<syncstream>是C++20中新增加的头文件,提供了对同步输出流的支持,即在多个线程中可安全地进行输出操作,此头文件是Input/Output库的一部分。包括:
1.std::basic_syncbuf:是std::basic_streambuf的包装器(wrapper),它将输出累积在其自己的内部缓冲区中,并在销毁时以及明确请求时将其全部内容自动传输到包装的缓冲区,以便它们显示为连续的字符序列。此类模板主要用于创建与多线程程序无缝协作的自定义流类。如果多个线程试图同时写入同一个流,std::basic_syncbuf将确保每个线程的输出都以正确的顺序写入流,而不会出现任何交错。
cpp
using syncbuf = basic_syncbuf<char>;
using wsyncbuf = basic_syncbuf<wchar_t>;
2.std::basic_osyncstream:是std::basic_syncbuf的便捷包装器。它提供了一种机制来同步写入同一流的线程。
cpp
using osyncstream = basic_osyncstream<char_t>;
using wosyncstream = basic_osyncstream<wchar_t>;
测试代码如下:
cpp
namespace {
void func1(int id, std::ostream& sync_out)
{
std::osyncstream sync_stream(sync_out);
sync_stream << "thread " << id << " is running" << std::endl;
}
void func2(int id, std::ostream& out)
{
out << "thread " << id << " is running" << std::endl;
}
void sync(std::ofstream& file_out)
{
std::syncbuf sync_buf(file_out.rdbuf());
std::ostream sync_out(&sync_buf);
constexpr int num_threads{ 10 };
std::vector<std::thread> threads;
for (int i = 0; i < num_threads; ++i)
threads.emplace_back(func1, i, std::ref(sync_out));
for (auto& t : threads)
t.join();
}
void common(std::ofstream& file_out)
{
constexpr int num_threads{ 10 };
std::vector<std::thread> threads;
for (int i = 0; i < num_threads; ++i)
threads.emplace_back(func2, i, std::ref(file_out));
for (auto& t : threads)
t.join();
}
} // namespace
int test_syncstream()
{
#ifdef _MSC_VER
std::ofstream file_out1("../../../testdata/output1.txt");
std::ofstream file_out2("../../../testdata/output2.txt");
#else
std::ofstream file_out1("testdata/output1.txt");
std::ofstream file_out2("testdata/output2.txt");
#endif
if (!file_out1 || ! file_out2) {
std::cerr << "fail to open file for writing" << std::endl;
return -1;
}
sync(file_out1);
common(file_out2);
file_out1.close();
file_out2.close();
return 0;
}
执行结果如下图所示:每次输出并不完全一致:output2.txt中输出是乱的,而output1.txt中的输出是正常的