# 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 the header files `MathFunctions.h` and `mysqrt.h`. Their respective source files `MathFunctions.cxx` and `mysqrt.cxx` are also provided. We will not need to modify any of these files. `mysqrt.cxx` has one function called `mysqrt` that provides similar functionality to the compiler's `sqrt` function. `MathFunctions.cxx` contains one function `sqrt` which serves to hide the implementation details of `sqrt`.

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 files for the library are passed as an argument to `add_library()`. This looks like the following line:

TODO 1: MathFunctions/CMakeLists.txt

    add_library(MathFunctions MathFunctions.cxx 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 `mathfunctions::mysqrt`.

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

      const double outputValue = mathfunctions::sqrt(inputValue);

## Exercise 2 - Adding an Option

Now let us add an option in the MathFunctions library to allow developers to select either the custom square root implementation or the built in standard implementation. 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()`

- `option()`

- `target_compile_definitions()`

### Files to Edit

- `MathFunctions/CMakeLists.txt`

- `MathFunctions/MathFunctions.cxx`

### Getting Started

Start with the resulting files from Exercise 1. Complete `TODO 7` through `TODO 14`.

First create a variable `USE_MYMATH` using the `option()` command in `MathFunctions/CMakeLists.txt`. In that same file, use that option to pass a compile definition to the `MathFunctions` library.

Then, update `MathFunctions.cxx` to redirect compilation based on `USE_MYMATH`.

Lastly, prevent `mysqrt.cxx` from being compiled when `USE_MYMATH` is on by making it its own library inside of the `USE_MYMATH` block of `MathFunctions/CMakeLists.txt`.

### 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 `MathFunctions/CMakeLists.txt`. 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: MathFunctions/CMakeLists.txt

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

Next, make building and linking our library with `mysqrt` function conditional using this new option.

Create an `if()` statement which checks the value of `USE_MYMATH`. Inside the `if()` block, put the `target_compile_definitions()` command with the compile definition `USE_MYMATH`.

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

    if (USE_MYMATH)
      target_compile_definitions(MathFunctions PRIVATE "USE_MYMATH")
    endif()

When `USE_MYMATH` is `ON`, the compile definition `USE_MYMATH` will be set. We can then use this compile definition to enable or disable sections of our source code.

The corresponding changes to the source code are fairly straightforward. In `MathFunctions.cxx`, we make `USE_MYMATH` control which square root function is used:

TODO 9: Click to show/hide answer
TODO 9: MathFunctions/MathFunctions.cxx

    #ifdef USE_MYMATH
      return detail::mysqrt(x);
    #else
      return std::sqrt(x);
    #endif

Next, we need to include `mysqrt.h` if `USE_MYMATH` is defined.

TODO 10: Click to show/hide answer
TODO 10: MathFunctions/MathFunctions.cxx

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

Finally, we need to include `cmath` now that we are using `std::sqrt`.

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

    #include <cmath>

At this point, if `USE_MYMATH` is `OFF`, `mysqrt.cxx` would not be used but it will still be compiled because the `MathFunctions` target has `mysqrt.cxx` listed under sources.

There are a few ways to fix this. The first option is to use `target_sources()` to add `mysqrt.cxx` from within the `USE_MYMATH` block. Another option is to create an additional library within the `USE_MYMATH` block which is responsible for compiling `mysqrt.cxx`. For the sake of this tutorial, we are going to create an additional library.

First, from within `USE_MYMATH` create a library called `SqrtLibrary` that has sources `mysqrt.cxx`.

TODO 12: Click to show/hide answer
TODO 12 : MathFunctions/CMakeLists.txt

      add_library(SqrtLibrary STATIC
                  mysqrt.cxx
                  )

Next, we link `SqrtLibrary` onto `MathFunctions` when `USE_MYMATH` is enabled.

TODO 13: Click to show/hide answer
TODO 13 : MathFunctions/CMakeLists.txt

      target_link_libraries(MathFunctions PUBLIC SqrtLibrary)

Finally, we can remove `mysqrt.cxx` from our `MathFunctions` library source list because it will be pulled in when `SqrtLibrary` is included.

TODO 14: Click to show/hide answer
TODO 14 : MathFunctions/CMakeLists.txt

    add_library(MathFunctions MathFunctions.cxx)

With these changes, the `mysqrt` function is now completely optional to whoever is building and using the `MathFunctions` library. Users can toggle `USE_MYMATH` to manipulate what library is used in the build.