c++ - 为什么在调用 std::call_once() 时需要这个指针？

Question

在“C++ Concurrency in Action”一书§3.3.1 中，在介绍使用std::call_once() 对类成员进行线程安全延迟初始化时，给出了以下示例:

#include <mutex>

struct connection_info
{};

struct data_packet
{};

struct connection_handle
{
    void send_data(data_packet const&)
    {}
    data_packet receive_data()
    {
        return data_packet();
    }
};

struct remote_connection_manager
{
    connection_handle open(connection_info const&)
    {
        return connection_handle();
    }
} connection_manager;


class X
{
private:
    connection_info connection_details;
    connection_handle connection;
    std::once_flag connection_init_flag;

    void open_connection()
    {
        connection=connection_manager.open(connection_details);
    }
public:
    X(connection_info const& connection_details_):
        connection_details(connection_details_)
    {}
    void send_data(data_packet const& data)
    {
        std::call_once(connection_init_flag,&X::open_connection,this);
        connection.send_data(data);
    }
    data_packet receive_data()
    {
        std::call_once(connection_init_flag,&X::open_connection,this);
        return connection.receive_data();
    }
};

int main()
{}

来自 its doc ，第三个参数是传递给函数X::open_connection()的参数。鉴于 X::open_connection() 没有输入参数，为什么调用 std::call_once() 时这里需要 this 指针？

std::call_once(connection_init_flag,&X::open_connection,this);

P.S.:删除this指针会导致C2064错误:

error C2064: term does not evaluate to a function taking 0 arguments

---

更新:在引入类似函数时，“C++ Concurrency in Action”一书的 §4.2.1 中进一步明确解决了这个问题，即 std::async:

If the first argument (should be the second one for std::call_once) is a pointer to a member function, the second argument (should be the third one for std::call_once) provides the object on which to apply the member function (either directly, or via a pointer, or wrapped in std::ref), and the remaining arguments are passed as arguments to the member function. Otherwise, the second (should be the third one for std::call_once) and subsequent arguments are passed as arguments to the function or callable object specified as the first argument.

Answer 1

Why is this pointer needed when calling std::call_once()?

因为open_connection 是一个非静态数据成员。它必须在某些东西上被调用，并且那个东西是同一个实例，由 this 指向(从技术上讲，非静态成员函数有一个隐式的第一个参数用于 this。 )

它可以用不同的实例调用，尽管在这种情况下这没有意义:

X x;
std::call_once(connection_init_flag, &X::open_connection, &x);