function pointer as template argument

[vpadial]

Hello,

I want to build a library to help exporting c++ functions to a
scripting languagge. The scripting language provides a function to
register functions like:

ANY f0()
ANY f1(ANY)
ANY f2(ANY, ANY)
ANY f3(ANY, ANY, ANY)
...

where ANY is an opaque type, with functions available to convert from
and to basic C types.

I would like to avoid writing a wrapper for any function I have to
export.
My first try (restricted to functions of one argument, but can easily
be extended):

//------------------------------------------------

template <class T_return, class T_arg, T_return (*fn)(T_arg)>
struct Translator1 {
static ANY proc(ANY arg)
{
T_arg a = from_any<T_arg>(arg); // Conversion from ANY to arg type
T_return r = to_any<T_return>((*fn)(a)); // Conversion from return
type to ANY
return r;
}
};

int foo(int a) { return a*10; }

typedef Translator1<int, int, foo> FN;
c_reg_function( FN:roc );

//-----------------------------------------------------

Ok, that works ! But it would be fine to let the compiler guess the
type of the arguments:

//--------------------------------------------------------

template <class T_return, class T_arg>
void export_fn(T_return (*fn)(T_arg))
{
c_reg_function(Translator1<T_return, T_arg, fn>:roc(arg));
}

export_fn(foo);

//-----------------------------------------------------------------

But now, something goes wrong (g++ v3.3) !!!

test.cc: En function `void export_fn(T_return (*)(T_arg)) [with
T_return = int,
T_arg = int]':
test.cc:43: instantiated from here
test.cc:33: error: `fn' is not a valid template argument
test.cc:33: error: it must be the address of a function with external
linkage
test.cc:33: error: `arg' undeclared (first use this function)
test.cc:33: error: (Each undeclared identifier is reported only once
for each
function it appears in.)
test.cc:33: error: `<type error>' is not a class type


I'm afraid that the compiler cannot expand a template class within a
template function with fewer template arguments ! I cannot use
functors because the register function expects C functions !

Any good idea ? I cannot think of any solution that avoids the use of
dirty preprocesor macros.

Thanks !!!


A reduced version of the code is provided so that you can try
yourself:

////////////////////////////////////////////
#include <iostream>

using namespace std;

typedef long ANY;

void c_reg_function(ANY (*fn)(ANY)); // external c library function

void c_reg_function(ANY (*fn)(ANY))
{
cout << "Registro: " << (*fn)(2) << endl;
}

template<class T> ANY to_any(T); // ANY conversion functions
template<class T> T from_any(ANY);

template<> int from_any(ANY a) { return a; }
template<> ANY to_any(int a) { return a; }

template <class T_return, class T_arg, T_return (*fn)(T_arg)>
struct Translator1 {
static ANY proc(ANY arg)
{
T_arg a = from_any<T_arg>(arg);
T_return r = to_any<T_return>((*fn)(a));
return r;
}
};

template <class T_return, class T_arg>
void export_fn(T_return (*fn)(T_arg))
{
c_reg_function(Translator1<T_return, T_arg, fn>:roc(arg));
}

int foo(int a) { return a*10; }

int main(int argc, char** argv)
{
typedef Translator1<int, int, foo> FN;
c_reg_function( FN:roc );

export_fn(foo);

}
/////////////////////////////////////7

 

[David Hilsee]

Hello,

I want to build a library to help exporting c++ functions to a
scripting languagge. The scripting language provides a function to
register functions like:

ANY f0()
ANY f1(ANY)
ANY f2(ANY, ANY)
ANY f3(ANY, ANY, ANY)
...

where ANY is an opaque type, with functions available to convert from
and to basic C types.

I would like to avoid writing a wrapper for any function I have to
export.
My first try (restricted to functions of one argument, but can easily
be extended):

//------------------------------------------------

template <class T_return, class T_arg, T_return (*fn)(T_arg)>
struct Translator1 {
static ANY proc(ANY arg)
{
T_arg a = from_any<T_arg>(arg); // Conversion from ANY to arg type
T_return r = to_any<T_return>((*fn)(a)); // Conversion from return
type to ANY
return r;
}
};

int foo(int a) { return a*10; }

typedef Translator1<int, int, foo> FN;
c_reg_function( FN:roc );

//-----------------------------------------------------

Ok, that works ! But it would be fine to let the compiler guess the
type of the arguments:

//--------------------------------------------------------

template <class T_return, class T_arg>
void export_fn(T_return (*fn)(T_arg))
{
c_reg_function(Translator1<T_return, T_arg, fn>:roc(arg));
}

export_fn(foo);

//-----------------------------------------------------------------

But now, something goes wrong (g++ v3.3) !!!

test.cc: En function `void export_fn(T_return (*)(T_arg)) [with
T_return = int,
T_arg = int]':
test.cc:43: instantiated from here
test.cc:33: error: `fn' is not a valid template argument
test.cc:33: error: it must be the address of a function with external
linkage
test.cc:33: error: `arg' undeclared (first use this function)
test.cc:33: error: (Each undeclared identifier is reported only once
for each
function it appears in.)
test.cc:33: error: `<type error>' is not a class type


I'm afraid that the compiler cannot expand a template class within a
template function with fewer template arguments ! I cannot use
functors because the register function expects C functions !

It's not complaining about the number of arguments. The value of "fn" won't
be known until runtime. You can't expect the compiler to instantiate a
template based on its value if it can't possibly know the value.

Why is it a template argument in the first place? Couldn't you just write
it like this:

template <class T_return, class T_arg>
struct Translator1 {
static ANY proc(ANY arg, T_return (*fn)(T_arg))
{
T_arg a = from_any<T_arg>(arg);
T_return r = to_any<T_return>((*fn)(a));
return r;
}
};

template <class T_return, class T_arg>
void export_fn(T_return (*fn)(T_arg))
{
c_reg_function(Translator1<T_return, T_arg, fn>:roc(arg,fn));
}

or do you plan on doing some specializations based on its value?

 

[David Hilsee]

[...]

c_reg_function(Translator1<T_return, T_arg, fn>:roc(arg,fn));

Sorry, I meant

c_reg_function(Translator1<T_return, T_arg>:roc(arg,fn));

 

[David Hilsee]

Hello,

I want to build a library to help exporting c++ functions to a
scripting languagge. The scripting language provides a function to
register functions like:

ANY f0()
ANY f1(ANY)
ANY f2(ANY, ANY)
ANY f3(ANY, ANY, ANY)
...

where ANY is an opaque type, with functions available to convert from
and to basic C types.

I would like to avoid writing a wrapper for any function I have to
export.
My first try (restricted to functions of one argument, but can easily
be extended):

//------------------------------------------------

template <class T_return, class T_arg, T_return (*fn)(T_arg)>
struct Translator1 {
static ANY proc(ANY arg)
{
T_arg a = from_any<T_arg>(arg); // Conversion from ANY to arg type
T_return r = to_any<T_return>((*fn)(a)); // Conversion from return
type to ANY
return r;
}
};

int foo(int a) { return a*10; }

typedef Translator1<int, int, foo> FN;
c_reg_function( FN:roc );

//-----------------------------------------------------

Ok, that works ! But it would be fine to let the compiler guess the
type of the arguments:

//--------------------------------------------------------

template <class T_return, class T_arg>
void export_fn(T_return (*fn)(T_arg))
{
c_reg_function(Translator1<T_return, T_arg, fn>:roc(arg));
}

export_fn(foo);

//-----------------------------------------------------------------

But now, something goes wrong (g++ v3.3) !!!

test.cc: En function `void export_fn(T_return (*)(T_arg)) [with
T_return = int,
T_arg = int]':
test.cc:43: instantiated from here
test.cc:33: error: `fn' is not a valid template argument
test.cc:33: error: it must be the address of a function with external
linkage
test.cc:33: error: `arg' undeclared (first use this function)
test.cc:33: error: (Each undeclared identifier is reported only once
for each
function it appears in.)
test.cc:33: error: `<type error>' is not a class type


I'm afraid that the compiler cannot expand a template class within a
template function with fewer template arguments ! I cannot use
functors because the register function expects C functions !

It's not complaining about the number of arguments. The value of "fn" won't
be known until runtime. You can't expect the compiler to instantiate a
template based on its value if it can't possibly know the value.

Why is it a template argument in the first place? Couldn't you just write
it like this:

template <class T_return, class T_arg>
struct Translator1 {
static ANY proc(ANY arg, T_return (*fn)(T_arg))
{
T_arg a = from_any<T_arg>(arg);
T_return r = to_any<T_return>((*fn)(a));
return r;
}
};

template <class T_return, class T_arg>
void export_fn(T_return (*fn)(T_arg))
{
c_reg_function(Translator1<T_return, T_arg, fn>:roc(arg,fn));
}
[...]

After rereading your post several times, I realized that I did not fully
understand what you were trying to do. I was confused by the sample code
that appeared to be an attempt at invoking "proc" (Translator1<T_return,
T_arg, fn>:roc(arg)). I think the line "c_reg_function( FN:roc );"
should have instead been "c_reg_function( &FN:roc );" For some reason, I
thought you had omitted some arguments to the function call on that line.
Sorry for any confusion my responses may have caused.

Even so, I can't think of any way that you could have the compiler easily
and automatically determine the return type and arguments in the way that
you need them to be determined. The function template you wrote won't work
for the reason a mentioned before: the value is not known at compile time.

 

[vpadial]

[...]

c_reg_function(Translator1<T_return, T_arg, fn>:roc(arg,fn));

Sorry, I meant

c_reg_function(Translator1<T_return, T_arg>:roc(arg,fn));

The problem is that c_reg_function expects a pointer to a function (ANY (*fn)(ANY))

 

[Tom Widmer]

Hello,

I want to build a library to help exporting c++ functions to a
scripting languagge. The scripting language provides a function to
register functions like:

ANY f0()
ANY f1(ANY)
ANY f2(ANY, ANY)
ANY f3(ANY, ANY, ANY)
...

where ANY is an opaque type, with functions available to convert from
and to basic C types.

I would like to avoid writing a wrapper for any function I have to
export.
My first try (restricted to functions of one argument, but can easily
be extended):

//------------------------------------------------

template <class T_return, class T_arg, T_return (*fn)(T_arg)>
struct Translator1 {
static ANY proc(ANY arg)
{
T_arg a = from_any<T_arg>(arg); // Conversion from ANY to arg type
T_return r = to_any<T_return>((*fn)(a)); // Conversion from return
type to ANY
return r;
}
};

int foo(int a) { return a*10; }

typedef Translator1<int, int, foo> FN;
c_reg_function( FN:roc );

//-----------------------------------------------------

Ok, that works ! But it would be fine to let the compiler guess the
type of the arguments:

//--------------------------------------------------------

template <class T_return, class T_arg>
void export_fn(T_return (*fn)(T_arg))
{
c_reg_function(Translator1<T_return, T_arg, fn>:roc(arg));
}

export_fn(foo);

//-----------------------------------------------------------------

But now, something goes wrong (g++ v3.3) !!!

test.cc: En function `void export_fn(T_return (*)(T_arg)) [with
T_return = int,
T_arg = int]':
test.cc:43: instantiated from here
test.cc:33: error: `fn' is not a valid template argument
test.cc:33: error: it must be the address of a function with external
linkage
test.cc:33: error: `arg' undeclared (first use this function)
test.cc:33: error: (Each undeclared identifier is reported only once
for each
function it appears in.)
test.cc:33: error: `<type error>' is not a class type


I'm afraid that the compiler cannot expand a template class within a
template function with fewer template arguments ! I cannot use
functors because the register function expects C functions !

Any good idea ? I cannot think of any solution that avoids the use of
dirty preprocesor macros.

The problem is that your function argument "fn" has a runtime value,
but template arguments are required to be known at compile time.

I can't think of a standard C++ solution, but I can think of a gcc
specific solution - typeof. e.g.

template <typename FuncType, FuncType func>
struct Translator1
{
//need to write FuncArgs, using partial specialization
typedef FuncArgs<FuncType>::first_argument_type T_arg;
typedef FuncArgs<FuncType>::return_type T_return;

static ANY proc(ANY arg)
{
T_arg a = from_any<T_arg>(arg); // Conversion from ANY to arg type
T_return r = to_any<T_return>((*fn)(a));
return r;
}
};

c_reg_function(&Translator1<typeof(&foo), foo>:roc);

Whether you can put up with the non-portability is up to you.

Tom

 

[Tom Widmer]

Hello,

I want to build a library to help exporting c++ functions to a
scripting languagge.

Actually, what you're doing has been done several times before. This
book would be helpful:
http://www.informit.com/title/0321227255

Here's an example of a library that binds C++ and Python:
http://www.boost.org/libs/python/doc/index.html

 

[vpadial]

The problem is that your function argument "fn" has a runtime value,
but template arguments are required to be known at compile time.

I can't think of a standard C++ solution, but I can think of a gcc
specific solution - typeof. e.g.

template <typename FuncType, FuncType func>
struct Translator1
{
//need to write FuncArgs, using partial specialization
typedef FuncArgs<FuncType>::first_argument_type T_arg;
typedef FuncArgs<FuncType>::return_type T_return;

static ANY proc(ANY arg)
{
T_arg a = from_any<T_arg>(arg); // Conversion from ANY to arg type
T_return r = to_any<T_return>((*fn)(a));
return r;
}
};

c_reg_function(&Translator1<typeof(&foo), foo>:roc);

Whether you can put up with the non-portability is up to you.

Thanks Tom, but I can't get it working. Could you explain better the
use of FuncArgs ? How can I specialize types of a template class ?
Must I specialize FuncArgs for any new function signature ?

Thanks in advance

 

[vpadial]

Thanks Tom, but I can't get it working. Could you explain better the
use of FuncArgs ? How can I specialize types of a template class ?
Must I specialize FuncArgs for any new function signature ?

Thanks in advance

Don't bother. I've seen Boost provides function_traits that I guess is
equivalent to your FuncArgs class:
http://www.boost.org/libs/type_traits/#function_traits

Thanks for your help