The example is taken from the openssl test package in the Conan Center Index.

Note that in call cases, linking OpenSSL is done with these two CMake lines:

find_package(OpenSSL REQUIRED)

target_link_libraries(digest OpenSSL::SSL)

Consume the system library using pure CMake.

• CMake listing

sudo apt-get install build_essential make gcc cmake libssl-dev

cd pure_cmake
mkdir build
cd build
cmake ..
make
./digest

Consume the openssl library via conan-center-index using conan and CMake.

• Conan recipe
• CMake listing

sudo apt-get install python3 python3-pip
pip3 install conan==1.43.2

cd conan_center_index
conan create .

Consumer cannot override to use the openssl provided by the distribution.

Consume the openssl library via conan-center-index using conan and CMake.

• Conan recipe
• CMake listing

cd conan_from_system
export CONAN_SYSREQUIRES_MODE=enabled
conan create .

Consumer cannot override to use conan.

Dummy openssl package:

• Conan recipe

Intermediate library:

• Conan recipe
• CMake listing

Consumer prefering conan:

• Conan recipe
• CMake listing

Consumer prefering system:

• Conan recipe
• CMake listing

cd dummy_system_package
cd openssl_system
conan create . # on Ubuntu 20.04, this will generate openssl/1.1.1f@system/stable
cd ..
cd digest_lib
conan create .
cd ..
cd consumer_cci
conan create .
cd ..
cd consumer_system
conan create . --build=digest_consumer_system --build=digest_lib # digest_lib should be re-built

Result:

• The consumer can decide where the OpenSSL library should come from. The difference is a single line of code: the requirement override in the consuming conan recipe.
• No a single adaption in the intermediate library needed!
• See the digest executable an the respective 'bin' subfolders of the produced packages. Check ldd of these and execute them.

Conditions:

• "system" package should have the same name as the CCI package so that override works
• the (most iumportant) options of the CCI package should be reflected in the dummy package (here: option shared)
• In order to have a real rebuild of the intermediate library, the intermediate lib should use the full_package_mode for the openssl lib. Alternatively, the version can be changed so something non-semver (I use "system" as version, not as channel). But then, the version reflection in openssl_version = tools.Version(self.deps_cpp_info["openssl"].version) would no longer work. And of course any version ranges specified by the consumer.

MIT License