std::thread::joinable
bool joinable() const noexcept;
Check if joinable Returns whether the thread object is joinable.
返回線程對象是否是joinable的。
A thread object is joinable if it represents a thread of execution.
如果是一個正在執行的線程，那麼它是joinable的。
A thread object is not joinable in any of these cases:
下列任一情況都是非joinable
if it was default-constructed.
默認構造器構造的。if it has been moved from (either constructing another thread object, or assigning to it).
通過移動構造獲得的。if either of its members join or detach has been called.
調用了join或者detach方法的。
例子：
#include
#include
#include
using namespace std;
void delay(double sec)
{
time_t start_time, cur_time; // 變量聲明
time(&start_time);
do {
time(&cur_time);
}while((cur_time - start_time) < sec );
};
void show(int n){
cout<運行結果：
用GDB調試發現一個好神奇的東西。
運行完了
cout<
之後，居然像棧析解一樣，好神奇阿，現在還不知道為什麼呢。。
先保留著，下次解決。
Parameters
none
Return value
true if the thread is joinable.
false otherwise.
Example
