multithreading - Why c++ threads are movable but not copiable? -

as title of question says, why c++ threads (std::thread , pthread) movable not copiable? consequences there, if make copiable?

regarding copying, consider following snippet:

void foo();  std::thread first (foo);      std::thread second = first; // (*) 

when line marked (*) takes place, presumably of foo executed. expected behavior be, then? execute foo start? halt thread, copy registers , state, , rerun there?

in particular, given function objects part of standard, it's easy launch thread performs same operation earlier thread, reusing function object.

there's not motivation begin this, therefore.

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regarding moves, though, consider following:

std::vector<std::thread> threads; 

without move semantics, problematic: when vector needs internally resize, how move elements buffer? see more on here.