[SOLVED] Access header files outside of current c++ file scope

Issue

I'm trying to set up a simple c++ project with CMake. I am trying to include Logger.h header residing under the utils folder from a/A.cpp file. Currently, I cannot configure my A.cpp file access utils/Logger.h header file. I have both A.cpp and Logger.h files in the executable in the CMakeLists.txt file, and added utils folder to be included directory for the executable target_include_directories(${PROJECT_NAME} PRIVATE utils a) but still I can't access Logger.h file. I get a/A.cpp:1:10: fatal error: 'utils/Logger.cpp' file not found

1. Do I need to add CMakeLists.txt to each subdirectory, one for each a and utils?
2. Can't I have just one parent that manages what files I can include for each file?
3. I want to have absolute path access from a.cpp #include "utils/Logger.h", how to obtain that?

Here is what my folder structure looks like

├── CMakeLists.txt
├── a
│   └── A.cpp
├── main.cpp
└── utils
    └── Logger.h

a/A.cpp

#include "utils/Logger.cpp"

class A {
public:
  void hello_a() {
    Logger logger;
    logger.print();
  }
}

utils/Logger.h

#include "iostream"

class Logger {
public:
  void print() { std::cout << "hello from Logger" << std::endl; }
}

CMakeLists.txt

project(MyProject)

cmake_minimum_required(VERSION 3.0)
add_executable(${PROJECT_NAME} main.cpp a/A.cpp utils/Logger.cppo)
target_include_directories(${PROJECT_NAME} PRIVATE utils a)

Solution

Includes in C++ use paths relative to the include directories (or the source file in case you use includes with ""-quotes).

For

#include "utils/Logger.cpp"

to work in a/A.cpp either there needs to be a file a/utils/Logger.cpp or the top directory needs to be one of the include directories.

To achieve the latter, you can simply do

target_include_directories(${PROJECT_NAME} PRIVATE .)

in addition to including the header instead of the source file. Note that for public/interface headers of exported targets you need to replace ., but in case of private headers it doesn't matter.

---

A note on #include "" vs #include <> though: When encountering the "" version, usually compilers look for a file relative to the source file and if not found they do same lookup as for the <> version. For this reason you'll never see

#include "iostream"

in any examples and I strongly recommend not making a habit of including standard library headers using the "" version of #include.

---

Furthermore including a .cpp usually indicates an error; Avoid the necessity to do this, if you're tempted to create a program like this or at the very least document this clearly in the code. Include the header not the source.

Answered By - fabian

Answer Checked By - David Goodson (WPSolving Volunteer)