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most simple but complete cmake example

Posted by: admin November 29, 2017 Leave a comment

Questions:

Somehow I am totally confused by how CMake works. Every time I think that I am getting closer to understand how CMake is meant to be written, it vanishes in the next example I read. All I want to know is, how should I structure my project, so that my CMake requires the least amount of maintainance in the future. For example, I don’t want to update my CMakeList.txt when I am adding a new folder in my src tree, that works exactly like all other src folders.

This is how I imagine my project’s structure, but please this is only an example. If the recommended way differs, please tell me, and tell me how to do it.

myProject
    src/
        module1/
            module1.h
            module1.cpp
        module2/
            [...]
        main.cpp
    test/
        test1.cpp
    resources/
        file.png
    bin
        [execute cmake ..]

By the way, it is important that my program knows where the resources are. I would like to know the recommended way of managing resources. I do not want to access my resources with “../resources/file.png”

Answers:

after some research I have now my own version of the most simple but complete cmake example. Here it is, and it tries to cover most of the basics, including resources and packaging.

one thing it does non-standard is resource handling. By default cmake wants to put them in /usr/share/, /usr/local/share/ and something equivalent on windows. I wanted to have a simple zip/tar.gz that you can extract anywhere and run. Therefore resources are loaded relative to the executable.

the basic rule to understand cmake commands is the following syntax:
<function-name>(<arg1> [<arg2> ...]) without comma or semicolor. Each argument is a string. foobar(3.0) and foobar("3.0") is the same. you can set lists/variables with set(args arg1 arg2). With this variable set foobar(${args}) and foobar(arg1 arg2) are effectively the same. A non existent variable is equivalent to an empty list. A list is internally just a string with semicolons to separate the elements. Therefore a list with just one element is by definition just that element, no boxing takes place. Variables are global. Builtin functions offer some form of named arguments by the fact that they expect some ids like PUBLIC or DESTINATION in their argument list, to group the arguments. But that’s not a language feature, those ids are also just strings, and parsed by the function implementation.

you can clone everything from github

cmake_minimum_required(VERSION 3.0)
project(example_project)

###############################################################################
## file globbing ##############################################################
###############################################################################

# these instructions search the directory tree when cmake is
# invoked and put all files that match the pattern in the variables 
# `sources` and `data`
file(GLOB_RECURSE sources      src/main/*.cpp src/main/*.h)
file(GLOB_RECURSE sources_test src/test/*.cpp)
file(GLOB_RECURSE data resources/*)
# you can use set(sources src/main.cpp) etc if you don't want to
# use globing to find files automatically

###############################################################################
## target definitions #########################################################
###############################################################################

# add the data to the target, so it becomes visible in some IDE
add_executable(example ${sources} ${data})

# just for example add some compiler flags
target_compile_options(example PUBLIC -std=c++1y -Wall -Wfloat-conversion)

# this lets me include files relative to the root src dir with a <> pair
target_include_directories(example PUBLIC src/main)

# this copies all resource files in the build directory
# we need this, because we want to work with paths relative to the executable
file(COPY ${data} DESTINATION resources)

###############################################################################
## dependencies ###############################################################
###############################################################################

# this defines the variables Boost_LIBRARIES that contain all library names
# that we need to link to
find_package(Boost 1.36.0 COMPONENTS filesystem system REQUIRED)

target_link_libraries(example PUBLIC
  ${Boost_LIBRARIES}
  # here you can add any library dependencies
)

###############################################################################
## testing ####################################################################
###############################################################################

# this is for our testing framework
# we don't add REQUIRED because it's just for testing
find_package(GTest)

if(GTEST_FOUND)
  add_executable(unit_tests ${sources_test} ${sources})

  # we add this define to prevent collision with the main
  # this might be better solved by not adding the source with the main to the
  # testing target
  target_compile_definitions(unit_tests PUBLIC UNIT_TESTS)

  # this allows us to use our executable as a link library
  # therefore we can inherit all compiler options and library dependencies
  set_target_properties(example PROPERTIES ENABLE_EXPORTS on)

  target_link_libraries(unit_tests PUBLIC
    ${GTEST_BOTH_LIBRARIES}
    example
  )

  target_include_directories(unit_tests PUBLIC
    ${GTEST_INCLUDE_DIRS} # doesn't do anything on Linux
  )
endif()

###############################################################################
## packaging ##################################################################
###############################################################################

# all install commands get the same destination. this allows us to use paths
# relative to the executable.
install(TARGETS example DESTINATION example_destination)
# this is basically a repeat of the file copy instruction that copies the
# resources in the build directory, but here we tell cmake that we want it
# in the package
install(DIRECTORY resources DESTINATION example_destination)

# now comes everything we need, to create a package
# there are a lot more variables you can set, and some
# you need to set for some package types, but we want to
# be minimal here
set(CPACK_PACKAGE_NAME "MyExample")
set(CPACK_PACKAGE_VERSION "1.0.0")

# we don't want to split our program up into several things
set(CPACK_MONOLITHIC_INSTALL 1)

# This must be last
include(CPack)

Questions:
Answers:

The most basic but complete example can be find in the cmake tutorial :

cmake_minimum_required (VERSION 2.6)
project (Tutorial)
add_executable(Tutorial tutorial.cxx)

For your project example you may have:

cmake_minimum_required (VERSION 2.6)
project (MyProject)
add_executable(myexec src/module1/module1.cpp src/module2/module2.cpp src/main.cpp)
add_executable(mytest test1.cpp)

For your additional question, one way to go is again in the tutorial: create a configurable header file that you include in your code. For this, make a file configuration.h.in with the following contents:

#define RESOURCES_PATH "@[email protected]"

Then in your CMakeLists.txt add:

set(RESOURCES_PATH "${PROJECT_SOURCE_DIR}/resources/"
# configure a header file to pass some of the CMake settings
# to the source code
configure_file (
  "${PROJECT_SOURCE_DIR}/configuration.h.in"
  "${PROJECT_BINARY_DIR}/configuration.h"
)

# add the binary tree to the search path for include files
# so that we will find TutorialConfig.h
include_directories("${PROJECT_BINARY_DIR}")

Finally, where you need the path in your code, you can do:

#include "configuration.h"

...

string resourcePath = string(RESOURCE_PATH) + "file.png";