[SOLVED] C++: passing list of functions as argument to function

rdx · 10-20-2019, 09:10 AM

This is really a pure C++ question (g++).
I want to pass a list of functions to another function so that it can call those in the list in sequence and accumulate their outputs in a matrix. The functions in the list are "double (*func)( vec x)" in the syntax of the Armadillo linear algebra library.

This is similar I think to the way hardware drivers used to have a list of function calls to access "read," "write," and control functions but I haven't seen that usage in a long time.

What I am trying to do is pass the pointers as "uwords" since that is a type acceptable to Armadillo and then cast the unsigned long ints to function calls. If this seems too awkward I would fall back on a STL <vector> to contain the list. Other ideas welcomed. The key feature here is that the number of functions in the list is arbitrary.

rdx · 10-20-2019, 06:07 PM

After a few hours study I have found the magic syntax.
First I use Armadillo "uvec" type (a column vector of "uword" or unsigned long long int), I cast the functions to these uwords and then cast them back in the called function.

Code:

#include <armadillo>
using namespace arma ;

double f1( vec x ) ;
double f2( vec x ) ;
double f3( vec x ) ;

int main()
{
    vec  x = ones<vec>(3) ;
    uvec U ;
    U << (uword) f1 << (uword) f2 << (uword) f3 ;

    vec V = my_function( U, x ) ;
       ----
}

vec my_function( uvec U, vec x )
{
    int M = U.n_item ;
    double  (*func)(vec) = (double (*)(vec)) U(0) ;
    double y = func(x) ;
        ....
}

It appears that the STL <vector> will also work.

Code:

#include <vector>
using namespace std ;

int main()
{
       ....
    vector<double (*)(arma::vec)>  V ;
    V.push_back( f1 ) ;
    V.push_back( f2 ) ;
    V.push_back( f3 ) ;
    arma::mat J = my_function( V, x ) ;
       ....
}

arma::mat my_function( vector<double (*)(arma::vec)> V, 
          arma::vec x )
{
    double (*func)(arma::vec) = V.at(0) ;
       ....
}

Personally I prefer using the Armadillo uvec but the g++ compiler is accepting both approaches.