# Step 2: Adding a Library

At this point, we have seen how to create a basic project using CMake. In this step, we will learn how to create and use a library in our project. We will also see how to make the use of our library optional.

## Exercise 1 - Creating a Library

To add a library in CMake, use the `add_library()` command and specify which source files should make up the library.

Rather than placing all of the source files in one directory, we can organize our project with one or more subdirectories. In this case, we will create a subdirectory specifically for our library. Here, we can add a new `CMakeLists.txt` file and one or more source files. In the top level `CMakeLists.txt` file, we will use the `add_subdirectory()` command to add the subdirectory to the build.

Once the library is created, it is connected to our executable target with `target_include_directories()` and `target_link_libraries()`.

### Goal

Add and use a library.

### Helpful Resources

- `add_library()`

- `add_subdirectory()`

- `target_include_directories()`

- `target_link_libraries()`

- `PROJECT_SOURCE_DIR`

### Files to Edit

- `CMakeLists.txt`

- `tutorial.cxx`

- `MathFunctions/CMakeLists.txt`

### Getting Started

In this exercise, we will add a library to our project that contains our own implementation for computing the square root of a number. The executable can then use this library instead of the standard square root function provided by the compiler.

For this tutorial we will put the library into a subdirectory called `MathFunctions`. This directory already contains a header file, `MathFunctions.h`, and a source file `mysqrt.cxx`. We will not need to modify either of these files. The source file has one function called `mysqrt` that provides similar functionality to the compiler's `sqrt` function.

From the `Help/guide/tutorial/Step2` directory, start with `TODO 1` and complete through `TODO 6`.

First, fill in the one line `CMakeLists.txt` in the `MathFunctions` subdirectory.

Next, edit the top level `CMakeLists.txt`.

Finally, use the newly created `MathFunctions` library in `tutorial.cxx`

### Build and Run

Run the `cmake` executable or the `cmake-gui` to configure the project and then build it with your chosen build tool.

Below is a refresher of what that looks like from the command line:

    mkdir Step2_build
    cd Step2_build
    cmake ../Step2
    cmake --build .

Try to use the newly built `Tutorial` and ensure that it is still producing accurate square root values.

### Solution

In the `CMakeLists.txt` file in the `MathFunctions` directory, we create a library target called `MathFunctions` with `add_library()`. The source file for the library is passed as an argument to `add_library()`. This looks like the following line:

TODO 1: Click to show/hide answer
TODO 1: MathFunctions/CMakeLists.txt

    add_library(MathFunctions mysqrt.cxx)

To make use of the new library we will add an `add_subdirectory()` call in the top-level `CMakeLists.txt` file so that the library will get built.

TODO 2: Click to show/hide answer
TODO 2: CMakeLists.txt

    add_subdirectory(MathFunctions)

Next, the new library target is linked to the executable target using `target_link_libraries()`.

TODO 3: Click to show/hide answer
TODO 3: CMakeLists.txt

    target_link_libraries(Tutorial PUBLIC MathFunctions)

Finally we need to specify the library's header file location. Modify `target_include_directories()` to add the `MathFunctions` subdirectory as an include directory so that the `MathFunctions.h` header file can be found.

TODO 4: Click to show/hide answer
TODO 4: CMakeLists.txt

    target_include_directories(Tutorial PUBLIC
                              "${PROJECT_BINARY_DIR}"
                              "${PROJECT_SOURCE_DIR}/MathFunctions"
                              )

Now let's use our library. In `tutorial.cxx`, include `MathFunctions.h`:

TODO 5: Click to show/hide answer
TODO 5 : tutorial.cxx

    #include "MathFunctions.h"

Lastly, replace `sqrt` with our library function `mysqrt`.

TODO 6: Click to show/hide answer
TODO 6 : tutorial.cxx

    const double outputValue = mysqrt(inputValue);

## Exercise 2 - Making Our Library Optional

Now let us make the MathFunctions library optional. While for the tutorial there really isn't any need to do so, for larger projects this is a common occurrence.

CMake can do this using the `option()` command. This gives users a variable which they can change when configuring their cmake build. This setting will be stored in the cache so that the user does not need to set the value each time they run CMake on a build directory.

### Goal

Add the option to build without `MathFunctions`.

### Helpful Resources

- `if()`

- `list()`

- `option()`

- `cmakedefine`

### Files to Edit

- `CMakeLists.txt`

- `tutorial.cxx`

- `TutorialConfig.h.in`

### Getting Started

Start with the resulting files from Exercise 1. Complete `TODO 7` through `TODO 13`.

First create a variable `USE_MYMATH` using the `option()` command in the top-level `CMakeLists.txt` file. In that same file, use that option to determine whether to build and use the `MathFunctions` library.

Then, update `tutorial.cxx` and `TutorialConfig.h.in` to use `USE_MYMATH`.

### Build and Run

Since we have our build directory already configured from Exercise 1, we can rebuild by simply calling the following:

    cd ../Step2_build
    cmake --build .

Next, run the `Tutorial` executable on a few numbers to verify that it's still correct.

Now let's update the value of `USE_MYMATH` to `OFF`. The easiest way is to use the `cmake-gui` or `ccmake` if you're in the terminal. Or, alternatively, if you want to change the option from the command-line, try:

    cmake ../Step2 -DUSE_MYMATH=OFF

Now, rebuild the code with the following:

    cmake --build .

Then, run the executable again to ensure that it still works with `USE_MYMATH` set to `OFF`. Which function gives better results, `sqrt` or `mysqrt`?

### Solution

The first step is to add an option to the top-level `CMakeLists.txt` file. This option will be displayed in the `cmake-gui` and `ccmake` with a default value of `ON` that can be changed by the user.

TODO 7: Click to show/hide answer
TODO 7: CMakeLists.txt

    option(USE_MYMATH "Use tutorial provided math implementation" ON)

Next, make building and linking the `MathFunctions` library conditional.

Start by creating a `list()` of the optional library targets for our project. At the moment, it is just `MathFunctions`. Let's name our list `EXTRA_LIBS`.

Similarly, we need to make a `list()` for the optional includes which we will call `EXTRA_INCLUDES`. In this list, we will `APPEND` the path of the header file needed for our library.

Next, create an `if()` statement which checks the value of `USE_MYMATH`. Inside the `if()` block, put the `add_subdirectory()` command from Exercise 1 with the additional `list()` commands.

When `USE_MYMATH` is `ON`, the lists will be generated and will be added to our project. When `USE_MYMATH` is `OFF`, the lists stay empty. With this strategy, we allow users to toggle `USE_MYMATH` to manipulate what library is used in the build.

The top-level CMakeLists.txt file will now look like the following:

TODO 8: Click to show/hide answer
TODO 8: CMakeLists.txt

    if(USE_MYMATH)
      add_subdirectory(MathFunctions)
      list(APPEND EXTRA_LIBS MathFunctions)
      list(APPEND EXTRA_INCLUDES "${PROJECT_SOURCE_DIR}/MathFunctions")
    endif()

Now that we have these two lists, we need to update `target_link_libraries()` and `target_include_directories()` to use them. Changing them is fairly straightforward.

For `target_link_libraries()`, we replace the written out library names with `EXTRA_LIBS`. This looks like the following:

TODO 9: Click to show/hide answer
TODO 9: CMakeLists.txt

    target_link_libraries(Tutorial PUBLIC ${EXTRA_LIBS})

Then, we do the same thing with `target_include_directories()` and `EXTRA_INCLUDES`.

TODO 10: Click to show/hide answer
TODO 10 : CMakeLists.txt

    target_include_directories(Tutorial PUBLIC
                               "${PROJECT_BINARY_DIR}"
                               ${EXTRA_INCLUDES}
                               )

Note that this is a classic approach when dealing with many components. We will cover the modern approach in the Step 3 of the tutorial.

The corresponding changes to the source code are fairly straightforward. First, in `tutorial.cxx`, we include the `MathFunctions.h` header if `USE_MYMATH` is defined.

TODO 11: Click to show/hide answer
TODO 11 : tutorial.cxx

    #ifdef USE_MYMATH
    #  include "MathFunctions.h"
    #endif

Then, in the same file, we make `USE_MYMATH` control which square root function is used:

TODO 12: Click to show/hide answer
TODO 12 : tutorial.cxx

    #ifdef USE_MYMATH
      const double outputValue = mysqrt(inputValue);
    #else
      const double outputValue = sqrt(inputValue);
    #endif

Since the source code now requires `USE_MYMATH` we can add it to `TutorialConfig.h.in` with the following line:

TODO 13: Click to show/hide answer
TODO 13 : TutorialConfig.h.in

    #cmakedefine USE_MYMATH

With these changes, our library is now completely optional to whoever is building and using it.

### Bonus Question

Why is it important that we configure `TutorialConfig.h.in` after the option for `USE_MYMATH`? What would happen if we inverted the two?

### Answer

Click to show/hide answer

We configure after because `TutorialConfig.h.in` uses the value of `USE_MYMATH`. If we configure the file before calling `option()`, we won't be using the expected value of `USE_MYMATH`.