std::thread  joinable()函数，用于检测线程是否有效。

joinable ： 代表该线程是可执行线程。

not-joinable ：通常一下几种情况会导致线程成为not-joinable

     1） 由thread的缺省构造函数构造而成(thread()没有参数)。

     2） 该thread被move过（包括move构造和move赋值）

     3） 该线程调用过join或者detach

bool joinable() const noexcept;

check if joinable

Returns whether the thread object is joinable.

A thread object is joinable if it represents a thread of execution.

A thread object is not joinable in any of these cases:

• if it was default-constructed.
• if it has been moved from (either constructing another thread object, or assigning to it).
• if either of its members join or detach has been called.

代码示例

// example for thread::joinable
#include <iostream>       // std::cout
#include <thread>         // std::thread
 
void mythread() 
{
  // do stuff...
}
 
int main() 
{
  std::thread foo;
  std::thread bar(mythread);
 
  std::cout << "Joinable after construction:\n" << std::boolalpha;
  std::cout << "foo: " << foo.joinable() << '\n';
  std::cout << "bar: " << bar.joinable() << '\n';
 
  if (foo.joinable()) foo.join();
  if (bar.joinable()) bar.join();
 
  std::cout << "Joinable after joining:\n" << std::boolalpha;
  std::cout << "foo: " << foo.joinable() << '\n';
  std::cout << "bar: " << bar.joinable() << '\n';
 
  return 0;
}

输出结果
Joinable after construction:
foo: false
bar: true
Joinable after joining:
foo: false
bar: false