Artem Zefirov Artem Zefirov - 2 months ago 6
C++ Question

passing pointer to multiply classes to a member function

I have the next classes:

"Integrator.h"

#include <vector>
#include <array>
using namespace std;

class Integrator {
public:
using coord_type = array<double, 3>;
protected:
void base_integrate_callback(const coord_type, double t_k) {
//does nothing
}
};

class MyIntegrator :public Integrator {
public:
template <class T>
void integrate(int mp_id, int t_span, int step ,
void(T::*callback)(const coord_type, double) = (Integrator::*)(const coord_type, double)){
//calls callback here
}
};

"main.cpp"

#include Integrator.h"

struct caller {
void callback(const Integrator::coord_type coord, double t_k) {
//does smth
}
};

int main(){
MyIntegrator integrator_1;
caller A;
int mp_id = 1;
int span = 365;
int step = 1;
integrator_1.integrate<caller>(mp_id,span,step,&A.callback);
return 0;
}


Trying to compile it I get an error:


file:integration.h, line 18, syntax error: '< tag>::*'


How can I call a callback which could belong to any class?

And the second question: when I try to call it without explicit template specification like

integrator_1.integrate(mp_id,span,step,&A.callback);


I get an error


file: main.cpp , line 65, 'MyIntegrator::integrate': no matching overloaded function found


So, why this function can not deduce its argument from its parameter?

Also I get the same error when calling it without the last parameter relying on the default parameter.

integrator_1.integrate(mp_id,span,step);

Answer

Decrypting what you have here with a little indentation

template <class T>
void integrate(int mp_id, 
               int t_span, 
               int step ,
               void(T::*callback)(const coord_type, double) = (Integrator::*)(const coord_type, double))
{
    //calls callback here
}

it looks like you are trying to declaring a method that takes a callback function as a parameter and assigning a default value. Unfortunately the default value looks like the declaration of another method pointer and not a method. You need to use a pointer to a method of T.

template <class T>
void integrate(int mp_id, 
               int t_span, 
               int step ,
               void(T::*callback)(const coord_type, double) = &Integrator::base_integrate_callback)
{
    //calls callback here
}

but I don't think this will be kosher as there is no way to ensure that T and Integrator are in any way related.

For example, after cleaning up

integrator_1.integrate < caller > (mp_id, span, step, &A.callback);

to

integrator_1.integrate < caller > (mp_id, span, step, &caller::callback);

because you need to provide a pointer to a method, not an object referring to a method. This exposes another problem we'll get to in a moment, but it will compile for now and let us continue.

But this would not

integrator_1.integrate < caller > (mp_id, span, step);

because the signature of Integrator::base_integrate_callback, void Integrator::base_integrate_callback(const coord_type, double), does not match the signature of void(caller::*callback)(const coord_type, double). They look the same, don't they? What's missing is the hidden this parameter all methods have. caller::*callbackexpects a caller *, but Integrator::base_integrate_callback provides Integrator *.

You can fix this by making caller and it's ilk inherit Integrator rather than MyIntegrator, but moving base_integrate_callback to a new struct Integrated and having caller and friends inherit Integrated would make more sense.

And back to the other problem I mentioned earlier. In

template <class T>
void integrate(int mp_id, 
               int t_span, 
               int step ,
               void(T::*callback)(const coord_type, double) = &Integrated::base_integrate_callback)
{
    coord_type x; // junk for example
    double y; //junk for example
    callback(x,y); //KABOOM!
}

On what object is callback being invoked? integrate will need one more parameter, a reference to T to provide context for callback.

template <class T>
void integrate(int mp_id, 
               int t_span, 
               int step,
               T & integrated,
               void(T::*callback)(const coord_type, double) = &Integrated::base_integrate_callback)
{
    coord_type x; // junk for example
    double y; //junk for example
    integrated.callback(x,y);
}

Then you have to use the correct syntax to invoke the function pointer because the above will always call caller::callback.

template <class T>
void integrate(int mp_id, 
               int t_span, 
               int step,
               T & integrated,
               void(T::*callback)(const coord_type, double) = &Integrated::base_integrate_callback)
{
    coord_type x; // junk for example
    double y; //junk for example
    (integrated.*callback)(x,y); //std::invoke would be preferred if available
}

All together:

#include <array>
#include <iostream>

class Integrator
{
public:
    using coord_type = std::array<double, 3>;
};

struct Integrated
{
    void base_integrate_callback(const Integrator::coord_type, double t_k)
    {
        std::cout << "made it to default" << std::endl;
    }
};

class MyIntegrator: public Integrator
{
public:
    template <class T>
    void integrate(int mp_id,
                   int t_span,
                   int step,
                   T & integrated,
            void(T::*callback)(const coord_type, double) = &Integrated::base_integrate_callback)
    {
        coord_type x; // junk for example
        double y = 0; //junk for example
        (integrated.*callback)(x,y);
    }
};


struct caller:public Integrated
{
    char val; // for test purposes
    caller(char inval): val(inval) // for test purposes
    {

    }
    void callback(const Integrator::coord_type coord, double t_k)
    {
        std::cout << "made it to " << val << std::endl;
    }
};

int main()
{
    MyIntegrator integrator_1;
    caller A {'A'};
    caller B {'B'};
    caller C {'C'};
    int mp_id = 1;
    int span = 365;
    int step = 1;
    integrator_1.integrate < caller > (mp_id, span, step, A, &caller::callback);
    integrator_1.integrate < caller > (mp_id, span, step, B, &caller::callback);
    integrator_1.integrate < caller > (mp_id, span, step, C);
    return 0;
}

Recommendation: Step into 2011 and see what std::function and lambda expressions can do for for you.

Here's an example:

#include <array>
#include <iostream>
#include <functional>

class Integrator
{
public:
    using coord_type = std::array<double, 3>;
};

// no need for integrated to get default callback

class MyIntegrator: public Integrator
{
public:
    template <class T>
    void integrate(int mp_id,
                   int t_span,
                   int step,
                   // no need to provide object instance for callback. packed with std::bind
                   std::function<void(const coord_type, double)> callback =
                           [](const coord_type, double) { std::cout << "made it to default" << std::endl; })
                           // default callback is now lambda expression
    {
        coord_type x; // junk for example
        double y = 0; //junk for example
        callback(x,y); // no weird syntax. Just call a function
    }
};


struct caller
{
    char val; // for test purposes
    // no need for test constructor
    void callback(const Integrator::coord_type coord, double t_k)
    {
        std::cout << "made it to " << val << std::endl;
    }
};

int main()
{
    MyIntegrator integrator_1;
    caller A {'A'};
    caller B {'B'};
    // no need for test object C
    int mp_id = 1;
    int span = 365;
    int step = 1;
    using namespace std::placeholders; // shorten placeholder names
    integrator_1.integrate < caller > (mp_id, 
                                       span, 
                                       step, 
                                       std::bind(&caller::callback, A, _1, _2));
    // std bind bundles the object and the callback together into one callable package

    integrator_1.integrate < caller > (mp_id, 
                                       span, 
                                       step, 
                                       [B](const Integrator::coord_type p1, 
                                           double p2) mutable // lambda captures default to const 
                                       { 
                                           B.callback(p1, p2); // and callback is not a const method
                                       });
    // Using lambda in place of std::bind. Bit bulkier, but often swifter and no 
    //need for placeholders

    integrator_1.integrate < caller > (mp_id,
                                       span,
                                       step,
                                       [](const Integrator::coord_type p1,
                                           double p2)
                                       {
                                           std::cout << "Raw Lambda. No callback object at all." << std::endl;
                                       });
    //custom callback without a callback object

    integrator_1.integrate < caller > (mp_id, span, step);
    //call default

    return 0;
}
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