Using boost::json static library with cmake

Question

I have this CMakeLists.txt

set(Boost_USE_STATIC_LIBS   ON)
find_package(Boost REQUIRED COMPONENTS json)
include_directories(${BOOST_INCLUDE_DIRS})
target_link_libraries(<Target> PRIVATE Boost::json)

But I get cmake warnings

[cmake] CMake Warning at C:/Program Files/CMake/share/cmake3.20/Modules/FindBoost.cmake:2185 (message):
[cmake]   No header defined for json; skipping header check (note: header-only
[cmake]   libraries have no designated component)

The project can configure and build tho. ~~But the compile_command.json I found didn't actually link to the static library, which clearly indicate it is not found and was compiled as header only library.~~

EDIT: I found it actually links to libboost_json-mt.a in the generated build.ninja file, so it is weird the fact that cmake does not find the static library in the find_package call.

build MyTest.exe MyTest[1]_tests.cmake: CXX_EXECUTABLE_LINKER__MyTest_Debug CMakeFiles/MyTest.dir/test.cpp.obj CMakeFiles/MyTest.dir/BasicStructures/Uri.cpp.obj | C$:/msys64/mingw64/lib/libgtest_main.dll.a C$:/msys64/mingw64/lib/libboost_json-mt.a C$:/msys64/mingw64/lib/libgtest.dll.a C$:/msys64/mingw64/lib/libboost_container-mt.a
  FLAGS = -g
  LINK_LIBRARIES = C:/msys64/mingw64/lib/libgtest_main.dll.a  C:/msys64/mingw64/lib/libboost_json-mt.a  C:/msys64/mingw64/lib/libgtest.dll.a  C:/msys64/mingw64/lib/libboost_container-mt.a  -lkernel32 -luser32 -lgdi32 -lwinspool -lshell32 -lole32 -loleaut32 -luuid -lcomdlg32 -ladvapi32
  OBJECT_DIR = CMakeFiles\MyTest.dir
  POST_BUILD = cmd.exe /C "cd /D C:\Users\Peter\Desktop\LspCpp\build && "C:\Program Files\CMake\bin\cmake.exe" -D TEST_TARGET=MyTest -D TEST_EXECUTABLE=C:/Users/Peter/Desktop/LspCpp/build/MyTest.exe -D TEST_EXECUTOR= -D TEST_WORKING_DIR=C:/Users/Peter/Desktop/LspCpp/build -D TEST_EXTRA_ARGS= -D TEST_PROPERTIES= -D TEST_PREFIX= -D TEST_SUFFIX= -D NO_PRETTY_TYPES=FALSE -D NO_PRETTY_VALUES=FALSE -D TEST_LIST=MyTest_TESTS -D CTEST_FILE=C:/Users/Peter/Desktop/LspCpp/build/MyTest[1]_tests.cmake -D TEST_DISCOVERY_TIMEOUT=5 -D TEST_XML_OUTPUT_DIR= -P "C:/Program Files/CMake/share/cmake-3.20/Modules/GoogleTestAddTests.cmake""
  PRE_LINK = cd .
  RESTAT = 1
  TARGET_FILE = MyTest.exe
  TARGET_IMPLIB = libMyTest.dll.a
  TARGET_PDB = MyTest.exe.dbg

Answer 1

Instead of the static link option, I think you're supposed to include

 #include <boost/json/src.hpp>

in one of your translation units. This makes Boost JSON effectively header-only.

Relevant documentation: https://github.com/boostorg/json#header-only

Next up, you can use this method to create your own static library. Keep in mind to optionally define visibility macros if your toolchain requires them (MSVC):

https://github.com/boostorg/json#standalone-shared-library

Answer 2

The latest commit to the findboost package at https://gitlab.kitware.com/cmake/cmake/-/commits/master/Modules/FindBoost.cmake dated June 4 describes itself thusly:

FindBoost: Add check for json component header in Boost 1.75+

The previous commit added support for boost 1_76. As of this writing the latest is 1_77_0... so if you use 1_77 you'll get a bunch (one per component?) of warning messages about unsupported versions. Unless you use 1_76 or 1_75 I suppose.

So the support you want is available now, but was not when you wrote this question. I don't know if the currently available version of cmake (3.21.2) includes that update or not. The docs certainly don't mention it... but incomplete docs are hardly a new thing under the sun.

So the support we need is out there now/finally, and according to the gitlab site, that change was included in cmake 3.21.2. Dude! Sweet!

So all we need is the latest cmake and we should be good to go.