# Step 5: In-Depth CMake Library Concepts

While executables are mostly one-size-fits-all, libraries come in many different forms. There are static archives, shared objects, modules, object libraries, header-only libraries, and libraries which describe advanced CMake properties to be inherited by other targets, just to name a few.

In this step you will learn about some of the most common kinds of libraries that CMake can describe. This will cover most of the in-project uses of `add_library()`. Libraries which are imported from dependencies (or exported by the project to be consumed as a dependency) will be covered in later steps.

## Background

As we learned in `Step1`, the `add_library()` command accepts the name of the library target to be created as its first argument. The second argument is an optional `<type>` for which the following values are valid:

> `STATIC`

>   A Static Library: an archive of object files for use when linking other targets.

> `SHARED`

>   A Shared Library: a dynamic library that may be linked by other targets and loaded at runtime.

> `MODULE`

>   A Module Library: a plugin that may not be linked by other targets, but may be dynamically loaded at runtime using dlopen-like functionality.

> `OBJECT`

>   An Object Library: a collection of object files which have not been archived or linked into a library.

> `INTERFACE`

>   An Interface Library: a library target which specifies usage requirements for dependents but does not compile sources and does not produce a library artifact on disk.

In addition, there are `IMPORTED` libraries which describe library targets from foreign projects or modules, imported into the current project. We will cover these briefly in later steps.

`MODULE` libraries are most commonly found in plugin systems, or as extensions to runtime-loading languages like Python or Javascript. They act very similar to normal shared libraries, except they cannot be directly linked by other targets. They are sufficiently similar that we won't cover them in further depth here.

## Exercise 1 - Static and Shared

While the `add_library()` command supports explicitly setting `STATIC` or `SHARED`, and this is sometimes necessary, it is best to leave the second argument empty for most "normal" libraries which can operate as either.

When not given a type, `add_library()` will create either a `STATIC` or `SHARED` library depending on the value of `BUILD_SHARED_LIBS`. If `BUILD_SHARED_LIBS` is true, a `SHARED` library will be created, otherwise it will be `STATIC`.

    add_library(MyLib-static STATIC)
    add_library(MyLib-shared SHARED)

    # Depends on BUILD_SHARED_LIBS
    add_library(MyLib)

This is desirable behavior, as it allows packagers to determine what kind of library will be produced, and ensure dependents link to that version of the library without needing to modify their source code. In some contexts, fully static builds are appropriate, and in others shared libraries are desirable.

Note

CMake does not define the `BUILD_SHARED_LIBS` variable by default, meaning without project or user intervention `add_library()` will produce `STATIC` libraries.

By leaving the second argument to `add_library()` blank, projects provide additional flexibility to their packagers and downstream dependents.

### Goal

Build `MathFunctions` as a shared library.

Note

On Windows, you might see warnings about an empty DLL, as `MathFunctions` doesn't export any symbols.

### Helpful Resources

- `BUILD_SHARED_LIBS`

### Files to Edit

There are no files to edit.

### Getting Started

The `Help/guide/tutorial/Step5` directory contains the complete, recommended solution to `Step4`. This step is about building the `MathFunctions` library, there are no `TODOs` necessary. You can proceed directly to the build step.

### Build and Run

We can configure using our preset, turning on `BUILD_SHARED_LIBS` with a `-D` flag.

    cmake --preset tutorial -DBUILD_SHARED_LIBS=ON

Then we can build only the `MathFunctions` library with `-t`.

    cmake --build build -t MathFunctions

Verify a shared library is produced for `MathFunctions` then reset `BUILD_SHARED_LIBS`, either by reconfiguring with `-DBUILD_SHARED_LIBS=OFF` or deleting the `CMakeCache.txt`.

### Solution

There are no changes to the project for this exercise.

## Exercise 2 - Interface Libraries

Interface libraries are those which only communicate usage requirements for other targets, they do not build or produce any artifacts of their own. As such all the properties of an interface library must themselves be interface properties, specified with the `INTERFACE` scope keywords.

    add_library(MyInterface INTERFACE)
    target_compile_definitions(MyInterface INTERFACE MYINTERFACE_COMPILE_DEF)

The most common kind of interface library in C++ development is a header-only library. Such libraries do not build anything, only providing the flags necessary to discover their headers.

### Goal

Add a header-only library to the tutorial project, and use it inside the `Tutorial` executable.

### Helpful Resources

- `add_library()`

- `target_sources()`

### Files to Edit

- `MathFunctions/MathLogger/CMakeLists.txt`

- `MathFunctions/CMakeLists.txt`

- `MathFunctions/MathFunctions.cxx`

### Getting Started

In our previous discussions of `target_sources(FILE_SET)`, we noted we can omit the `TYPE` parameter if the file set's name is the same as the file set's type. We also said we can omit the `BASE_DIRS` parameter if we want to use the current source directory as the only base directory.

We're ready to introduce a third shortcut, we only need to include the `FILES` parameter if the headers are intended to be installed, such as public headers of a library.

The `MathLogger` headers in this exercise are only used internally by the `MathFunctions` implementation. They will not be installed. This should make for a very abbreviated call to `target_sources(FILE_SET)`.

Note

The headers will be discovered by the compiler's dependency scanner to ensure correct incremental builds. It can be useful to list header files in these contexts anyway, as the list can be used to generate metadata some IDEs rely on.

You can begin editing the `Step5` directory. Complete `TODO 1` through `TODO 7`.

### Build and Run

The preset has already been updated to use `mathfunctions::sqrt` instead of `std::sqrt`. We can build and configure as usual.

    cmake --preset tutorial
    cmake --build build

Verify that the `Tutorial` output now uses the logging framework.

### Solution

First we add a new `INTERFACE` library named `MathLogger`.

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

    add_library(MathLogger INTERFACE)

Then we add the appropriate `target_sources()` call to capture the header information. We give this file set the name `HEADERS` so we can omit the `TYPE`, we don't need `BASE_DIRS` as we will use the default of the current source directory, and we can exclude the `FILES` list because we don't intend to install the library.

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

    target_sources(MathLogger
      INTERFACE
        FILE_SET HEADERS
    )

Now we can add the `MathLogger` library to the `MathFunctions` linked libraries, and at the `MathLogger` folder to the project.

TODO 3-4: Click to show/hide answer
TODO 3: MathFunctions/CMakeLists.txt

    target_link_libraries(MathFunctions
      PRIVATE
        MathLogger
    )

TODO 4: MathFunctions/CMakeLists.txt

    add_subdirectory(MathLogger)

Finally we can update `MathFunctions.cxx` to take advantage of the new logger.

TODO 5-7: Click to show/hide answer
TODO 5: MathFunctions/MathFunctions.cxx

    #include <cmath>
    #include <format>

    #include <MathLogger.h>

TODO 6: MathFunctions/MathFunctions.cxx

    mathlogger::Logger Logger;

TODO 7: MathFunctions/MathFunctions.cxx

    Logger.Log(std::format("Computing sqrt of {} to be {}\n", x, result));

## Exercise 3 - Object Libraries

Object libraries have several advanced uses, but also tricky nuances which are difficult to fully enumerate in the scope of this tutorial.

    add_library(MyObjects OBJECT)

The most obvious drawback to object libraries is the objects themselves cannot be transitively linked. If an object library appears in the `INTERFACE_LINK_LIBRARIES` of a target, the dependents which link that target will not "see" the objects. The object library will act like an `INTERFACE` library in such contexts. In the general case, object libraries are only suitable for `PRIVATE` or `PUBLIC` consumption via `target_link_libraries()`.

A common use case for object libraries is coalescing several library targets into a single archive or shared library object. Even within a single project libraries may be maintained as different targets for a variety of reasons, such as belonging to different teams within an organization. However, it may be desirable to distribute these as a single consumer-facing binary. Object libraries make this possible.

### Goal

Add several object libraries to the `MathFunctions` library.

### Helpful Resources

- `target_link_libraries()`

- `add_subdirectory()`

### Files to Edit

- `MathFunctions/CMakeLists.txt`

- `MathFunctions/MathFunctions.h`

- `Tutorial/Tutorial.cxx`

### Getting Started

Several extensions for our `MathFunctions` library have been made available (we can imagine these coming from other teams in our organization). Take a minute to look at the targets made available in `MathFunctions/MathExtensions`. Then complete `TODO 8` through `TODO 11`.

### Build and Run

There's no reconfiguration needed, we can build as usual.

    cmake --build build

Verify the output of `Tutorial` now includes the verification message. Also take a minute to inspect the build directory under `build/MathFunctions/MathExtensions`. You should find that, unlike `MathFunctions`, no archives are produced for any of the object libraries.

### Solution

First we will add links for all the object libraries to `MathFunctions`. These are `PUBLIC`, because we want the objects to be added to the `MathFunctions` library as part of its own build step, and we want the headers to be available to consumers of the library.

Then we add the `MathExtensions` subdirectoy to the project.

TODO 8-9: Click to show/hide answer
TODO 8: MathFunctions/CMakeLists.txt

    target_link_libraries(MathFunctions
      PRIVATE
        MathLogger

      PUBLIC
        OpAdd
        OpMul
        OpSub
    )

TODO 9: MathFunctions/CMakeLists.txt

    add_subdirectory(MathExtensions)

To make the extensions available to consumers, we include their headers in the `MathFunctions.h` header.

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

    #include <OpAdd.h>
    #include <OpMul.h>
    #include <OpSub.h>

Finally we can take advantage of the extensions in the `Tutorial` program.

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

    double const checkValue = mathfunctions::OpMul(outputValue, outputValue);
    std::cout << std::format("The square of {} is {}\n", outputValue,
                             checkValue);