning that any output operation on {{tt|std::cerr}} first executes {{c|std::cout.flush()}} (via {{lc|std::basic_ostream::sentry}}'s constructor).
+===Notes===
+The 'c' in the name refers to "character" ([https://www.stroustrup.com/bs_faq2.html#cout stroustrup.com FAQ]); {{tt|cerr}} means "character error (stream)" and {{tt|wcerr}} means "wide character error (stream)".
 ===Example===
 {{example
- | output to stderr via cerr flushes out the pending output on cout, while output to stderr via clog does not
+|Output to {{lc|stderr}} via {{tt|std::cerr}} flushes out the pending output on {{lc|std::cout}}, while output to {{lc|stderr}} via {{lc|std::clog}} does not.
- | code=
+|code=
-#include <thread>
-#include <iostream>
 #include <chrono>
+#include <iostream>
+#include <thread>
+using namespace std::chrono_literals;
 void f()
 {
      std::cout << "Output from thread...";
-     std::this_thread::sleep_for(std::chrono::seconds(2));
+     std::this_thread::sleep_for(2s);
      std::cout << "...thread calls flush()" << std::endl;
 }
@@ Line 37: / Line 42: @@
 int main()
 {
-     std::thread t1(f);
+     std::jthread t1{f};
-     std::this_thread::sleep_for(std::chrono::seconds(1));
+     std::this_thread::sleep_for(1000ms);
      std::clog << "This output from main is not tie()'d to cout\n";
      std::cerr << "This output is tie()'d to cout\n";
-    t1.join();
 }
- | output=
+|p=true
+|output=
 This output from main is not tie()'d to cout
 Output from thread...This output is tie()'d to cout
 ...thread calls flush()
 }}
+===Defect reports===
+{{dr list begin}}
+{{dr list item|wg=lwg|dr=455|std=C++98|before={{c|std::cerr.tie()}} and<br>{{c|std::wcerr.tie()}} returned null pointers|after=they return {{c|&std::cout}} and<br>{{c|&std::wcout}} respectively}}
+{{dr list end}}
 ===See also===
-{{dcl list begin}}
+{{dsc begin}}
-{{dcl list template | cpp/io/ios_base/dcl list Init}}
+{{dsc inc|cpp/io/ios_base/dsc Init}}
-{{dcl list template | cpp/io/dcl list clog}}
+{{dsc inc|cpp/io/dsc clog}}
-{{dcl list template | cpp/io/dcl list cout}}
+{{dsc inc|cpp/io/dsc cout}}
-{{dcl list end}}
+{{dsc inc|cpp/io/c/dsc std streams}}
+{{dsc end}}
+{{langlinks|de|es|fr|it|ja|pt|ru|zh}}

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Difference between revisions of "cpp/io/cerr"

Latest revision as of 11:29, 11 September 2023

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[edit] Notes

[edit] Example

[edit] Defect reports

[edit] See also

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