How to compile a C++ program using eigen without specifying the -I flag?

Question

I have the following test program using eigen:

#include <iostream>
#include <Eigen/Dense>

using namespace std;
using Eigen::MatrixXd;

int main() {
  MatrixXd m(2, 2);
  m(0, 0) = 3;
  m(1, 0) = 2.5;
  m(0, 1) = -1;
  m(1, 1) = m(1, 0) + m(0, 1);
  cout << m << endl;
}

and I can compile it with g++ -I/usr/include/eigen3/ test1.cpp.

However, the compile command doesn't work if I don't specify the include flag. This seems strange to me because I thought that any headers under /usr/include will be picked up automatically by the compiler (e.g. Boost headers, also located under /usr/include, work perfectly fine without having to tell the compiler where to look for them). What changes do I need to make to the eigen setup so I don't have to specify the -I flag in the compile command?

Answer 1

If you replace

#include <Eigen/Dense>

by

#include <eigen3/Eigen/Dense>

your code will compile. In other words, the problem is that you are including <Eigen/Dense> which is in the directory /usr/include/eigen3, but the compiler only searches in /usr/include by default.

I would recommend using include flags, instead of including <eigen3/Eigen/Dense>, because this is more portable across distributions, operating systems, etc. and generally, it is easier to configure compilation for other environments. Eigen3 comes with pkg-config files, which are very easy to use and portable. Compiling with

g++ $(pkg-config --cflags eigen3) test1.cpp

will work on all platforms, where pkg-config is available and it is a great alternative for you, if you want to avoid the hard-coded include paths.