Use a function pointer as template function type parameter?

Question

#include <iostream>
#include <unordered_map>
#include <utility>
#include <typeinfo>

using namespace std;
class Handle{
    public:
    int val;
    bool getAskPrice(int& tmp) const
    {
        tmp = val;
        return true;
    }
    
    bool setAskPrice(int& tmp)
    {
        val = tmp;
        return true;
    }
};

template<class RT, class ARG>
struct convertToAFL{
    static RT to_afl(ARG);
};

template<class RT, class ARG>
struct convertFromAFL{
    static RT from_afl(ARG);
};

template<>
struct convertToAFL<float, int>
{
    static float to_afl(int& value)
    {
        return static_cast<float>(value);
    }
};

template<>
struct convertFromAFL<int, float>
{
    static int from_afl(float& val)
    {
        return static_cast<int>(val);
    }
};

struct Getter{
    template<typename TICK_D, bool (Handle::*Getter)(TICK_D&) const, typename AFL_D>
    static AFL_D getter(const Handle& handle)
    {
        TICK_D temp;
        bool exists;
        exists = (handle.*Getter)(temp);
        AFL_D x = convertToAFL<AFL_D, TICK_D>::to_afl(temp);
        return exists ? x : -1;
    }
};

struct Setter{
    template<typename TICK_D, bool (Handle::*Setter)(TICK_D&), typename AFL_D>
    static void setter(Handle& handle, AFL_D& val)
    {   
        TICK_D x;
        x = convertFromAFL<TICK_D, AFL_D>::from_afl(val);
        (handle.*Setter)(x);
    }
};

int main()
{   
    Handle h;
    float val = 20.0;
    Setter::setter<int, &Handle::setAskPrice, float>(h, val);
    std::cout<<Getter::getter<int, &Handle::getAskPrice, float>(h);
    
    //std::pair<, &Setter::setter<int, &Handle::setAskPrice, float>> x;
    return 0;
}

The above code works as expected, however, in the main() instead of calling the functions, how can I store the pointer to the templatized Setter:setter() and Getter::getter() ? I m trying something like

std::pair<&Setter::setter<int, &Handle::setAskPrice, float>, &Getter::getter<int, &Handle::getAskPrice, float>(h)> func_pair;

And be able to call the functions later.

But i get an error saying

main.cpp: In function ‘int main()’:
main.cpp:85:118: error: type/value mismatch at argument 1 in template parameter list for ‘template struct std::pair’
     std::pair<&Setter::setter<int, &Handle::setAskPrice, float>, &Getter::getter<int, &Handle::getAskPrice, float>(h)> func_pair;
                                                                                                                      ^
main.cpp:85:118: note:   expected a type, got ‘& setter’
main.cpp:85:118: error: template argument 2 is invalid

Antoni · Accepted Answer · 2020-09-27 19:59:56Z

3

Static member functions are just a normal functions. You can store these pointers like this:

std::pair<void (*)(Handle& handle, float& val), float (*)(const Handle& handle)> 
    func_pair(&Setter::setter<int, &Handle::setAskPrice, float>, &Getter::getter<int, &Handle::getAskPrice, float>);

answered Sep 27, 2020 at 19:59

Antoni

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Comments

Eugene · Accepted Answer · 2020-09-27 19:54:21Z

1

Your problem is that template parameter is a type, but you are passing a value (pointer) as an argument. Instead, you could use auto like this:

auto func_pair = std::make_pair(&Setter::setter<int, &Handle::setAskPrice, float>, &Getter::getter<int, &Handle::getAskPrice, float>);

Edit: if you are using C++03, std::make_pair() is still available, but not auto. You will need to describe the type manually with a series of typedefs.

edited Sep 27, 2020 at 19:54

answered Sep 27, 2020 at 19:35

Eugene

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Comments

Ted Lyngmo · Accepted Answer · 2020-09-27 20:39:24Z

1

You can use decltype to get the pointer type.

Example:

std::pair<int, decltype(&Setter::setter<int, &Handle::setAskPrice, float>)> x = {
    1, &Setter::setter<int, &Handle::setAskPrice, float>
};

Prior to C++11:

std::pair<int, void(*)(Handle&, float&)> x(
    1, &Setter::setter<int, &Handle::setAskPrice, float>
);

edited Sep 27, 2020 at 20:39

answered Sep 27, 2020 at 19:33

Ted Lyngmo

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2 Comments

Sujith Over a year ago

I am using C++ 03

Ted Lyngmo Over a year ago

@Sujith Ok, I added a version without decltype.

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