指向void *函数的指针C ++

Question

I am trying to call pointer to a void * function inside the main method and the compiler is saying assigning to 'funcptr<g>' from incompatible type 'void *(void *) . hello function is actually an argument to pthread_create function. That's why it is void * function. How can I create a function pointer to a void * function?

#include <iostream> 
#include <pthread.h> 

using namespace std; 

template<typename T> 
using funcptr = void (*T::*)(void *); // I think it is wrong here.

class m { 
public: 
    template <typename T> 
    struct my_struct { 
        funcptr<T> ptr; 
    };
}; 

class g { 
public: 
    static void *hello(void *); 
}; 

int main() { 
    struct m::my_struct<g> h; 
    h.ptr = g::hello; // Error here

    return 0; 
}

Answer 1

How can I create a function pointer to a void * function? hello is not a member function, but it's a static function.

So your funcptr should be as follows:

// No template needed.
using funcptr = void* (*)(void *)

Note that hello is declared with static , meaning that it's no longer a member function to g .

Static members of a class are not associated with the objects of the class.

So using void (*T::*)(void *) to cull non-member functions is incorrect.

If you're allowed to use a compiler that supports C++11, you don't even need to manually deduct its type anymore, using decltype :

// decltype deducts its exact type for you.
using funcptr = decltype(&g::hello);

class m 
{ 
public: 
    struct my_struct 
    { 
        funcptr ptr; 
    };
}; 

FYI, since hello does not have its definition, you might encounter a linkage error. To prevent that, I assumed that there's some implementation inside:

static void *hello(void *) 
{ 
    // Meaningless, but..
    return nullptr;
}

Answer 2

if you're using C++11, you can use std::function<> which just bothers about the return type and parameters of the function and not where they are defined and what are its type.

Here is the code using std::function<>

#include <iostream> 
#include <functional>
#include <pthread.h> 

using namespace std; 

class m { 
public: 
    template <typename T> 
    struct my_struct { 
        function<void*(void*)> ptr;
    };
}; 

class g { 
public: 
    static void *hello(void *) {
        cout<<"Hello.."<<endl;
    }
}; 

int main() { 
    struct m::my_struct<g> h; 
    h.ptr = g::hello;
    h.ptr(nullptr);

    return 0; 
}