Using template class method outside class in dispatch table

Question

Im writing this small template class inheriting from an interface. Inside my class I declared a variable I'd like to use outside and put it into a dispatch table. when I try to compile my program it throws me an error

This is my source code:

template <typename T> class Operand;
typedef struct  s_typeInfo
{
    int     enum_nb;
    bool    (*fct_cast)(void);
}               t_typeInfo;


t_typeInfo typeInfo[] =
{
    {0, Operand::castInt8},
};

template <typename T>
class Operand : public IOperand {
    ...
    bool    castInt8(void) {...}
}

I have been trying to solve this problem in many different ways, but no one them work. How could I fix it? Thank you in advance :)

Answer 1

There is number of things that cause erroer with compilation of Your code.

1. First of all, this construction Operand::castInt8 makes no sense to compiler as Operand is not a class/struct but a class template. To get pointer to function You need a concrete type not a template of it. Therefore something like this would be more reasonable Operand<int>::castInt8 for example.

2. The type of bool castInt8(void) is not bool (*)(void) as it appears to be. Non-static member functions have more complicated types. In Your case it would be bool (Operand<sometype>::*)(void).

3. One last thing - the compiler does not know Operand template has member castInt8 before the definition. So You should reorder it like this:

   template <typename T>
   class Operand : public IOperand {
       ...
       bool    castInt8(void) {...}
   }
   
   t_typeInfo typeInfo[] =
   {
       {0, &Operand<sometype>::castInt8},
   };
   

Putting it all together it would look like this:

template <typename T> class Operand;

typedef struct  s_typeInfo
{
    int     enum_nb;
    bool    (Operand<int>::*fct_cast)(void);
}               t_typeInfo;


template <typename T>
class Operand {
    public:
    bool    castInt8(void) {}
};


t_typeInfo typeInfo[] =
{
    {0, &Operand<int>::castInt8},
};