Defining include path for mingw compiler

I'm trying to use Mingw compiler on Linux OS for cross-compiling to Windows binaries.

In the very simple project I use one include:

#include <nlohmann/json.hpp>

The json.hpp is located in /usr/include/nlohmann.
When I compile the project via g++, the compiler looks for its default include paths and is able to find the header file without problems.

However, when I compile the project via mingw, it uses different include paths, so I have to specify the include path explicitly. I use this command:

x86_64-w64-mingw32-g++ -I/usr/include/nlohmann --static main.cpp 

But I get this error:

fatal error: nlohmann/json.hpp: No such file or directory
    2 | #include <nlohmann/json.hpp>

Can anybody help me please?
NOTE: the json.hpp file depends on other files from nlohmann folder, so including only the json.hpp file will not work.

MinGW not searching /usr/include is actually a good thing. Libraries in there might not work with MinGW.

Now, nlohmann/json itself, being header-only and without any platform-specific generated headers, might work as is. But trying to add <s>-I/usr/include</s> to flags (which would let the compiler find the file) would cause huge problems, as all the other headers in there might get used as well.

Instead, what you should do is to clone nlohmann/json yourself, and add the include/ directory in the repo to the search path (with -I).

Also there's quasi-msys2, which has a package manager for MinGW libraries. Though for header-only libraries that don't need to be compiled, the benefit of using it is rather small (not having to manually clone the repo and set the include path). (I'm the author.)

If the file is located at /usr/include/nlohmann/json.hpp and the include line is

#include <nlohmann/json.hpp>

what you need is -I/usr/include.

Then it will find nlohmann/json.hpp under /usr/include.

Note that it's only okay to do this if this is a header-only library that is not different for the specific target platform you are cross-compiling to.

In general you need to have dependency libraries compiled for the same target platform and link with them. That's also the reason the cross compiler won't look at the stuff in /usr/include.