Passing a struct function to a function

[nirakara]

Hi all. I wonder if you have an answer for me for this curious little pointer problem. I have an all purpose function that I typically pass a function to. I'm having to pass it a function from someone else's struct and I can't seem to figure out how to do it.

struct someStruct{
  int someStructFunction(int x){
    return ( x + 1 );
  }
}

int myCoolFunction(int(*someFunction)(int)){
  return ( someFunction(5) * 2 );
{

void main{
  int the_result;
  someStruct structInstance;
  the_result = myCoolFunction( structInstance.someStructFunction );
  return 0;
}

What I get when I compile though is

error: argument of type 'int (someStruct::)(int)' does not match 'int (*)(double)

' Perhaps I should change my function-passing system or something but I can't seem to figure it out. Any ideas? Thanks!

[firedraco]

Functions from a class are different from functions in the global space:

//notice the someStruct::
//also note you need an object since the function is not static
int myCoolFunction(someStruct obj, int (someStruct::*someFunction)(int)) {
    return (obj->(*someFunction)(5) * 2); //don't forget to deference the function pointer before calling it
}

I suggest reading this: http://www.parashift.com/c++-faq-lite/pointers-to-members.html#faq-33.1

Oh btw, you need int main().

[jsmith]

To add to the above, to solve your problem you need either std::mem_fun and std::bind1st, or boost::bind and boost::function. I prefer the latter as being easier to read.

#include <boost/bind.hpp>
#include <boost/function.hpp>

struct someStruct {
  int memfunc( int x ) const
    { return x + 1; }
};

int free_func( int x )
   { return x + 2; }

int call_it( const boost::function<int(int)>& fn )
    { return fn( 5 ) * 2; }

int main() {
   someStruct inst;
   int result = call_it( boost::bind( &someStruct::memfunc, &inst, _1 ) );
   int result2 = call_it( free_func );
}