std::cerr,std::wcerr(3)

🎀 NAME 🎀

okay so bestie, welcome to std::cerr,std::wcerr from the C++ Standard Libary manual 💖. The NAME is literally std::cerr,std::wcerr - std::cerr,std::wcerr — yes, it says the same thing twice, and honestly? iconic. This is the page for the global error-stream girlies, and we're getting into every single detail, no cap. ✨

---

✨ Synopsis ✨

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. fr fr, multi-thread safe from the jump. 🧚

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). the way it just flushes instantly?? obsessed. 🔥

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). that tie() energy is real. 💖

---

💅 Notes 💅

The 'c' in the name refers to "character" (stroustrup.com FAQ); cerr means "character error (stream)" and wcerr means "wide character error (stream)". 🎀

---

🌈 Example 🌈

Output to stderr via std::cerr flushes out the pending output on std::cout, while output to stderr via std::clog does not. iykyk, that's the whole point. 🦋

// Run this code

#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()

---

🧚 Defect reports 🧚

The following behavior-changing defect reports were applied retroactively to previously published C++ standards. history homework but make it cute. 💜

• DR: LWG 455
  • Applied to: C++98
  • Behavior as published: std::cerr.tie() and std::wcerr.tie() returned null pointers
  • Correct behavior: they return &std::cout and &std::wcout respectively

---

🫶 See also 🫶

• Init — initializes standard stream objects (public member class of std::ios_base)
• clog — writes to the standard C error stream stderr
• wclog — (global object)
• cout — writes to the standard C output stream stdout
• wcout — (global object)
• stdin — expression of type FILE* associated with the input stream
• stdout — expression of type FILE* associated with the output stream
• stderr — expression of type FILE* associated with the error output stream (macro constant)

---

k that's the whole page, bestie 💖 — source: http://cppreference.com; version/date: 2024.06.10; page: std::cerr,std::wcerr(3). mwah 💋✨