Get a pointer to object's member function

问题:

Here is the problem:

1) I have a class like so:

class some_class
{
public:
    some_type some_value;
    int some_function(double *a, double *b, int c, int d, void *e);
};

2) Inside some_function, I use some_values from some_class object to get a result.

3) So, I have a concrete object and I want to get a pointer to this object some_function.

Is it possible? I can't use some_fcn_ptr because the result of this function depends on the concrete some_value of an object.

How can I get a pointer to some_function of an object? Thanks.

typedef  int (Some_class::*some_fcn_ptr)(double*, double*, int, int, void*);

回答1:

You cannot, at least it won't be only a pointer to a function.

Member function is common for all instances of this class. All member functions have implicit (first) parameter - this. To call a member function for specific instance you need pointer to this member function and this instance.

class Some_class
{
public:
    void some_function() {}
};

int main()
{
    typedef void (Some_class::*Some_fnc_ptr)();
    Some_fnc_ptr fnc_ptr = &Some_class::some_function;

    Some_class sc;

    (sc.*fnc_ptr)();

    return 0;
}

More info here in C++ FAQ

Using Boost this can look like (C++11 provides similar functionality):

#include <boost/bind.hpp>
#include <boost/function.hpp>

boost::function<void(Some_class*)> fnc_ptr = boost::bind(&Some_class::some_function, _1);
Some_class sc;
fnc_ptr(&sc);

C++11's labdas:

#include <functional>

Some_class sc;
auto f = [&sc]() { sc.some_function(); };
f();
// or
auto f1 = [](Some_class& sc) { sc.some_function(); };
f1(sc);

回答2:

You may write some kind of Wrapper which is able to use both function or method as parameter.

I used following classes for launching functions (it was used in one my SDL program):

class CallbackFunction {
public:
    // Constructor, copy constructor and destructor

    virtual int execute( SDL_keysym* keysym) const;
    virtual int operator()( SDL_keysym* keysym) const;

protected:
    int( *callback)( SDL_keysym*));
}

int CallbackFunction::execute( SDL_keysym* keysym) const{
    return callback(keysym);
}

int CallbackFunction::operator()( SDL_keysym* keysym) const{
    return callback( keysym);
}

And this extension for "methods":

template<class T>
class CallbackMethod : public CallbackFunction {
public:
    // Constructor, copy constructor and destructor
    CallbackMethod( T *object, int(T::*callback)( SDL_keysym* keysym));

    int execute( SDL_keysym* keysym) const;
    int operator()(SDL_keysym* keysym) const;

protected:
    T *object;
    int(T::*method)( SDL_keysym* keysym);
};

// Object initialization (constructor)
template<class T>
CallbackMethod<T>::CallbackMethod( T *object, int(T::*callback)( SDL_keysym* keysym)):
    CallbackFunction( NULL),object(object),method(callback){
}


// Responsible for executing
template<class T>
int CallbackMethod<T>::execute( SDL_keysym* keysym) const {
    return (object->*method)(keysym);
}
template<class T>
int CallbackMethod<T>::operator()( keysym) const {
    return (object->*method)( keysym);
}

And then use it as:

CallbackFunction *callback;
callback = new CallbackFunction( myFunction);
callback = new CallbackMethod<A>( instanceOfA, instanceOfA::myMethod);
callback = new CallbackMethod<B>( instanceOfB, instanceOfB::myMethod);
...
callback( keysym);

I found macro as this:

CALLBACK(object,method) new CallbackMethod<typeof(*object)>( object, &method)

really useful

回答3:

No, you can't get a pointer to a C++ class method (unless the method is declared static). The reason is that a class method always has the pointer this, a pointer to the class instance. But were you to call the method through a pointer, that pointer could not encapsulate the this pointer, and then then there would be no instance attached, and therefore this behavior is not legal.

回答4:

While not exactly what you requested, if you can use C++11 the following might nevertheless suit your needs (untested):

std::function<int(double*, double*, int, int, void*)>
  some_function_of(Some_class& obj)
{
  return [&](double* a, double* b, int c, int d, void* e){
    return obj.some_func(a, b, c, d, e); };
}

回答5:

I used the Boost library. I included "boost/bind.hpp" in my code. Then a method named "fn" can be defined as

auto fn = boost::bind(ClassName::methodName, classInstanceName, boost::placeholders::_1);