Duplicate symbol definition linker error

[simone-gaiarin]

When I include <nlohmann/json.hpp> in two different header files I get the multiple linker errors regarding duplicate symbol definition (I just report one of them):

[build] A.cpp.obj : warning LNK4006: "void __cdecl nlohmann::detail::dtoa_impl::grisu2_round(char *,int,unsigned __int64,unsigned __int64,unsigned __int64,unsigned __int64)" (?grisu2_round@dtoa_impl@detail@nlohmann@@YAXPADH_K111@Z) already defined in B.cpp.obj; second definition ignored

[build] lld-link: error: duplicate symbol: void __cdecl nlohmann::detail::dtoa_impl::grisu2_round(char *, int, unsigned __int64, unsigned __int64, unsigned __int64, unsigned __int64)

I defined a file called "json.hpp" with the following content:

#pragma once

#include <nlohmann/json.hpp>

using json = nlohmann::json;

then I import it as follow:

A.hpp

#pragma once

#include "json.hpp"

B.hpp

#pragma once

#include "json.hpp"
#include "A.hpp"

Relevant part of CMakeLists.txt

set(JSON_MultipleHeaders ON)

include(FetchContent)

FetchContent_Declare(json
  GIT_REPOSITORY https://github.com/nlohmann/json
  GIT_TAG v3.7.3
)

FetchContent_GetProperties(json)
if(NOT json_POPULATED)
  FetchContent_Populate(json)
  add_subdirectory(${json_SOURCE_DIR} ${json_BINARY_DIR} EXCLUDE_FROM_ALL)
endif()

add_library(libsipnativemessaginghost STATIC 
	"A.cpp"
	"B.cpp"
)
target_link_libraries(myproject PUBLIC nlohmann_json::nlohmann_json)

Any idea what can cause the issue or what can I do to debug it further?

I tried to use json_fwd.hpp but given that I have a lines like this in both header, importing json_fwd.hpp is not enough.

void method(const json &param1= json::value_t::object) const;

I tried to replicate this isssue in a dummy project, but there everything works as expected, even if I include <nlohmann/json.hpp> multiple times in the headers.

[nlohmann]

I am working in the exact setup at work and don’t have such an error. 🤔

[simone-gaiarin]

Is it there a non-header only version of the library that I can use to avoid including it in the headers?

[nlohmann]

No, it’s header only. But I never heard of such problems.

[simone-gaiarin]

Found the source of the problem in an third party library

/* Define 'inline' to nothing, since the MSVC compiler doesn't support it.  */
#define inline

It was completely disrupting the header-only library mechanism.

[nlohmann]

Wow, that’s subtle! Thanks for sharing!

[gregmarr]

I was debating whether or not I was crazy to ask if there was something doing something insane like defining away inline, since that's the only way I could see to get that error. I guess I wasn't so crazy after all.