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

3.5 参考

https://thispointer.com//c11-multithreading-part-4-data-sharing-and-race-conditions/