passing pointer to multiply classes to a member function - c++

Question

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 1

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;
}