Using a class and a function from different header files C++ and CMake

Question

I am learning Modern C++ form a course by University of Bonn. I have declared a class in a grass.h file and its implementation in a corresponding grass.cpp file. Similarly in another tools.h-tools.cpp declaration-implementation pair is a function that takes in a reference to the class object defined above. And finally a main.cpp file initialises a Grass object from first pair and passes it to the function from the second pair. I am able to compile main.cpp and get the desired output from these command line instructions:

c++ -std=c++11 -g -c grass.cpp -o grass.o
c++ -std=c++11 -g -c tools.cpp -o tools.o
c++ -std=c++11 -g main.cpp grass.o tools.o -o main

but unable to do so with CMake. I do not know what the command line instructions are called (linking and compiling?).

Here is the code example "grass.h":

#include <string>
#pragma once

class Grass
{
private:
    int green_;
    std::string name_;
public:
    Grass(int green, std::string name): green_{green}, name_{name} {}

    int get_green() const;                        // getter functions for 
    const std::string& get_green_name() const;    // both private members
};

The functions just return the values and are implemented in "grass.cpp".

Then I have a "tools.h":

#include <iostream>
#include "grass.h"

#pragma once

void PrintGrass(Grass& grObj);

And this function just prints values from the getter functions of Grass.

Finally, the main.cpp contains:

int main ()
{
    Grass grObj{10, "ABC"};
    PrintGrass(grObj);
    cout << endl;
    return 0;
}

The CMake error occurs at 100% Linking CXX executable main. The error is in tools.cpp: undefined reference to `Grass::get_green_name[abi:cxx11]()' and undefined reference to `Grass::get_green()'. This means that the the program is unable to find to find the definition of these function, right? The CMake file looks like this:

set(CMAKE_CXX_STANDARD 11)
set(CMAKE_BUILD_TYPE Debug)
set(CMAKE_CXX_FLAGS "-Wall")
set(CMAKE_CXX_FLAGS_DEBUG "-g -O0")
add_library(grass grass.cpp)
add_library(tools tools.cpp)
add_executable(main main.cpp)
target_link_libraries(main grass tools)

How do I make this work with CMake? Thank you very much for your help.

Answer 1

I had a similar problem solved by adding the include files too into add_library:

add_library(grass grass.cpp grass.h)

Try this and see if it works

Answer 2

I will to summarise the comments section and based on @pptaszni's advice put the entire code and the verbose error here:
The entire code in order of dependency:

1. grass.h and grass.cpp:

#include <string>
#pragma once

class Grass
{
public:
    Grass(int green, std::string name): green_{green}, name_{name} {}

    int get_green ();
    std::string get_green_name ();
private:
    int green_ = 100;
    std::string name_ = "Joe";
};

and

#include "grass.h"

Grass::Grass(int green, std::string name): green_{green}, name_{name}{}

int Grass::get_green() {
    return green_;  
}

std::string Grass::get_green_name() { 
    return name_;
}

2. tools.h and tools.cpp:

#include <iostream>
#include "grass.h"

#pragma once

void PrintGrass(Grass& grObj);

and

#include <iostream>
#include "grass.h"
#include "tools.h"

void PrintGrass(Grass& grObj)
{
    std::cout << grObj.get_green_name() << ", " << grObj.get_green() << " ";
}

3. main.cpp

#include <iostream>
#include "grass.h"
#include "tools.h"
#include <string>

using std::string;
using std::cout;
using std::endl;


int main ()
{
    Grass grObj{10, "Jane"};
    PrintGrass(grObj);
    cout << endl;
    return 0;
}

4. CMakeLists.txt

project(Grasses)
cmake_minimum_required(VERSION 3.0)
set(CMAKE_CXX_STANDARD 11)
set(CMAKE_BUILD_TYPE Debug)
set(CMAKE_CXX_FLAGS "-Wall")
set(CMAKE_CXX_FLAGS_DEBUG "-g -O0")
add_library(grass grass.cpp)
add_library(tools tools.cpp)
add_executable(main main.cpp)
target_link_libraries(main grass tools)

The error output of make VERBOSE=1 being verbose, has been pasted in a Github Gist, so as to not make this post longer than it already is.

Summary
What worked? Writing

target_link_libraries(main tools grass)

instead of

target_link_libraries(main grass tools).

The take away message from the explanation given by @Yksisarvinen is to always place libraries without any dependencies at the very end. This is because the compiler might take a look at such libraries and discard them, if it does not find any use for it at the moment of processing it.

Another thing that works is just using add_executable(main main.cpp grass.cpp tools.cpp) and not adding libraries and then linking them. This was suggested by @molbdnilo and @Yksisarvinen.

Thank you all for helping me in understanding C++ a bit better.