Eigen3 header file in included directory not found when building with CMake

Question

I am trying to build a project using Eigen3 library of headers. My repo has the following structure

Eigen
    Dense
    ...
pricing
    CMakeLists.txt
    options.cpp
    options.h
CMakeLists.txt
main.cpp

main.cpp:

#include "pricing/options.h"


int main()
{
  fun();
}

CMakeLists.cpp:

cmake_minimum_required(VERSION 3.22.0)

project(my_application VERSION 1.0.0 LANGUAGES CXX)

set(CMAKE_CXX_STANDARD 17)
set(CMAKE_CXX_STANDARD_REQUIRED ON)

add_executable(quant_finance_app
    main.cpp
)

add_subdirectory(pricing)

target_link_libraries(quant_finance_app pricing_lib)

pricing/options.h:

#pragma once

#include <iostream>
#include <Eigen/Dense>
 
 
using Eigen::MatrixXd;
 

void fun();

pricing/options.cpp:

#include "options.h"

 
void fun()
{

  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);
  std::cout << m << std::endl;

}

pricing/CMakeLists.txt:

add_library(pricing_lib
options.cpp)

target_include_directories(pricing_lib  PRIVATE ../)

I run CMake and I get the following error when compiling "options.cpp" file:

fatal error: Eigen/Dense: No such file or directory
    4 | #include <Eigen/Dense>

Why can't the compiler find the included headers necessary to create "pricing_lib" library?

Solution

The solution is to rewrite the file "pricing/CMakeLists.txt" as follows:

add_library(pricing_lib
options.cpp)

target_include_directories(pricing_lib PUBLIC ../)

Note that the only difference is that instead of PRIVATE I am using the keyword PUBLIC.