原始C++标准仅支持单线程编程。新的C++标准(称为c++11或c++0x)于2011年发布。在c++11中，引入了新的线程库。因此运行本文程序需要C++至少符合c++11标准。
3 小心地将参数传递给线程
要将参数传递给线程的关联可调用对象或函数，只需将其他参数传递给std::thread构造函数。默认情况下，所有参数都复制到新线程的内部存储中。让我们看一个例子：
3.1 在c++11中将简单参数传递给std::thread
#include <iostream>
#include <string>
#include <thread>
void threadCallback(int x, std::string str)
{
std::cout << "Passed Number = " << x << std::endl;
std::cout << "Passed String = " << str << std::endl;
}
int main()
{
int x = 10;
std::string str = "Sample String";
std::thread threadObj(threadCallback, x, str);
threadObj.join();
return 0;
}
输出为：
Passed Number = 10
Passed String = Sample String
3.2 如何不将参数传递给c++11中的线程
不要将变量的地址从本地堆栈传递到线程的回调函数。因为线程1中的局部变量可能超出了作用域，但是线程2仍然试图通过它的地址访问它。在这种情况下，访问无效地址可能会导致意外行为。例如：
#include <iostream>
#include <thread>
void newThreadCallback(int * p)
{
std::cout << "Inside Thread:"":p = " << p << std::endl;
std::chrono::milliseconds dura(1000);
std::this_thread::sleep_for(dura);
*p = 19;
}
void startNewThread()
{
int i = 10;
std::cout << "Inside Main Thread:"":i = " << i << std::endl;
std::thread t(newThreadCallback, &i);
t.detach();
std::cout << "Inside Main Thread:"":i = " << i << std::endl;
}
int main()
{
startNewThread();
// 表示一段时间，这里是2000 毫秒
std::chrono::milliseconds dura(2000);
// 当前线程休眠一段时间
std::this_thread::sleep_for(dura);
return 0;
}
输出为：
Inside Main Thread::i = 10
Inside Thread::p = Inside Main Thread::i = 10
000000D9DD5BF4A4
程序崩溃
同样，将指向堆上内存的指针传递给线程时也要小心。因为在新线程试图访问该存储器之前，某些线程可能会删除该存储器。在这种情况下，访问无效地址可能导致意外行为。例如：
#include <iostream>
#include <thread>
void newThreadCallback(int * p)
{
std::cout << "Inside Thread : "" : p = " << p << std::endl;
std::chrono::milliseconds dura(1000);
std::this_thread::sleep_for(dura);
*p = 19;
}
void startNewThread()
{
int * p = new int();
*p = 10;
std::cout << "Inside Main Thread : "" : *p = " << *p << std::endl;
std::thread t(newThreadCallback, p);
t.detach();
delete p;
p = NULL;
}
int main()
{
startNewThread();
std::chrono::milliseconds dura(2000);
std::this_thread::sleep_for(dura);
return 0;
}
输出为：
Inside Main Thread :? : *p = 10
Inside Thread :? : p = 0000024AC61ECEA0
3.3 如何在c++11中传递对std::thread的引用
由于参数被复制到新的线程堆栈，因此，如果您需要以通用方式传递引用，即
#include <iostream>
#include <thread>
void threadCallback(int const & x)
{
int & y = const_cast<int &>(x);
y++;
std::cout << "Inside Thread x = " << x << std::endl;
}
int main()
{
int x = 9;
std::cout << "In Main Thread:Before Thread Start x = " << x << std::endl;
std::thread threadObj(threadCallback, x);
threadObj.join();
std::cout << "In Main Thread:After Thread Joins x = " << x << std::endl;
return 0;
}
输出为：
In Main Thread:Before Thread Start x = 9
Inside Thread x = 10
In Main Thread:After Thread Joins x = 9
即使threadCallback接受参数作为参考，但所做的更改仍在线程外部不可见。这是因为线程函数threadCallback中的x引用了在新线程的堆栈上复制的临时值。如何解决呢？使用std::ref()即可。std::ref 用于包装按引用传递的值。
#include <iostream>
#include <thread>
void threadCallback(int const & x)
{
int & y = const_cast<int &>(x);
y++;
std::cout << "Inside Thread x = " << x << std::endl;
}
int main()
{
int x = 9;
std::cout << "In Main Thread : Before Thread Start x = " << x << std::endl;
std::thread threadObj(threadCallback, std::ref(x));
threadObj.join();
std::cout << "In Main Thread : After Thread Joins x = " << x << std::endl;
return 0;
}
输出为：
In Main Thread : Before Thread Start x = 9
Inside Thread x = 10
In Main Thread : After Thread Joins x = 10
3.4 将指向类成员函数的指针分配为线程函数
将指针传递给成员函数作为回调函数，并将指针传递给Object作为第二个参数。例如：
#include <iostream>
#include <thread>
class DummyClass {
public:
DummyClass()
{}
DummyClass(const DummyClass & obj)
{}
void sampleMemberFunction(int x)
{
std::cout<<"Inside sampleMemberFunction "<<x<<std::endl;
}
};
int main() {
DummyClass dummyObj;
int x = 10;
std::thread threadObj(&DummyClass::sampleMemberFunction,&dummyObj, x);
threadObj.join();
return 0;
}
输出为：
Inside sampleMemberFunction 10
