Why armadillo changed after including RcppArmadillo?

Question

I'm working on a RcppArmadillo package. Most part of my algorithm myfun() is implemented in a C++ source file, called myfun.cpp with a header file myfun.h providing some declarations. My algorithm is built on top of pure armadillo (i.e., it just includes armadillo not RcppArmadillo.h) and gsl. In RcppExports.cpp there is also a wrapper function _mypackager_myfun() to convert R objects and call myfun() to get its results.

When compiling my package with command R CMD INSTALL, an error "undefined external symbol referenced in _mypackager_myfun()" occurs always if I include my own header file in the RcppExports.cpp file like this:

// RcppExports.cpp
#include <RcppArmadillo.h>
#include <Rcpp.h>
#include "myfun.h"

However, everything goes well if myfun.h is included before RcppArmadillo.h like this:

// RcppExports.cpp
#include "hlmgwr.h"
#include <RcppArmadillo.h>
#include <Rcpp.h>

To what I know, file RcppArmadillo.h also includes the armadillo header. But why could the order of these header files affect linking of symbols? Are mat classes in armadillo and RcppArmadillo not the same?