cmake undefined reference to function

Question

The project structure below is a simplified example. I tried to boil it down to the minimal amount of files to reproduce my issue.

.
├── CMakeLists.txt
├── subdir1
│   ├── CMakeLists.txt
│   └── subsubdir1
│       ├── CMakeLists.txt
│       ├── Example.cpp
│       └── Example.h
└── subdir2
    ├── CMakeLists.txt
    ├── main.cpp
    └── subsubdir1
        ├── CMakeLists.txt
        ├── ExampleCreator.cpp
        └── ExampleCreator.h

./CMakeLists.txt

cmake_minimum_required(VERSION 2.8)
project(test)

macro(add_sources)
    file (RELATIVE_PATH _relPath "${CMAKE_SOURCE_DIR}" "${CMAKE_CURRENT_SOURCE_DIR}")
    foreach (_src ${ARGN})
        if (_relPath)
            list (APPEND SRCS "${_relPath}/${_src}")
        else()
            list (APPEND SRCS "${_src}")
        endif()
    endforeach()
    if (_relPath)
        # propagate SRCS to parent directory
        set (SRCS ${SRCS} PARENT_SCOPE)
    endif()
endmacro()

add_subdirectory(subdir1)
add_subdirectory(subdir2)

add_executable(test ${SRCS})

subdir1/CMakeLists.txt

add_subdirectory(subsubdir1)

subdir1/subsubdir1/CMakeLists.txt

add_sources(Example.cpp)

subdir1/subsubdir1/Example.h

#ifndef EXAMPLE_H
#define EXAMPLE_H

class Example
{
public:
    Example();
    virtual ~Example();
};

#endif

subdir1/subsubdir1/Example.cpp

#include <stdio.h>
#include "Example.h"

Example::Example()
{
    printf("Inside Example constructor\n");
}

Example::~Example()
{
}

subdir2/CMakeLists.txt

add_subdirectory(subsubdir1)
add_sources(main.cpp)

subdir2/main.cpp

#include "subsubdir1/ExampleCreator.h"

int main(int argc, char** argv)
{
    ExampleCreator creator;
    return 0;
}

subdir2/subsubdir1/CMakeLists.txt

add_sources(ExampleCreator.cpp)

subdir2/subsubdir1/ExampleCreator.h

#ifndef EXAMPLE_CREATOR_H
#define EXAMPLE_CREATOR_H

class ExampleCreator
{
public:
    ExampleCreator();
    virtual ~ExampleCreator();
};

#endif

subdir2/subsubdir1/ExampleCreator.cpp

#include "ExampleCreator.h"
#include "../../subdir1/subsubdir1/Example.h"

ExampleCreator::ExampleCreator()
{
    Example* ex1 = new Example();
}

ExampleCreator::~ExampleCreator()
{
}

I'm hoping this is a really simple lack of understanding of how CMake handles dependencies. This compiles without error, but fails during linking. The make output below shows that Example.cpp isn't even compiling and I don't understand why.

user>:~/src/test/build$ make
Scanning dependencies of target test
[ 50%] Building CXX object CMakeFiles/test.dir/subdir2/subsubdir1/ExampleCreator.cpp.o
[100%] Building CXX object CMakeFiles/test.dir/subdir2/main.cpp.o
Linking CXX executable test
CMakeFiles/test.dir/subdir2/subsubdir1/ExampleCreator.cpp.o: In function `ExampleCreator::ExampleCreator()':
ExampleCreator.cpp:(.text+0x2b): undefined reference to `Example::Example()'
collect2: ld returned 1 exit status
make[2]: *** [test] Error 1
make[1]: *** [CMakeFiles/test.dir/all] Error 2
make: *** [all] Error 2

All the sources are appended to the SRCS variable in the root CMakeLists.txt file from what I can tell. So, why isn't Example.cpp getting compiled? or linked?

Solution

The directory subdir1 doesn't generate a binary. Add this:

project(Example)
add_library(Example Example.cpp)

instead of your add_sources. After this, you need to tell the project that uses it to link against it:

TARGET_LINK_LIBRARIES(subdir2 Example)

If your names differ, document yourself on the functionalities of these commands.

Comments

[E-rich]

This worked, but I don't understand why I need to make a library and link against as opposed to just adding the source directly to the executable. I didn't need the project(Example) btw.

[Luchian Grigore]

@E-rich you didn't add the sources in the project that actually needed the symbols.