C++多线程的使用，返回值和加锁

使用C++多线程可以进行并行计算，提高执行速率。
在创建thread对象时，要注意如果返回参数是引用类型，如果不使用std::ref前缀，会发生：

Error	C2672	'std::invoke': no matching overloaded function found;
Error	C2893	Failed to specialize function template 'unknown-type std::invoke(_Callable &&,_Types &&...) noexcept(<expr>)';

Reference: https://stackoverflow.com/questions/62187098/stdinvoke-no-matching-overloaded-function-found
正确的使用方式：

The arguments to the thread function are moved or copied by value.
If a reference argument needs to be passed to the thread function,
it has to be wrapped (e.g., with std::ref or std::cref).

std::thread my_thread(process_client, std::ref(client));

Example：

#include <thread>

void sum(int a, int b, int &c) {
	c = a + b;
}

int sum1 = 0;
int sum2 = 0;
std::thread fun1(sum, 1, 2, std::ref(sum1));
std::thread fun2(sum, 3, 4, std::ref(sum2));

fun1.join(); // wait for thread return
fun2.join();

当有其他需求如：加锁和同步等，参考：C++多线程详细讲解