Hello,
I'm rather new to Nim and I'm currently experimenting with ways to interact with C++. I really like the language and the ease of interacting with C++, but I noticed some weirdness taking place when using imported C++ template types as return types of non-imported procs. As a toy model I used the following code:
type
Map {.importcpp: "std::map", header: "<map>".} [T,U] = object
proc cInitMap(T: typedesc, U: typedesc): Map[T,U] {.importcpp: "std::map<'*1,'*2>()", nodecl.}
# This one alone works.
proc initMap[T, U](): Map[T, U] =
cInitMap(T, U)
# This one causes an error in N_NIMCALL
var x: Map[cstring, cint] = initMap[cstring, cint]()
This code manages to get up to the step of generating C++ code, but does not compile with the following error in g++:
c:\nim\nimcache\cppgentest.cpp:11:50: error: macro "N_NIMCALL" passed 3 arguments, but takes just 2
N_NIMCALL(std::map<NCSTRING, int> , initmap_94036)(void);
^
c:\nim\nimcache\cppgentest.cpp:51:50: error: macro "N_NIMCALL" passed 3 arguments, but takes just 2
N_NIMCALL(std::map<NCSTRING, int> , initmap_94036)(void) {
^
c:\nim\nimcache\cppgentest.cpp:11:57: error: expected constructor, destructor, or type conversion before ';' token
N_NIMCALL(std::map<NCSTRING, int> , initmap_94036)(void);
^
c:\nim\nimcache\cppgentest.cpp: In function 'int N_NIMCALL()':
c:\nim\nimcache\cppgentest.cpp:57:9: error: cannot convert 'std::map<char*, int>' to 'int' in return
return result;
^
c:\nim\nimcache\cppgentest.cpp: In function 'void cppgentestInit000()':
c:\nim\nimcache\cppgentest.cpp:102:26: error: 'initmap_94036' was not declared in this scope
x_94058 = initmap_94036();
^
In the generated cpp file, the offending line is the following:
N_NIMCALL(std::map<NCSTRING, int> , initmap_93036)(void);
What seems to be happening is that the preprocessor macro N_NIMCALL defined in nimbase.h cannot cope with a rettype which is a C++ template with more than one argument, as the arguments of the template are separated by commas. This then leads to the macro seeing three arguments
std::map<char*,
int>,
initmap_94036
Just using cInitMap works fine, as it doesn't use a calling convention macro such as N_NIMCALL.
Now, I'd like to ask if I was doing anything obviously wrong or unsupported in the above, or if this behaviour is indeed unintended / a bug (in which case I would file an issue on GitHub). Are there any obvious workarounds for this in Nim?
Though I'm fairly new to Nim, if this turns out to be a bug I would very much like to help, if I can.
You can try to add
#define COMMA ,
and replace the offending comma(s) with the preprocessor macro. If this works, the nim compiler should probably emit code with this protection.