thread 的join() 成员函数会告诉当前线程在这个线程执行完成之前不要继续执行，即在当前线程阻塞，直到这个线程执行完成，join() 本质上就是一个等待事件。例如：
#include <atomic>
#include <thread>
#include <chrono>
using namespace std::chrono;
void tick(int n)
{
for (int i = 0; i != n; ++i) {
this_thread::sleep_for(seconds{ 1 });
cout<<"Alive!"<<endl;
}
}
int _tmain(int argc, _TCHAR* argv[])
{
thread timer{ tick,10 };
timer.join(); //阻塞直到 timer 线程完成
system("pause");
return 0;
}
**join()**实现代码(windows平台)：
void join() {
if (!joinable()) {
_Throw_Cpp_error(_INVALID_ARGUMENT);
}
if (_Thr._Id == _Thrd_id()) {
_Throw_Cpp_error(_RESOURCE_DEADLOCK_WOULD_OCCUR);
}
if (_Thrd_join(_Thr, nullptr) != _Thrd_success) {
_Throw_Cpp_error(_NO_SUCH_PROCESS);
}
_Thr = {};
}
⇓
函数 _Thrd_join（）：
---cthread.cpp ------------------
00007FF7459BDC60 mov qword ptr [rsp+10h],rdx
00007FF7459BDC65 mov qword ptr [rsp+8],rcx
00007FF7459BDC6A sub rsp,38h
00007FF7459BDC6E xor r8d,r8d
00007FF7459BDC71 mov edx,0FFFFFFFFh
00007FF7459BDC76 mov rax,qword ptr [&thr]
00007FF7459BDC7B mov rcx,qword ptr [rax]
00007FF7459BDC7E call qword ptr [__imp_WaitForSingleObjectEx (07FF745B9A090h)]
00007FF7459BDC84 cmp eax,0FFFFFFFFh
00007FF7459BDC87 jne _Thrd_join+30h (07FF7459BDC90h)
00007FF7459BDC89 mov eax,4
00007FF7459BDC8E jmp _Thrd_join+89h (07FF7459BDCE9h)
00007FF7459BDC90 cmp qword ptr [code],0
00007FF7459BDC96 je _Thrd_join+61h (07FF7459BDCC1h)
00007FF7459BDC98 lea rdx,[rsp+24h]
00007FF7459BDC9D mov rax,qword ptr [&thr]
00007FF7459BDCA2 mov rcx,qword ptr [rax]
00007FF7459BDCA5 call qword ptr [__imp_GetExitCodeThread (07FF745B9A0A8h)]
00007FF7459BDCAB test eax,eax
00007FF7459BDCAD jne _Thrd_join+56h (07FF7459BDCB6h)
00007FF7459BDCAF mov eax,4
00007FF7459BDCB4 jmp _Thrd_join+89h (07FF7459BDCE9h)
00007FF7459BDCB6 mov rax,qword ptr [code]
00007FF7459BDCBB mov ecx,dword ptr [rsp+24h]
00007FF7459BDCBF mov dword ptr [rax],ecx
00007FF7459BDCC1 mov rax,qword ptr [&thr]
00007FF7459BDCC6 mov rcx,qword ptr [rax]
00007FF7459BDCC9 call qword ptr [__imp_CloseHandle (07FF745B9A088h)]
00007FF7459BDCCF test eax,eax
00007FF7459BDCD1 je _Thrd_join+7Dh (07FF7459BDCDDh)
00007FF7459BDCD3 mov dword ptr [rsp+20h],0
00007FF7459BDCDB jmp _Thrd_join+85h (07FF7459BDCE5h)
00007FF7459BDCDD mov dword ptr [rsp+20h],4
00007FF7459BDCE5 mov eax,dword ptr [rsp+20h]
00007FF7459BDCE9 add rsp,38h
00007FF7459BDCED ret
可以看出，实际上函数接收一个线程句柄，调用函数 WaitForSingleObjectEx 等待线程完成，然后释放线程句柄。
而线程在完成初始化后，直接启动一个新线程：
thread timer{ tick,10 } 内部调用 _beginthreadex 启动一个新的线程。