c++ - std::thread constructor with no parameter -

according cppreference.com, std::thread constructor no parameter means:

creates new thread object not represent thread.

my questions are:

1. why need constructor? , if create thread using constructor, how can "assign" thread function later?
2. why don't have "run(function_address)" method when constructed no parameter, can specify function "run" thread.
3. or, can construct thread callable parameter (function, functors, etc.) call "run()" method execute thread later. why std::thread not designed in way?

your question suggests there might confusion , helpful separate ideas of thread of execution std::thread type, , separate both idea of "thread function".

• a thread of execution represents flow of control through program, corresponding os thread managed kernel.
• an object of type std::thread can associated thread of execution, or can "empty" , not refer thread of execution.
• there no such concept "thread function" in standard c++. function can run in new thread of execution passing constructor of std::thread object.

1. why need constructor?

to construct empty state doesn't refer thread of execution. might want have member variable of class std::thread, not want associate thread of execution right away. default construct it, , later launch new thread of execution , associate std::thread member variable. or might want do:

std::thread t; if (some_condition) {   t = std::thread{ func1, arg1 }; } else {   auto result = some_calculation();   t = std::thread{ func2, arg2, result }; } 

the default constructor allows object t created without launching new thread of execution until needed.

and if create thread using constructor, how can "assign" thread function later?

you "assign" using "assignment" :-)

but don't assign "thread function" it, not std::thread for. assign std::thread it:

std::thread t; std::thread t2{ func, args }; t = std::move(t2); 

think in terms of creating new thread of execution not "assigning thread function" something. you're not assigning function, that's std::function used for. requesting runtime create new thread of execution, managed std::thread object.

2. why don't have "run(function_address)" method when constructed no parameter, can specify function "run" thread.

because don't need it. start new threads of execution constructing std::thread object arguments. if want thread of execution associated existing object can move-assigning or swapping.

3. or, can construct thread callable parameter(function, functors, etc.) call "run()" method execute thread later. why std::thread not designed in way?

why should designed way?

the std::thread type managing thread of execution not holding callable object later use. if want create callable object can later run on new thread of execution there lots of ways in c++ (using lambda expression, or std::bind, or std::function, or std::packaged_task, or custom functor type). job of std::thread manage thread of execution not hold onto callable object until want call it.