Missing include guard for nlohmann::detail::combine?

[kratsg]

Hi, thanks for the great library. We've been using it to compile code used in particle physics and we ran into a weird include error with the latest, I suspect related to #2269 . The #pragma once may not be working, but not entirely sure here (maybe include guards will fix it).

We were previously using v3.7.3 and bumping to master gives errors like

CMakeFiles/EquipConf.dir/PowerSupplyRegistry.cpp.o: In function `nlohmann::detail::combine(unsigned long, unsigned long)':
PowerSupplyRegistry.cpp:(.text+0x120): multiple definition of `nlohmann::detail::combine(unsigned long, unsigned long)'
CMakeFiles/EquipConf.dir/EquipConf.cpp.o:EquipConf.cpp:(.text+0x120): first defined here
CMakeFiles/EquipConf.dir/ComRegistry.cpp.o: In function `nlohmann::detail::combine(unsigned long, unsigned long)':
ComRegistry.cpp:(.text+0x120): multiple definition of `nlohmann::detail::combine(unsigned long, unsigned long)'
CMakeFiles/EquipConf.dir/EquipConf.cpp.o:EquipConf.cpp:(.text+0x120): first defined here
collect2: error: ld returned 1 exit status
make[2]: *** [lib/libEquipConf.so] Error 1
make[1]: *** [src/libEquipConf/CMakeFiles/EquipConf.dir/all] Error 2
make: *** [all] Error 2

---

CMakeLists.txt

cmake_minimum_required (VERSION 3.12)

project (testProject DESCRIPTION "For debugging.")

message("Using internal JSON library")
set(JSON_BuildTests OFF CACHE INTERNAL "")
set(JSON_Install OFF CACHE INTERNAL "")
add_subdirectory(nlohmann_json EXCLUDE_FROM_ALL)
set(JSON_LIBS "nlohmann_json::nlohmann_json")

add_library(Foo SHARED)
target_sources(Foo PRIVATE Foo.cpp Bar.cpp)
target_link_libraries(Foo PUBLIC ${JSON_LIBS})
target_include_directories(Foo PUBLIC ${CMAKE_CURRENT_SOURCE_DIR})

add_library(Bar SHARED)
target_sources(Bar PRIVATE Bar.cpp)
target_link_libraries(Bar PRIVATE Foo)
target_link_libraries(Bar PUBLIC ${JSON_LIBS})
target_include_directories(Bar PUBLIC ${CMAKE_CURRENT_SOURCE_DIR})

Foo.h

#pragma once
#include <nlohmann/json.hpp>

class Foo{};

Foo.cpp

#include "Foo.h"

class Foo;

Bar.h

#include <nlohmann/json.hpp>
#include "Foo.h"

class Bar : public Foo{};

Bar.cpp

#include "Bar.h"

class Bar;

Running cmake3; make produces

[root@460ee802fac4 build]# cmake3 ../
-- The C compiler identification is GNU 4.8.5
-- The CXX compiler identification is GNU 4.8.5
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc - works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ - works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
Using internal JSON library
-- Using the single-header code from /Users/kratsg/labRemote/example/nlohmann_json/single_include/
-- Configuring done
-- Generating done
-- Build files have been written to: /Users/kratsg/labRemote/example/build
[root@460ee802fac4 build]# make
Scanning dependencies of target Foo
[ 20%] Building CXX object CMakeFiles/Foo.dir/Foo.cpp.o
[ 40%] Building CXX object CMakeFiles/Foo.dir/Bar.cpp.o
[ 60%] Linking CXX shared library libFoo.so
CMakeFiles/Foo.dir/Bar.cpp.o: In function `nlohmann::detail::combine(unsigned long, unsigned long)':
Bar.cpp:(.text+0x0): multiple definition of `nlohmann::detail::combine(unsigned long, unsigned long)'
CMakeFiles/Foo.dir/Foo.cpp.o:Foo.cpp:(.text+0x0): first defined here
collect2: error: ld returned 1 exit status
make[2]: *** [libFoo.so] Error 1
make[1]: *** [CMakeFiles/Foo.dir/all] Error 2
make: *** [all] Error 2
[root@460ee802fac4 build]# exit

On latest of develop, but this does not reproduce on master which makes just fine:

[root@949b37647c4f build]# make
Scanning dependencies of target Foo
[ 20%] Building CXX object CMakeFiles/Foo.dir/Foo.cpp.o
[ 40%] Building CXX object CMakeFiles/Foo.dir/Bar.cpp.o
[ 60%] Linking CXX shared library libFoo.so
[ 60%] Built target Foo
Scanning dependencies of target Bar
[ 80%] Building CXX object CMakeFiles/Bar.dir/Bar.cpp.o
[100%] Linking CXX shared library libBar.so
[100%] Built target Bar
[root@949b37647c4f build]# exit

[nlohmann]

Thanks for reporting! Can you provide a small example to reproduce?

[kratsg]

Updated with a MWE. Thanks!

[nlohmann]

Thanks! Can you please check if #2285 fixes the issue?

[kratsg]

Works for me. I'm surprised simply inlining works here, and not also making it static as well. I'm clearly not good enough at C++.

[nlohmann]

#2301 adds a regression test for issue like this one.