非标准语法； 将成员函数分配给vector时，使用'＆'创建指向成员错误的指针

Question

Im trying to add a member function of Player Class to a vector of functions.

This is the vector of functions that require a Keyboard::Key as parameter.

vector<function<void(Keyboard::Key)>> KeyEvent;

Then i have the Players implementation

Player::Player() {
    KeyEvent.push_back(OnKey); //error: non-standard syntax; use '&' to create a pointer to member
}

void Player::OnKey(Keyboard::Key key) {
    //do something with key...
}

I have other instances where something code similair works fine, but not with member functions.

vector<function<void()>> UpdateEvent;

UpdateEvent.push_back(HandleInputUpdate);

void HandleInputUpdate() {
    for (size_t i = 0; i < pressedKeys.size(); i++) {
        for (size_t e = 0; e < KeyEvent.size(); e++) {
            KeyEvent[e](pressedKeys[i]);
        }
    }
}

Is it possible to add an a member function to this vector and if not, how could I create something similair? I am very new to C++ and I created the vector so I could replicate C# Actions. This is how far i have come until I ran into problems. This is an early concept as I still need to add a way to remove functions from the vector.

Answer 1

OnKey is a non-static member function of Player and thus requires an instance of Player to be called on. Even if you got the pointer-to-member-function syntax right, you'd still have that exact problem, because there is no automatic binding; in other words, the instance pointed to by this is not automatically used to produce a callable object usable as a std::function<void(Keyboard::Key)> .

A very easy way to achieve what you want to do here is to use a lambda:

KeyEvent.push_back([this](auto key) { OnKey(key); });

The OnKey(key); call in the function is equivalent to this->OnKey(key); .

---

A more complicated (in my opinion) way would be to use std::bind :

using namespace std::placeholders;
KeyEvent.push_back(std::bind(&Player::OnKey, this, _1));

Generally speaking, with lambdas and std::function in modern C++, use cases for pointers to member functions have become extremely rare.

Answer 2

UsePlayer :: OnKey（仅具有OnKey甚至＆OnKkey）不足以用于指针到成员的情况。

Answer 3

Is it possible to add an a member function to this vector

No. Non-static member function pointers are quite a bit different than non-member function pointers and static member function pointers.

and if not, how could I create something similair?

Use a lambda expression. In the lambda expression, make sure to capture the this pointer and use it in the function call.

KeyEvent.push_back([this](Keyboard::Key key) {this->OnKey(key);});