Linux `std：：cout` 是如何实现的

如何创建std::cout？

第一件事，从https://en.cppreference.com/w/cpp/io/ios_base/Init开始:

std::ios\U base::Init

此类用于确保默认的C++流(std::cin，

The header behaves as if it defines (directly or indirectly) an instance of std::ios\U base::Init with static storage duration: [...]

Meh，让我们做一个真实的代码示例.我将使用GCC C++ library.从https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/include/std/iostream#L73开始，这是重要的部分:

 // For construction of filebuffers for cout, cin, cerr, clog et. al.
 static ios_base::Init __ioinit;

现在我们跳到ios_base::Init类的contsructor，在https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/src/c%2B%2B98/ios_init.cc#L85中:

ios_base::Init::Init()
  {
    if (__gnu_cxx::__exchange_and_add_dispatch(&_S_refcount, 1) == 0)
      {
    // Standard streams default to synced with "C" operations.
    _S_synced_with_stdio = true;

    new (&buf_cout_sync) stdio_sync_filebuf<char>(stdout);
    new (&buf_cin_sync) stdio_sync_filebuf<char>(stdin);
    new (&buf_cerr_sync) stdio_sync_filebuf<char>(stderr);

    // The standard streams are constructed once only and never
    // destroyed.
    new (&cout) ostream(&buf_cout_sync);
    new (&cin) istream(&buf_cin_sync);
    new (&cerr) ostream(&buf_cerr_sync);
    new (&clog) ostream(&buf_cerr_sync);
    cin.tie(&cout);
    cerr.setf(ios_base::unitbuf);
    // _GLIBCXX_RESOLVE_LIB_DEFECTS
    // 455. cerr::tie() and wcerr::tie() are overspecified.
    cerr.tie(&cout);

_S_refcount用于从静态类的构造函数手动调用ios_base::Init::Init();时，它可以防止双重初始化.

stdio_sync_filebuf是用于存储输入/输出数据的内部缓冲器istream/ostream，在这里实现https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/include/ext/stdio_sync_filebuf.h#L56.它继承自std::basic_streambuf.

So cout以stdio_sync_filebuf<char>为参数构造in-place.这是这里提到的第一个构造函数https://en.cppreference.com/w/cpp/io/basic_ostream/basic_ostream.

现在，因为这些东西是就地构建的，您可能想知道内存是如何分配的？从https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/src/c%2B%2B98/globals_io.cc#L50开始:

  // Standard stream objects.
  // NB: Iff <iostream> is included, these definitions become wonky.
  typedef char fake_istream[sizeof(istream)]
  __attribute__ ((aligned(__alignof__(istream))));
  typedef char fake_ostream[sizeof(ostream)]
  __attribute__ ((aligned(__alignof__(ostream))));
  fake_istream cin;
  fake_ostream cout;
  fake_ostream cerr;
  fake_ostream clog;

这些对象只是char个大小适当、对齐适当的空缓冲区.

是的，您可以自己构建ostream，GCC上有__gnu_cxx::stdio_sync_filebuf个:

#include <fstream>
#include <ext/stdio_sync_filebuf.h>
int main() {
    __gnu_cxx::stdio_sync_filebuf<char> mybuf_cout_sync(stdout);
    std::ostream os(&mybuf_cout_sync);
    os << "Hello world!\n";
    return 0;
}

或者，为了便于移植，您可以编写自己的类，从std::streambuf继承，并自己构造ostream.网上有很多例子，比如这里的https://stackoverflow.com/a/51250032/9072753个例子.