# Step 4: Adding Generator Expressions

`Generator expressions` are evaluated during build system generation to produce information specific to each build configuration.

`Generator expressions` are allowed in the context of many target properties, such as `LINK_LIBRARIES`, `INCLUDE_DIRECTORIES`, `COMPILE_DEFINITIONS` and others. They may also be used when using commands to populate those properties, such as `target_link_libraries()`, `target_include_directories()`, `target_compile_definitions()` and others.

`Generator expressions` may be used to enable conditional linking, conditional definitions used when compiling, conditional include directories and more. The conditions may be based on the build configuration, target properties, platform information or any other queryable information.

There are different types of `generator expressions` including Logical, Informational, and Output expressions.

Logical expressions are used to create conditional output. The basic expressions are the `0` and `1` expressions. A `$<0:...>` results in the empty string, and `<1:...>` results in the content of `...`. They can also be nested.

## Exercise 1 - Setting the C++ Standard with Interface Libraries

Before we use `generator expressions` let's refactor our existing code to use an `INTERFACE` library. We will use that library in the next step to demonstrate a common use for `generator expressions`.

### Goal

Add an `INTERFACE` library target to specify the required C++ standard.

### Helpful Resources

- `add_library()`

- `target_compile_features()`

- `target_link_libraries()`

### Files to Edit

- `CMakeLists.txt`

- `MathFunctions/CMakeLists.txt`

### Getting Started

In this exercise, we will refactor our code to use an `INTERFACE` library to specify the C++ standard.

The starting source code is provided in the `Step4` directory. In this exercise, complete `TODO 1` through `TODO 3`.

Start by editing the top level `CMakeLists.txt` file. Construct an `INTERFACE` library target called `tutorial_compiler_flags` and specify `cxx_std_11` as a target compiler feature.

Modify `CMakeLists.txt` and `MathFunctions/CMakeLists.txt` so that all targets have a `target_link_libraries()` call to `tutorial_compiler_flags`.

### Build and Run

Make a new directory called `Step4_build`, run the `cmake` executable or the `cmake-gui` to configure the project and then build it with your chosen build tool or by using `cmake --build .` from the build directory.

Here's a refresher of what that looks like from the command line:

    mkdir Step4_build
    cd Step4_build
    cmake ../Step4
    cmake --build .

Next, use the newly built `Tutorial` and verify that it is working as expected.

### Solution

Let's update our code from the previous step to use interface libraries to set our C++ requirements.

To start, we need to remove the two `set()` calls on the variables `CMAKE_CXX_STANDARD` and `CMAKE_CXX_STANDARD_REQUIRED`. The specific lines to remove are as follows:

CMakeLists.txt

    set(CMAKE_CXX_STANDARD 11)
    set(CMAKE_CXX_STANDARD_REQUIRED True)

Next, we need to create an interface library, `tutorial_compiler_flags`. And then use `target_compile_features()` to add the compiler feature `cxx_std_11`.

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

    add_library(tutorial_compiler_flags INTERFACE)
    target_compile_features(tutorial_compiler_flags INTERFACE cxx_std_11)

Finally, with our interface library set up, we need to link our executable `Target` and our `MathFunctions` library to our new `tutorial_compiler_flags` library. Respectively, the code will look like this:

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

    target_link_libraries(Tutorial PUBLIC ${EXTRA_LIBS} tutorial_compiler_flags)

and this:

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

    target_link_libraries(MathFunctions tutorial_compiler_flags)

With this, all of our code still requires C++ 11 to build. Notice though that with this method, it gives us the ability to be specific about which targets get specific requirements. In addition, we create a single source of truth in our interface library.

## Exercise 2 - Adding Compiler Warning Flags with Generator Expressions

A common usage of `generator expressions` is to conditionally add compiler flags, such as those for language levels or warnings. A nice pattern is to associate this information to an `INTERFACE` target allowing this information to propagate.

### Goal

Add compiler warning flags when building but not for installed versions.

### Helpful Resources

- `cmake-generator-expressions(7)`

- `cmake_minimum_required()`

- `set()`

- `target_compile_options()`

### Files to Edit

- `CMakeLists.txt`

### Getting Started

Start with the resulting files from Exercise 1. Complete `TODO 4` through `TODO 7`.

First, in the top level `CMakeLists.txt` file, we need to set the `cmake_minimum_required()` to `3.15`. In this exercise we are going to use a generator expression which was introduced in CMake 3.15.

Next we add the desired compiler warning flags that we want for our project. As warning flags vary based on the compiler, we use the `COMPILE_LANG_AND_ID` generator expression to control which flags to apply given a language and a set of compiler ids.

### Build and Run

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

    cd Step4_build
    cmake --build .

### Solution

Update the `cmake_minimum_required()` to require at least CMake version `3.15`:

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

    cmake_minimum_required(VERSION 3.15)

Next we determine which compiler our system is currently using to build since warning flags vary based on the compiler we use. This is done with the `COMPILE_LANG_AND_ID` generator expression. We set the result in the variables `gcc_like_cxx` and `msvc_cxx` as follows:

TODO 5: Click to show/hide answer
TODO 5: CMakeLists.txt

    set(gcc_like_cxx "$<COMPILE_LANG_AND_ID:CXX,ARMClang,AppleClang,Clang,GNU,LCC>")
    set(msvc_cxx "$<COMPILE_LANG_AND_ID:CXX,MSVC>")

Next we add the desired compiler warning flags that we want for our project. Using our variables `gcc_like_cxx` and `msvc_cxx`, we can use another generator expression to apply the respective flags only when the variables are true. We use `target_compile_options()` to apply these flags to our interface library.

TODO 6: Click to show/hide answer
TODO 6: CMakeLists.txt

    target_compile_options(tutorial_compiler_flags INTERFACE
      "$<${gcc_like_cxx}:-Wall;-Wextra;-Wshadow;-Wformat=2;-Wunused>"
      "$<${msvc_cxx}:-W3>"
    )

Lastly, we only want these warning flags to be used during builds. Consumers of our installed project should not inherit our warning flags. To specify this, we wrap our flags in a generator expression using the `BUILD_INTERFACE` condition. The resulting full code looks like the following:

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

    target_compile_options(tutorial_compiler_flags INTERFACE
      "$<${gcc_like_cxx}:$<BUILD_INTERFACE:-Wall;-Wextra;-Wshadow;-Wformat=2;-Wunused>>"
      "$<${msvc_cxx}:$<BUILD_INTERFACE:-W3>>"
    )