使用成员函数作为线程人口函数现象观察

#include <iostream>
#include <string>
using namespace std;
class A {
public:
	A(int a) {
		std::cout << "构造函数执行了" << "this = " << this << " threadid = " << std::this_thread::get_id() << std::endl;
	}
	A(const A& a) {
		std::cout << "拷贝构造函数执行了" << "this = " << this << " threadid = " << std::this_thread::get_id() << std::endl;
	}
	~A() {
		std::cout << "析构函数执行了" << "this = " << this << " threadid = " << std::this_thread::get_id() << std::endl;
	}
public:
	void run(int ab) {
		std::cout << ab << std::endl;
		std::cout << "子线程ID = " << std::this_thread::get_id() <<std::endl;
	}
};

int main()
{
	A a(1);
	std::thread obj(&A::run, a, 1);
	std::cout << "主线程ID = " << std::this_thread::get_id() << std::endl;
	obj.join();
}

在上面的例程当中使用了非静态成员函数作为线程入口函数，同时传递了一个类对象和一个参数给线程对象obj。

来看下打印结果：

从结果可以看到，调用了拷贝构造函数，所以是复制了一个类对象到子线程当中执行的。并且复制的动作是在主线程当中执行的，而析构的动作是在子线程当中执行的。

如果传递的是&a我们再来看下现象如何：

#include <iostream>
#include <string>
using namespace std;
class A {
public:
	A(int a) {
		std::cout << "构造函数执行了" << "this = " << this << " threadid = " << std::this_thread::get_id() << std::endl;
	}
	A(const A& a) {
		std::cout << "拷贝构造函数执行了" << "this = " << this << " threadid = " << std::this_thread::get_id() << std::endl;
	}
	~A() {
		std::cout << "析构函数执行了" << "this = " << this << " threadid = " << std::this_thread::get_id() << std::endl;
	}
public:
	void run(int ab) {
		std::cout << ab << std::endl;
		std::cout << "子线程ID = " << std::this_thread::get_id() <<std::endl;
	}
};

int main()
{
	A a(1);
	std::thread obj(&A::run, &a, 1);
	std::cout << "主线程ID = " << std::this_thread::get_id() << std::endl;
	obj.join();
}

从结果可以看出，没有调用拷贝构造函数，所以要注意了，必须要使用join了，避免主线程执行结束了，子线程还在使用类a对象。

我们再来看看使用std::ref

#include <iostream>
#include <string>
using namespace std;
class A {
public:
	A(int a) {
		std::cout << "构造函数执行了" << "this = " << this << " threadid = " << std::this_thread::get_id() << std::endl;
	}
	A(const A& a) {
		std::cout << "拷贝构造函数执行了" << "this = " << this << " threadid = " << std::this_thread::get_id() << std::endl;
	}
	~A() {
		std::cout << "析构函数执行了" << "this = " << this << " threadid = " << std::this_thread::get_id() << std::endl;
	}
public:
	void run(int ab) {
		std::cout << ab << std::endl;
		std::cout << "子线程ID = " << std::this_thread::get_id() <<std::endl;
	}
};

int main()
{
	A a(1);
	std::thread obj(&A::run, std::ref(a), 1);
	std::cout << "主线程ID = " << std::this_thread::get_id() << std::endl;
	obj.join();
}

结果和使用&a是一样的。