I need to store a templated function in a function pointer, how can I do this?


e.g.,

void function(double) myfuncptr;
void myfunc(double d) { }
myfuncptr = myfunc;

Now I would like to use a template parameter instead of double.

In C++ one can do this by using boosts binding's and function types.

For example, I want something like this

void function(F)(F) funcptr;

void Bind(T)(void function(T)(T) v)
{
    funcptr = v; // F is sort of deduced automatically as being T. Obviously problematic but effectively what I want to do.
}

This way I can bind to any function that takes a single type parameter and returns a void.


void function(F) myfuncptr;  // F is undefined
void myfunc(T)(T d) { }      //
myfuncptr = myfunc;

To do this using boost I would simply bind the parameter so myfuncptr would not depend on an arbitrary type.

I need a rather performant way to do this too.

On 11/29/2012 03:17 PM, js.mdnq wrote:
>
>
> I need to store a templated function in a function pointer, how can I do
> this?
>
>
> e.g.,
>
> void function(double) myfuncptr;
> void myfunc(double d) { }
> myfuncptr = myfunc;
>
> Now I would like to use a template parameter instead of double.

std.functional may have something:

  http://dlang.org/phobos/std_functional.html

Are you looking for something like the following?

import std.traits;

auto myDelegate(F : T function(T), T)(F f)
    if (isFunctionPointer!F)  // <-- probably unnecessary
{
    return (T a) => f(a * 2);
}

double func_double(double d)
{
    return d;
}

int func_int(int i)
{
    return i;
}

void main()
{
    auto d = myDelegate(&func_double);
    auto i = myDelegate(&func_int);

    assert(d(1.5) == 3.0);
    assert(i(42) == 84);
}

Ali

It seems one can accomplish this using delegates assigned by generic functions. The delegate will end hold holding the "state" of the function. It sort of acts like a binder. It seems to work but more testing needs to be done. I'm not sure how efficient it is or if there is a better way but basically it seems rather elegant(rather than trying to create some new type to deal with it).

js.mdnq:

> It seems one can accomplish this using delegates assigned by generic functions.

If you care so much for performance, when you work with D delegates it's useful to know the difference between static/scope delegates (that are just a fat pointer) and closures (that often induce a heap allocation).

Bye,
bearophile