Implementing R function inside Qt creator

Question

I have this function to connect R with MYSQL and return the connection as global variable to be used in other functions later on. The parameters should be taken from the user. I would like to call this function from Qt creator using RInside and Rcpp libraries.

connection <- function(username,passwords,databaseName){ 
 #' create connection with selected database
ConnectedDb <- DBI::dbConnect(RMariaDB::MariaDB(), user=username,
                              password=passwords, dbname=databaseName,
                              host='localhost')
  con <<- ConnectedDb
     #' assign the connection as global variable
  assign("con", ConnectedDb, envir = .GlobalEnv)  
  #' return the connection
return(con)}

I added RInside and Rcpp to my Qt project and applied the project suggested in https://github.com/eddelbuettel/rinside/tree/master/inst/examples/qt . The project worked fine.

I appreciate any suggestions.

UPDATE: I understand that I need to put the function inside R.parseEvalQ and then call it using R.parseEval. A summary of what I did:

    #include <R.h>
    #include <RInside.h>
    #include <Rcpp.h>
    #include <mysql.h>
    #include <driver.h>
    int main(int argc, const char * argv[]) {
    RInside R(argc, argv);
    string username,passwords,databaseName;
    R["username"] = "root";
    R["passwords"] = "pass";
    R["databaseName"] = "Test";
   
    // define R function in C++ environment 
    R.parseEvalQ("connection <- function(username, 
     passwords,databaseName){ConnectedDb <<- 
 DBI::dbConnect(RMariaDB::MariaDB(),user=username,password=passwords,   
     dbname=databaseName,host='localhost'); con <<- ConnectedDb; 
     assign('con', ConnectedDb, envir = .GlobalEnv); con}");
     // call R function in C++ environment 
     R.parseEval("connection(username,passwords,databaseName)");
     return 0;
}

Now I would like to test the function (if it work probably or not). I need to assign variable equal to R.parseEval to print it out. I added the following code:

sql::Connection *con;
con = R.parseEval("connection(username,passwords,databaseName)");

It gave the following error:

Cannot initialize a member subobject of type 'sql::Connection *' with an lvalue of type 'SEXP' (aka 'SEXPREC *')

When I use Rcpp::List as follow:

 Rcpp::List c;
 c = R.parseEval("connection(username,passwords,databaseName)");

I got the following error:

/Library/Frameworks/R.framework/Versions/4.0/Resources/library/Rcpp/include/Rcpp/internal/wrap.h:503:18 No viable conversion from 'const RInside::Proxy' to 'SEXP' (aka 'SEXPREC *')

Which object in Rcpp or c++ will be equal to returned connection ? because I tried so many types and nothing worked. For now I do not guarantee that the connection is established !