Rcpp: ambiguous overload for 'operator=' Matrix and List - c++

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>