Rcpp: ambiguous overload for 'operator=' Matrix and List

Question

The following Rcpp code is the minimal reproducible example for a much larger code that generates the identical compilation error. It seems that I cannot asign a numeric matrix to a list and the list then again to another matrix.

#include <Rcpp.h>
using namespace Rcpp;

//[[Rcpp::export]]
List return_a(NumericMatrix a, NumericMatrix b){
    //the function only returns the input matrix a
    List result(1);
    result(0) = a;
    return(result);
}


//[[Rcpp::export]]
List wrapper_cpp(NumericMatrix a, NumericMatrix b){
    //the function is a dummy wrapper for much more code
    List Step1(1);
    List results(1);    
    Step1 = return_a(a,b);
    a = Step1(0);   
    results(0) = a;
    return(results);
}

The code above gives the following compilation error that I shortened:

error: ambiguous overload for 'operator=' (operand types are 'Rcpp::NumericMatrix {aka Rcpp::Matrix<14>}' and 'Rcpp::Vector<19>::Proxy ...
a = Step1(0);

My real function is much more complex. I need to manipulate matrices in several loops and in each step the matrices are returned by each function within a list. I then need to extract these lists to manipulate the matrices further. How can this be done?

Answer 1

Besides the error that @Ralf already mentioned, you were simply trying too much. Sometimes we need an intermediate step as the template magic is ... finicky. The following works.

Code

#include <Rcpp.h>
using namespace Rcpp;

//[[Rcpp::export]]
List return_a(NumericMatrix a, NumericMatrix b){
  //the function only returns the input matrix a
  List result(1);
  result(0) = a;
  return(result);
}


//[[Rcpp::export]]
List wrapper_cpp(NumericMatrix a, NumericMatrix b){
  //the function is a dummy wrapper for much more code
  List results(1);
  List Step1 = return_a(a,b);
  NumericMatrix tmp = Step1(0);
  results(0) = tmp;
  return(results);
}

Output

R> Rcpp::sourceCpp("~/git/stackoverflow/54771818/answer.cpp")
R> wrapper_cpp(matrix(1:4,2,2), matrix(4:1,2,2))
[[1]]
     [,1] [,2]
[1,]    1    3
[2,]    2    4

R>