Cmake generator expressions

Question

I'm trying for long time to understand the benefit of generator expressions such as $<xxx:yy> in CMake, when and how to use them. Can anybody explain it clearly with some examples. Many thank in advance

`Can anybody explain it clearly` - too broad. Should someone repeat [CMake documentation](https://cmake.org/cmake/help/v3.9/manual/cmake-generator-expressions.7.html)? Or other manual? Otherwise, explain **what exactly** you don't understand, e.g. which expression, or which example. — Tsyvarev, Sep 13 '17 at 20:52
@Tsyvarev I added the documentation of the part, that I didn't understand — Mauri, Sep 13 '17 at 21:07
`$` is nearly a half of all usage of generator expressions. Its meaning in short: if generator expression `xxx` is evaluated to TRUE (as boolean), then value of generator expression `yy` is used. Otherwise, empty string is used. Usefullness of this kind of expression is: when value of the left generator expressions depends on configuration option, and build system supports multiconfiguration (e.g. Visual Studio), then such expression cannot be rewritten with `if`, as concrete configuration isn't known at this stage. — Tsyvarev, Sep 13 '17 at 21:56

Florian · Accepted Answer · 2017-09-15T18:44:10.897

CMake does first parse the CMakeLists.txt files in your project - named "Configuration Phase" - and then it generates your build environment - named "Generation Phase".

So basically the generator expressions are for everything only the generator could know:

The name and path of target outputs (mainly when cross-compiling and in multi-configuration environments)
Or more generally any target property that the generator evaluates to mingle together the compiler/linker calls

Here are examples where I use generator expressions in my project:

Copying files next to the executable (in multi-configuration environments you can't just use variables like CMAKE_CURRENT_BINARY_DIR)

add_custom_command(
    TARGET library1 
    POST_BUILD
    COMMAND ${CMAKE_COMMAND} -E copy
        "$<TARGET_FILE:library1>"
        "$<TARGET_FILE_DIR:mainProject>/$<TARGET_FILE_NAME:library1>"
)

CMake post-build-event: copy compiled libraries

add_custom_command(
    TARGET myBinary 
    POST_BUILD
        COMMAND ${CMAKE_COMMAND} -E copy  
                     "${CMAKE_CURRENT_SOURCE_DIR}/myTest.txt" 
                     "$<TARGET_FILE_DIR:myBinary>/myTest.txt"
)

how do I add external test files to a cmake project

Differentiate e.g. DEBUG or RELEASE configurations
```
add_compile_options("$<$<CONFIG:DEBUG>:/MDd>")
```
For Cmake, can you modify the release/debug compiler flags with `add_compiler_flags()` command?

Modern way to set compiler flags in cross-platform cmake project
With the TARGET_PROPERTY generator expression you could do a lot of things e.g.
```
file(GENERATE 
    OUTPUT "includes.txt" 
    CONTENT "$<TARGET_PROPERTY:motor,INCLUDE_DIRECTORIES>\n"
)
```
CMake doesn't pick up INTERFACE_INCLUDE_DIRECTORIES of linked library

In 1 you're saying that "in multi-configuration environments you can't just use variables like `CMAKE_CURRENT_BINARY_DIR`", why? — Shlomo Gottlieb, Apr 30 '22 at 11:33

Cmake generator expressions

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