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 :)

since `Operand` is a class template, you need `Operand::castInt8` — Sander De Dycker, Jul 13 '18 at 11:12
You also need to place the complete definition of `Operand` *before* the use of `Operand::castInt8`. In fact this is what your error message is telling you. — john, Jul 13 '18 at 11:14
You also need to make castInt8 public and the fct_cast member in the typeinfo struct has to be a pointer to member function instead of a plain function pointer — phön, Jul 13 '18 at 11:15
@PaulinaSprawka Does IOperand has virtual `castInt8` method? — bartop, Jul 13 '18 at 11:29
@bartop no, this is not a virtual method. Should it be virtual? If yes, why? — Paulina Sprawka, Jul 13 '18 at 12:30
@PaulinaSprawka If it's inherited it is, in most cases, intended to be used polimorphically. But I do not know Your exact use case — bartop, Jul 13 '18 at 12:33
@bartop yes, it's about polymorphism, but `castInt8` is not a member of `IOperand` class, it's `Operand`'s method — Paulina Sprawka, Jul 13 '18 at 13:12

bartop · Answer 1 · 2018-07-13T11:21:37.600

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

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.
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).

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},
};

@PaulinaSprawka I had to put something, it's just an example — bartop, Jul 13 '18 at 13:28

Using template class method outside class in dispatch table

1 Answers1