Add generator expression

Review the past and learn the new
Add compiler warning flags using generator expressions
- cmake-generator-expressions
- cmake_minimum_required
- set function
- target_compile_options

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Review the past and learn the new

After the first exercise, everyone should have mastered the basic usage. Since there are too many codes, it would be too lengthy to introduce them in detail in the document. You can download the code for reference.

Exercise 1 Build a basic project
- cmake_minimum_required() is a function used to specify the minimum CMake version required to build a specific project.
- The project() function is used to define a project and specify the name and related properties of the project.
- add_executable() is used to specify the target to generate an executable file.
- CMAKE_CXX_STANDARD is used to specify the C++ standard that the C++ compiler in the project should follow.
- CMAKE_CXX_STANDARD_REQUIRED is used to indicate whether the compiler is required to enforce the use of the specified C++ standard.
- The set() function is used to set the value of a variable.
- _VERSION_MAJOR is typically used to access the project’s major version number.
- _VERSION_MINOR is typically used to access the project’s minor version number.
- configure_file() is used to copy a file and replace variable values in it when generating the build system.
- target_include_directories() is used to specify the header file inclusion path of a target (usually an executable file or library).
Exercise 2 Adding a library
- add_library() is used to create and configure a library target.
- add_subdirectory() is used to add other source code directories (usually subdirectories) to the current CMake project and build other CMake projects in these subdirectories.
- target_link_libraries() is used for a target (usually an executable file or library) to specify other libraries that it depends on.
- PROJECT_SOURCE_DIRIt contains the path to the source code directory of the current CMake project.
- if() is used to execute different CMake code blocks based on whether the condition is true or false.
- The option() command is used to define user-configurable options.
- target_compile_definitions() is used to set compile-time macro definitions (preprocessor definitions) for a specific target (usually an executable file or library).
Exercise 3 Adding library usage requirements
- target_compile_options() is used to set compiler options for a specific target (usually an executable file or library)
- The target_link_directories() command is used to specify link directories for a specific target (usually an executable file or library) to help the build system find the location of shared libraries or library files when linking the target.
- target_link_options() is a command in CMake used to set linker options for a specific target (usually an executable or library).
- target_precompile_headers() is a feature introduced in CMake version 3.16, which is used to precompile header files (precompiled headers) for targets (usually executable files or libraries) to improve build performance.
- target_sources() is used to add source files to a specific target (usually an executable or library) so that they can be compiled and linked into the target.
- target_compile_features() is a command in CMake that sets the C++ features supported by the compiler for a specific target (usually an executable or library).

Generator expressions are evaluated during build system generation to generate 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 can also be used when populating these properties with commands such as target_link_libraries(), target_include_directories(), target_compile_definitions() and others.

Generator expressions can be used to enable conditional linking, conditional definitions used at compile time, conditional include directories, etc. These conditions can be based on build configurations, target properties, platform information, or any other queryable information.

There are different types of generator expressions including logical, informational and output expressions.

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Add compiler warning flags using generator expressions

A common use of generator expressions is to conditionally add compiler flags, such as language level or warning flags. INTERFACE A good pattern is to associate this information with a goal that allows this information to spread.

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cmake-generator-expressions

cmake-generator-expressions() (CMake generator expressions) is a special expression used in the CMake build system to generate build files. They allow you to generate different compilation and linking options at build time based on different generator and target properties.

Generator Expressions can be used in different commands of CMake to give more granular control over the build process. These expressions are usually enclosed in $< > tags to distinguish them from normal CMake variables and properties.

Here are some common uses of Generator Expressions:

Conditional compilation: You can use if expressions to conditionally compile parts of your code based on different build configurations or target properties. For example:
```
target_compile_definitions(mytarget PRIVATE $<$<CONFIG:Debug>:DEBUG_MODE>)
```
This will add the DEBUG_MODE macro definition under the Debug build configuration.
Conditional link libraries: You can use Generator Expressions in the target_link_libraries command to link different libraries based on different conditions. For example:
```
target_link_libraries(mytarget PRIVATE $<$<CONFIG:Release>:optimized some_lib>)
```
This will link against the some_lib library under the Release build configuration.
Select source files: You can use Generator Expressions to select source files to compile based on different conditions. For example:
```
set(sources
    source1.cpp
    $<$<CONFIG:Debug>:debug_source.cpp>
)
add_executable(mytarget ${sources})
```
This will compile debug_source.cpp under the Debug build configuration.
Target properties: You can use Generator Expressions to access a target’s properties to configure the target in different situations. For example:
```
target_compile_options(mytarget PRIVATE $<TARGET_PROPERTY:mylib,INTERFACE_COMPILE_OPTIONS>)
```
This will set the same interface compilation options for mytarget as for mylib.

Generator Expressions in CMake provide a flexible way to customize the build process based on different conditions, adapting the build system to different needs and platforms. They often provide fine-grained control over conditional compilation, different build configurations, and target properties.

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cmake_minimum_required

cmake_minimum_required() has been introduced in Exercise 1 to build a basic project and will not be repeated.

Next article: Exercise 5 Installation and Testing, Previous article: Exercise 3 Adding Library Usage Requirements, Table of Contents | Home Page

set function

In CMake, set() has been introduced in Exercise 1 to build a basic project and will not be repeated.

Next article: Exercise 5 Installation and Testing, Previous article: Exercise 3 Adding Library Usage Requirements, Table of Contents | Home Page

target_compile_options

target_compile_options() The usage requirements for adding libraries have been introduced in Exercise 3 and will not be repeated here.

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