std::bind 和 std::function 具有多态性，基数 class

Question

I'm trying to bind an object to a function, but the base's class function is being called. The code it's just an example for the real code. I will only have the string later in the real code.

 class A {
    public:
        virtual void f(const std::string &s) {
            std::cout << "A " << s << std::endl;
        }
    };

    class B : public A {
    public:
        void f(const std::string &s) override {
            std::cout << "B " << s << std::endl;
        }
    };

    void main() {
        std::unique_ptr<A> a = std::make_unique<B>();

        std::function<void(const std::string &)> callback
                =  std::bind(&A::f, *a, std::placeholders::_1);
        callback("my string");
    }

The result is

 A my string

I tried to change it to &B:f, but got an error.

No viable conversion from 'typename _Bind_helper<__is_socketlike<void (B::*)(const basic_string<char> &)>::value, void (B::*)(const basic_string<char> &), A &, const _Placeholder<1> &>::type' (aka '_Bind<void (Centerity::Cluster::Correlation::B::*(Centerity::Cluster::Correlation::A, std::_Placeholder<1>))(const std::basic_string<char> &)>') to 'std::function<void (const std::string &)>' (aka 'function<void (const basic_string<char> &)>')

I tried to change it to A* a = B(); same result. Couldn't find a similar code anywhere. Of course creating B b will call B::f , but that's not helping me.

I can lose the bind and use lambda, but I don't know how.

Answer 1

NOTE:

I don't know which compiler you are using but main shouldn't be void. I have fixed it in my code below:

#include <memory>
#include <iostream>
#include <functional>

class A {
public:
    virtual void f(const std::string &s)
    {
        std::cout << "A " << s << std::endl;
    }
};

class B: public A {
public:
    void f(const std::string &s) override 
    {
        std::cout << "B " << s << std::endl;
    }
};

int main() 
{
    std::unique_ptr<A> a = std::make_unique<B>();
    auto callback = std::bind(&A::f, a.get(), std::placeholders::_1);

    callback("my string");
    return 0;
}

Outputs:

B my string

The catch is to pass pointer to the base class (pointing to an object of derived class) to std::bind(&A::f, a.get(), std::placeholders::_1); .

If you pass by value, then a new object of your base class A will be created (by copy constructor) and the f version of base class will be called as you have observed in your program.

Answer 2

As you ask for the lambda version:

class A { 
    public:
        virtual void f(const std::string &s) {
            std::cout << "A " << s << std::endl;
        }   
};  

class B : public A { 
    public:
        void f(const std::string &s) override {
            std::cout << "B " << s << std::endl;
        }
};  

int main() {
    std::unique_ptr<A> a = std::make_unique<B>();

    auto callback = [&a](const std::string& s){ return a->f(s); };

    callback("my string");
}