Go语言中的sync.Once，用于实现"只执行一次"的功能。功能与pthread_once()差不多。但pthread_once()可不通用，不过现在C++11终于有了std::call_once来实现这一功能。
虽然一个面向协程，一个面向线程，但这个点要实现的功能是一样的，，把两种语言的实现记录一下。
在 有C++11的相关例子，但用了很多新特性。
简化下，便于做对比.
没有传任何参数的 C++/Go例 子:
// call_once example#include <iostream>// std::cout#include <thread>// std::thread, std::this_thread::sleep_for#include <mutex>// std::call_once, std::once_flagstd::once_flag flag;int Gid = 0;void setup(){Gid++;std::cout << "Called once " << Gid << std::endl;}void doprint(){std::call_once(flag, setup);}int main(){std::thread t1(doprint);std::thread t2(doprint);std::thread t3(doprint);std::thread t4(doprint);t1.join();t2.join();t3.join();t4.join();std::cout << "Gid: " << Gid << std::endl;}
Go例子:
package mainimport ("fmt""sync""time")var once sync.Oncevar Gid intfunc setup() {Gid++fmt.Println("Called once")}func doprint() {once.Do(setup)fmt.Println("doprint()…")}func main() {go doprint()go doprint()go doprint()go doprint()time.Sleep(time.Second)fmt.Println("Gid:", Gid)} 关于参数传递，C++是很容易解决的，直接在call_once后面加就行了 。// call_once example#include <iostream>// std::cout#include <thread>// std::thread, std::this_thread::sleep_for#include <mutex>// std::call_once, std::once_flagstd::once_flag flag;int Gid = 0;void setup(std::string parm){Gid++;std::cout << "Called once! " << Gid << " parm:" << parm.c_str() << std::endl;}void doprint(std::string parm){std::call_once(flag, setup, parm);}int main(){std::thread t1(doprint, "1");std::thread t2(doprint, "2");std::thread t3(doprint, "3");std::thread t4(doprint, "4");t1.join();t2.join();t3.join();t4.join();std::cout << "Gid: " << Gid << std::endl;} Go语言的sync.Once后面不允许直接传参数，但可以通过以下方法来变通。package mainimport ("fmt""sync""time")var once sync.Oncevar Gid intfunc doprint(parm string) {setup := func() {Gid++fmt.Println("Called once! parm:", parm)}once.Do(setup)fmt.Println("doprint()…")}func main() {go doprint("1")go doprint("2")go doprint("3")go doprint("4")time.Sleep(time.Second)fmt.Println("Gid:", Gid)} 完毕!
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