# 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 - 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

Open the file `Step4/CMakeLists.txt` and complete `TODO 1` through `TODO 4`.

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

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.

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

### Solution

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

TODO 1: Click to show/hide answer
TODO 1: 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 2: Click to show/hide answer
TODO 2: 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 3: Click to show/hide answer
TODO 3: 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 from TODO 3 in a generator expression using the `BUILD_INTERFACE` condition. The resulting full code looks like the following:

TODO 4: Click to show/hide answer
TODO 4: 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>>"
    )