# Step 4: In-Depth CMake Target Commands

There are several target commands within CMake we can use to describe requirements. As a reminder, a target command is one which modifies the properties of the target it is applied to. These properties describe requirements needed to build the software, such as sources, compile flags, and output names; or properties necessary to consume the target, such as header includes, library directories, and linkage rules.

Note

As discussed in `Step1`, properties required to build a target should be described with the `PRIVATE` scope keyword, those required to consume the target with `INTERFACE`, and properties needed for both are described with `PUBLIC`.

In this step we will go over all the available target commands in CMake. Not all target commands are created equal. We have already discussed the two most important target commands, `target_sources()` and `target_link_libraries()`. Of the remaining commands, some are almost as common as these two, others have more advanced applications, and a couple should only be used as a last resort when other options are not available.

## Background

Before going any further, let's name all of the CMake target commands. We'll split these into three groups: the recommended and generally useful commands, the advanced and cautionary commands, and the "footgun" commands which should be avoided unless necessary.

Common/Recommended | Advanced/Caution | Esoteric/Footguns
--- | --- | ---
`target_compile_definitions()` `target_compile_features()` `target_link_libraries()` `target_sources()` | `get_target_property()` `set_target_properties()` `target_compile_options()` `target_link_options()` `target_precompile_headers()` | `target_include_directories()` `target_link_directories()`

Note

There's no such thing as a "bad" CMake target command. They all have valid use cases. This categorization is provided to give newcomers a simple intuition about which commands they should consider first when tackling a problem.

We'll demonstrate most of these in the following exercises. The three we won't be using are `get_target_property()`, `set_target_properties()` and `target_precompile_headers()`, so we will briefly discuss their purpose here.

The `get_target_property()` and `set_target_properties()` commands give direct access to a target's properties by name. They can even be used to attach arbitrary property names to a target.

    add_library(Example)
    set_target_properties(Example
      PROPERTIES
        Key Value
        Hello World
    )

    get_target_property(KeyVar Example Key)
    get_target_property(HelloVar Example Hello)

    message("Key: ${KeyVar}")
    message("Hello: ${HelloVar}")

    $ cmake -B build
    ...
    Key: Value
    Hello: World

The full list of target properties which are semantically meaningful to CMake are documented at `cmake-properties(7)`, however most of these should be modified with their dedicated commands. For example, it is unnecessary to directly manipulate `LINK_LIBRARIES` and `INTERFACE_LINK_LIBRARIES`, as these are handled by `target_link_libraries()`.

Conversely, some lesser-used properties are only accessible via these commands. The `DEPRECATION` property, used to attach deprecation notices to targets, can only be set via `set_target_properties()`; as can the `ADDITIONAL_CLEAN_FILES`, for describing additional files to be removed by CMake's `clean` target; and other properties of this sort.

The `target_precompile_headers()` command takes a list of header files, similar to `target_sources()`, and creates a precompiled header from them. This precompiled header is then force included into all translation units in the target. This can be useful for build performance.

## Exercise 1 - Features and Definitions

In earlier steps we cautioned against globally setting `CMAKE_<LANG>_STANDARD` and overriding packagers' decision concerning which language standard to use. On the other hand, many libraries have a minimum required feature set they need in order to build, and for these it is appropriate to use the `target_compile_features()` command to communicate those requirements.

    target_compile_features(MyApp PRIVATE cxx_std_20)

The `target_compile_features()` command describes a minimum language standard as a target property. If the `CMAKE_<LANG>_STANDARD` is above this version, or the compiler default already provides this language standard, no action is taken. If additional flags are necessary to enable the standard, these will be added by CMake.

Note

`target_compile_features()` manipulates the same style of interface and non-interface properties as the other target commands. This means it is possible to inherit a language standard requirement specified with `INTERFACE` or `PUBLIC` scope keywords.

If language features are used only in implementation files, then the respective compile features should be `PRIVATE`. If the target's headers use the features, then `PUBLIC` or `INTERFACE` should be used.

For C++, the compile features are of the form `cxx_std_YY` where `YY` is the standardization year, e.g. `14`, `17`, `20`, etc.

The `target_compile_definitions()` command describes compile definitions as target properties. It is the most common mechanism for communicating build configuration information to the source code itself. As with all properties, the scope keywords apply as we have discussed.

    target_compile_definitions(MyLibrary
      PRIVATE
        MYLIBRARY_USE_EXPERIMENTAL_IMPLEMENTATION

      PUBLIC
        MYLIBRARY_EXCLUDE_DEPRECATED_FUNCTIONS
    )

It is neither required nor desired that we attach `-D` prefixes to compile definitions described with `target_compile_definitions()`. CMake will determine the correct flag for the current compiler.

### Goal

Use `target_compile_features()` and `target_compile_definitions()` to communicate language standard and compile definition requirements.

### Helpful Resources

- `target_compile_features()`

- `target_compile_definitions()`

- `option()`

- `if()`

### Files to Edit

- `Tutorial/CMakeLists.txt`

- `MathFunctions/CMakeLists.txt`

- `MathFunctions/MathFunctions.cxx`

- `CMakePresets.json`

### Getting Started

The `Help/guide/tutorial/Step4` directory contains the complete, recommended solution to `Step3` and relevant `TODOs` for this step. Complete `TODO 1` through `TODO 8`.

### Build and Run

We can run CMake using our `tutorial` preset, and then build as usual.

    cmake --preset tutorial
    cmake --build build

Verify that the output of `Tutorial` is what we would expect for `std::sqrt`.

### Solution

First we add a new option to the top-level CML.

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

    option(TUTORIAL_BUILD_UTILITIES "Build the Tutorial executable" ON)
    option(TUTORIAL_USE_STD_SQRT "Use std::sqrt" OFF)

Then we add the compile feature and definitions to `MathFunctions`.

TODO 2-3: Click to show/hide answer
TODO 2-3: MathFunctions/CMakeLists.txt

    target_compile_features(MathFunctions PRIVATE cxx_std_20)

    if(TUTORIAL_USE_STD_SQRT)
      target_compile_definitions(MathFunctions PRIVATE TUTORIAL_USE_STD_SQRT)
    endif()

And the compile feature for `Tutorial`.

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

    target_compile_features(Tutorial PRIVATE cxx_std_20)

Now we can modify `MathFunctions` to take advantage of the new definition.

TODO 5-6: Click to show/hide answer
TODO 5: MathFunctions/MathFunctions.cxx

    #include <cmath>
    #include <format>
    #include <iostream>

TODO 6: MathFunctions/MathFunctions.cxx

    double sqrt(double x)
    {
    #ifdef TUTORIAL_USE_STD_SQRT
      return std::sqrt(x);
    #else
      return mysqrt(x);
    #endif
    }

Finally we can update our `CMakePresets.json`. We don't need to set `CMAKE_CXX_STANDARD` anymore, but we do want to try out our new compile definition.

TODO 7-8: Click to show/hide answer
TODO 7-8: CMakePresets.json

    "cacheVariables": {
      "TUTORIAL_USE_STD_SQRT": "ON"
    }

## Exercise 2 - Compile and Link Options

Sometimes, we need to exercise specific control over the exact options being passed on the compile and link line. These situations are addressed by `target_compile_options()` and `target_link_options()`.

    target_compile_options(MyApp PRIVATE -Wall -Werror)
    target_link_options(MyApp PRIVATE -T LinksScript.ld)

There are several problems with unconditionally calling `target_compile_options()` or `target_link_options()`. The primary problem is compiler flags are specific to the compiler frontend being used. In order to ensure that our project supports multiple compiler frontends, we must only pass compatible flags to the compiler.

We can achieve this by checking the `CMAKE_<LANG>_COMPILER_FRONTEND_VARIANT` variable which tells us the style of flags supported by the compiler frontend.

Note

Prior to CMake 3.26, `CMAKE_<LANG>_COMPILER_FRONTEND_VARIANT` was only set for compilers with multiple frontend variants. In versions after CMake 3.26 checking this variable alone is sufficient.

However this tutorial targets CMake 3.23. As such, the logic is more complicated than we have time for here. This tutorial step already includes correct logic for checking the compiler variant for MSVC, GCC, Clang, and AppleClang on CMake 3.23.

Even if a compiler accepts the flags we pass, the semantics of compiler flags change over time. This is especially true with regards to warnings. Projects should not turn warnings-as-error flags by default, as this can break their build on otherwise innocuous compiler warnings included in later releases.

Note

For errors and warnings, consider placing flags in `CMAKE_<LANG>_FLAGS` for local development builds and during CI runs (via preset or `-D` flags). We know exactly which compiler and toolchain are being used in these contexts, so we can customize the behavior precisely without risking build breakages on other platforms.

### Goal

Add appropriate warning flags to the `Tutorial` executable for MSVC-style and GNU-style compiler frontends.

### Helpful Resources

- `target_compile_options()`

### Files to Edit

- `Tutorial/CMakeLists.txt`

### Getting Started

Continue editing files in the `Step4` directory. The conditional for checking the frontend variant has already been written. Complete `TODO 9` and `TODO 10` to add warning flags to `Tutorial`.

### Build and Run

Since we have already configured for this step, we can build with the usual command.

    cmake --build build

This should reveal a simple warning in the build. You can go ahead and fix it.

### Solution

We need to add two compile options to `Tutorial`, one MSVC-style flag and one GNU-style flag.

TODO 9-10: Click to show/hide answer
TODO 9-10: Tutorial/CMakeLists.txt

    if(
      (CMAKE_CXX_COMPILER_ID STREQUAL "MSVC") OR
      (CMAKE_CXX_COMPILER_FRONTEND_VARIANT STREQUAL "MSVC")
    )

      target_compile_options(Tutorial PRIVATE /W3)

    elseif(
      (CMAKE_CXX_COMPILER_ID STREQUAL "GNU") OR
      (CMAKE_CXX_COMPILER_ID MATCHES "Clang")
    )

      target_compile_options(Tutorial PRIVATE -Wall)

    endif()

## Exercise 3 - Include and Link Directories

Note

This exercise requires building an archive using a compiler directly on the command line. It is not used in later steps. It is included only to demonstrate a use case for `target_include_directories()` and `target_link_directories()`.

If you cannot complete this exercise for whatever reason feel free to treat it as informational-only, or skip it entirely.

It is generally unnecessary to directly describe include and link directories, as these requirements are inherited when linking together targets generated within CMake, or from external dependencies imported into CMake with commands we will cover in later steps.

If we happen to have some libraries or header files which are not described by a CMake target which we need to bring into the build, perhaps pre-compiled binaries provided by a vendor, we can incorporate with the `target_link_directories()` and `target_include_directories()` commands.

    target_link_directories(MyApp PRIVATE Vendor/lib)
    target_include_directories(MyApp PRIVATE Vendor/include)

These commands use properties which map to the `-L` and `-I` compiler flags (or whatever flags the compiler uses for link and include directories).

Of course, passing a link directory doesn't tell the compiler to link anything into the build. For that we need `target_link_libraries()`. When `target_link_libraries()` is given an argument which does not map to a target name, it will add the string directly to the link line as a library to be linked into the build (prepending any appropriate flags, such a `-l`).

### Goal

Describe a pre-compiled, vendored, static library and its headers inside a project using `target_link_directories()` and `target_include_directories()`.

### Helpful Resources

- `target_link_directories()`

- `target_include_directories()`

- `target_link_libraries()`

### Files to Edit

- `Vendor/CMakeLists.txt`

- `Tutorial/CMakeLists.txt`

### Getting Started

You will need to build the vendor library into a static archive to complete this exercise. Navigate to the `Help/guide/tutorial/Step4/Vendor/lib` directory and build the code as appropriate for your platform. On Unix-like operating systems the appropriate commands are usually:

    g++ -c Vendors.cxx
    ar rvs libVendor.a Vendor.o

Then complete `TODO 11` through `TODO 14`.

Note

`VendorLib` is an `INTERFACE` library, meaning it has no build requirements (because it has already been built). All of its properties should also be interface properties.

We'll discuss `INTERFACE` libraries in greater depth during the next step.

### Build and Run

If you have successfully built `libVendor`, you can rebuild `Tutorial` using the normal command.

    cmake --build build

Running `Tutorial` should now output a message about the acceptability of the result to the vendor.

### Solution

We need to use the target link and include commands to describe the archive and its headers as `INTERFACE` requirements of `VendorLib`.

TODO 11-13: Click to show/hide answer
TODO 11-13: Vendor/CMakeLists.txt

    target_include_directories(VendorLib
      INTERFACE
        include
    )

    target_link_directories(VendorLib
      INTERFACE
        lib
    )

    target_link_libraries(VendorLib
      INTERFACE
        Vendor
    )

Then we can add `VendorLib` to `Tutorial`'s linked libraries.

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

    target_link_libraries(Tutorial
      PRIVATE
        MathFunctions
        VendorLib
    )