C++11多线程例子.docx

C++11多线程例子.docx

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C++11多线程例子

Threads

The std:

:

thread classrepresentsathreadofexecutionandisavailableinthe  header. std:

:

thread canworkwithregularfunctions,lambdasandfunctors（aclassimplementingoperator（））.Moreoveritallowsyoutopassanynumberofparameterstothethreadfunction. 

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#include

voidfunc（）

{

//dosomework

}

intmain（）

{

std:

:

threadt（func）;

t.join（）;

return0;

}

Inthisexample t isathreadobjectrepresentingthethreadunderwhichfunction func（） runs.Thecallto join blocksthecallingthread（inthiscasethemainthread）untilthejoinedthreadfinishesexecution.Ifthethreadfunctionreturnsavalue,itisignored.However,thefunctioncantakeanynumberofparameters.

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voidfunc（inti,doubled,conststd:

:

string&s）

{

std:

:

cout<

:

endl;

}

intmain（）

{

std:

:

threadt（func,1,12.50,"sample"）;

t.join（）;

return0;

}

Eventhoughit'spossibletopassanynumberofparameterstothethreadfunction,allparametersarepassedbyvalue.Ifthefunctionneedstotakeparametersbyreference,thepassedargumentsmustbewrappedina std:

:

reforstd:

:

creflikeinthefollowingexample.

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voidfunc（int&a）

{

a++;

}

intmain（）

{

inta=42;

std:

:

threadt（func,std:

:

ref（a））;

t.join（）;

std:

:

cout<

:

endl;

return0;

}

Theprogramprints43,butwithoutwrappingaina std:

:

ref theoutputwouldbe42.

Apartfromthejoinmethod,thethreadclassprovidesacouplemoreoperations:

∙swap:

exchangestheunderlyinghandlesoftwothreadobjects

∙detach:

allowsathreadofexecutiontocontinueindependentlyofthethreadobject.Detachedthreadsarenolongerjoinable（youcannotwaitforthem）.

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intmain（）

{

std:

:

threadt（funct）;

t.detach（）;

return0;

}

Animportantthingtonoteisthatifathreadfunctionthrowsanexceptionitwon'tbecaughtwitharegulartry-catchblock.Inotherwords,thiswon'twork:

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try

{

std:

:

threadt1（func）;

std:

:

threadt2（func）;

t1.join（）;

t2.join（）;

}

catch（conststd:

:

exception&ex）

{

std:

:

cout<

:

endl;

}

Topropagateexceptionsbetweenthreadsyoucouldcatchtheminthethreadfunctionandstoretheminaplacewhereitcanbeaccessedlater.

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std:

:

mutexg_mutex;

std:

:

vector

:

exception_ptr>g_exceptions;

voidthrow_function（）

{

throwstd:

:

exception（"somethingwronghappened"）;

}

voidfunc（）

{

try

{

throw_function（）;

}

catch（...）

{

std:

:

lock_guard

:

mutex>lock（g_mutex）;

g_exceptions.push_back（std:

:

current_exception（））;

}

}

intmain（）

{

g_exceptions.clear（）;

std:

:

threadt（func）;

t.join（）;

for（auto&e:

g_exceptions）

{

try

{

if（e!

=nullptr）

{

std:

:

rethrow_exception（e）;

}

}

catch（conststd:

:

exception&e）

{

std:

:

cout<

:

endl;

}

}

return0;

}

Formoreinformationaboutcatchingandpropagatingexceptionsyoucanread HandlingC++exceptionsthrownfromworkerthreadinthemainthread and HowcanIpropagateexceptionsbetweenthreads?

.

Beforegoingfurtheritworthnotingthatthe  headerprovidessomehelperfunctionsinnamespacestd:

:

this_thread:

∙get_id:

returnstheidofthecurrentthread 

∙yield:

tellstheschedulertorunotherthreadsandcanbeusedwhenyouareinabusywaitingstate

∙sleep_for:

blockstheexecutionofthecurrentthreadforatleastthespecifiedperiod

∙sleep_util:

blockstheexecutionofthecurrentthreaduntilthespecifiedmomentoftimehasbeenreached

Locks

InthelastexampleIneededtosynchronizetheaccesstothe g_exceptions vectortomakesureonlyonethreadatatimecanpushanewelement.ForthisIusedamutexandalockonthemutex.AmutexisacoresynchronizationprimitiveanditC++11itcomesinfourflavorsinthe  header.

∙mutex:

providesthecorefunctions lock（） and unlock（） andthenon-blocking try_lock（） methodthatreturnsifthemutexisnotavailable.

∙recursive_mutex:

allowsmultipleacquisitionsofthemutexfromthesamethread.

∙timed_mutex:

similartomutex,butitcomeswithtwomoremethods try_lock_for（） and try_lock_until（） thattrytoacquirethemutexforaperiodoftimeoruntilamomentintimeisreached.

∙recursive_timed_mutex:

isacombinationoftimed_mutexandrecusive_mutex.

Hereisanexampleforusinga std:

:

mutex （noticetheuseofthe get_id（） and sleep_for（） helperfunctionsmentionedearlier）. 

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#include

#include

#include

#include

std:

:

mutexg_lock;

voidfunc（）

{

g_lock.lock（）;

std:

:

cout<<"enteredthread"<

:

this_thread:

:

get_id（）<

:

endl;

std:

:

this_thread:

:

sleep_for（std:

:

chrono:

:

seconds（rand（）%10））;

std:

:

cout<<"l