can not create a function pointer of class

[cplusplusstudent]

I have one templated class in one test1.h file.
template<Data_T>
class test
{
.....
.....
}
In another header file test2.h i am creating a structure
like below

struct example
{
int a,
float b,
test (*fun_ptr)(const test &a);
};

But compiler gives the error as "test" not declared.I have included the test1.h file in test2.h.

[Seraphimsan]

Its a template class so you need to specify what is substituted in for Data_T. When creating an object


ex:

struct example
{
int a
float b
test<int> (*fun_ptr)(const test &a);
}

Also

template<Data_T>
class test

should look like

template<typename Data_T>
class test

//or

template<class Data_T>
class test

[cplusplusstudent]

I have mentioned it like that only. I forgot to mention that in my example. Means i have declared my class as
template<typename Data_T>
class test
{
.....
.....
}

But in structure i don't want to specify the type(as mentioned in above post "int") because i want it to be decided at runtime.

[Galik]

On what basis is the type of the template parameter going to be decided?

[cplusplusstudent]

Actuallt the whole logic or implementation is below

in "test" class(templated) there is a function like

template<typename Data_T >
test<Data_T>
test<Data_T>::Convert(const Unit &unit)
{
......
......
func1() //this function resides in another .cpp file whose corresponding file is test2.h where that "example" structure is defined
}

convert function is called upon a "test" object.

Now in func1,
func1(...,...)
{
check for the mapping in the table of type "example",wherever match is found call the corresponding "fun_ptr" function
}

table is of type "example"

const example table[] =
{
{1,2,f1},
(2,3,f2},
.........
};

[cplusplusstudent]

I am still waiting for the solution guys :(

[moorecm]

The problem still isn't very clear. Could you explain what you're actually trying to do (the big picture)?

[cplusplusstudent]

i want to create a templated function pointer, and i don't want to supply the type as suggested by "Seraphimsan". Because i don't want to restrict the function pointer at compile time. Whenever user creates a object of any type(int,float, complex) then this function should be invoked.

[jsmith]

Templates allow compile-time polymorphism only.
Inheritance and virtual functions allow runtime polymorphism.

[cplusplusstudent]

That's true but the whole idea to provide template function or class is that you generalize the class or function so that whatever type of object is created of a template class that version of the object is created of the template class. Tht's waht i want to achieve from the function template here.
I don't want to specify the type in fucntion as below,otherwise i have to write 4 different structures of the same type( for float,complex etc)

struct example
{
int a
float b
test<int> (*fun_ptr)(const test &a);
}

[cplusplusstudent]

Hi Guys i think nobody is getting my point clearly..here i am explaining it at length

Firstly there is one .h file which consists of a template class,all the definitions of functions are defined in this class itself because its a template class.

//test.h
template<Data_T>
class test
{
test test_func_1(const test &var);
.....
};



There is another header file which has a structure "example" which should have a fucntion pointer as mentioned below,As "test" class is a template class i want this function pointer "fun_ptr" to behave like that


//test2.h
i can declare this structure like this (only one version,int only)

1. struct example
{
int a
float b
test<int> (*fun_ptr)(const test &a);
}

OR

2. template<typename Data_T>
struct example
{
int a
float b
test<Data_T> (*fun_ptr)(const test &a);
}

both of these version compiles successfully.

Now i want to declare an array of this templated structure e.g.

example example_array[]
{
{1,2,test_func_1}, //test_func_1 is defined in test.h file
{3,4,test_func_2}, //test_func_2 is defined in test.h file
};

But compiler gives an error at this point. If i use "template<typename Data_T>" before "example_array[]" then also compiler gives error.
Please tell me how to achieve this. Because of templates it is giving me errors.

[Galik]

Okay, I think your syntax for declaring member function pointers needs some work ;o)

template<typename Data_T>
class test
{
public:
	test<Data_T> test_func_1(const test &var)
	{
		// ... stuff ...
	}
};

template<typename Data_T>
struct example
{
	int a;
	float b;
	test<Data_T> (test<Data_T>::*fun_ptr)(const test<Data_T> &a);
};

example<int> example_array[] =
{
	{1, 2, &test<int>::test_func_1}
};

[cplusplusstudent]

Hi Galik,
Thanks for the detailed reply. But isn't the "example<int> example_array[]" restricting it to be only of type "int". What if i need to define for complex and doucble also? Then do i need to define them separately?

[Galik]

You have to declare the type of the array. There is no getting around that. C++ conspires to make you use the correct type from compile time. So you would need a different array for each type.

[jsmith]

Or use boost::function<> to hold the pointers.

[cplusplusstudent]

Because of "test<Data_T> (test<Data_T>::*fun_ptr)(const test<Data_T> &a)" be a templated function i had to declare the "struct example" as template structure. That's why i have to give type while declaring that array.
Is there any other way if i want to keep my "fun_ptr" as templated but the structure holding it to be a normal structure(non-template)?

[Galik]

You could create a templatized array by placing it in a template class or a template function:

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

#include <iostream> #include <cstdlib> template<typename Data_T> class test { public: void test_func_1(const Data_T &var) { std::cout << "test_func_1: " << var << std::endl; } }; template<typename Data_T> struct example { int a; float b; void (test<Data_T>::*fun_ptr)(const Data_T& a); }; template<typename Data_T> example<Data_T>& get_example(const Data_T&, size_t i) { static example<Data_T> example_array[] = { {1, 2, &test<Data_T>::test_func_1} }; return example_array[i]; } int main() { int i = 3; double d = 23.78; test<int> a; test<double> b; (a.*get_example(int(), 0).fun_ptr)(i); (b.*get_example(double(), 0).fun_ptr)(d); }

test_func_1: 3 test_func_1: 23.78

But you still have to know the type before calling it.

[Galik]

cplusplusstudent wrote:

Is there any other way if i want to keep my "fun_ptr" as templated but the structure holding it to be a normal structure(non-template)?

To be honest, when things get this convoluted, you might want to consider another approach to whatever problem you are trying to solve.

[cplusplusstudent]

Okey,i'll try another approach,but template will still be there :(

[Galik]

What are you actually trying to achieve with all this?