无法将'void（myClass :: *）（）转换为void（*）（）

Question

So, I know this question exists in so many places. But none of the examples helped me solve my issue.

I'm trying to create a method pointer (within a class), so it will address one of several methods of the class according to specific conditions.

I tried, unsuccessfully to use a static function (guess I misunderstood the instructions how to do so...).

here is the header file:

class myClass
{
public:
    myClass(int value);

    void methodA(const string &msg);
    void methodB(const string &msg);

    void (*send_msg)(const string &msg);
};

and the cpp:

myClass::myClass(int value){
    if(value > 0){
        send_msg = &methodA
    }
    else{
        send_msg = &methodB
    }
}

and the errors, as some of you already know:

error: ISO C++ forbids taking the address of an unqualified or parenthesized non-static member function to form a pointer to member function. Say '&myClass::methodA' [-fpermissive]

error: cannot convert 'void (myClass:: )(const string&) {aka void (myClass:: )(const std::basic_string&)}' to 'void ( )(const string&) {aka void ( )(const std::basic_string&)}' in assignment

any help will be much appreciated.

Edit Thanks to Jose, it can be compile:

void (myClass::*send_msg)(const string &msg);

and the assignment:

send_msg = &myClass::methodA;

But now, as I'm trying to use the code I cannot call the function-pointer:

this->myClass_instance.send_msg(line); //*****getting error*****

when this is an instance of other class. The error is:

error: must use '. ' or '-> ' to call pointer-to-member function in '((otherClass*)this)->otherClass::myClass_instance.myClass::send_msg (...)', eg '(... ->* ((otherClass*)this)->otherClass::myClass_instance.myClass::send_msg) (...)'

Answer 1

My demo: http://ideone.com/Vx4x88

Make send_msg a pointer to a class method by adding myClass:: .

void (myClass::*send_msg)(const string &msg);

Also, it seems like you are missing myClass:: and a semicolon ; at the end of

    send_msg = &myClass::methodA;
else
    send_msg = &myClass::methodB;

Edit:

Since your requested how to call it from outside the class in your comment:

myClass c(1);
(c.*c.send_msg)("hi");

or

myClass * p = new myClass(1);
(p->*p->send_msg)("hi");

Answer 2

void (*send_msg)(const string &msg); is declaration of pointer for free function or static member function, not non-static member function. You might want:

void (myClass::*send_msg)(const string &msg);

LIVE1

Or you could make the functions to be static member function:

static void methodA(const string &msg);
static void methodB(const string &msg);

LIVE2

For the 1st compile error, you should qualify name of non-static member function when taking its address (for the 1st solution only):

myClass::myClass(int value){
    if(value > 0){
        send_msg = &myClass::methodA;
                    ~~~~~~~~~
    }
    else{
        send_msg = &myClass::methodB;
                    ~~~~~~~~~
    }
}

Answer 3

you can also use std::function

#include <functional>

class myClass
{
public:
    myClass(int value){
     if(value > 0){
        send_msg = [this](const std::string& str){ methodA(str); };
     }
     else{
         send_msg = [this](const std::string& str){ methodB(str); };
     }
    }

    void methodA(const string &msg);
    void methodB(const string &msg);

    std::function<void(const string &msg)> send_msg;
};

to call the function, simply use () :

myClass obj(10);
obj.send_msg("hi there");