clib.nim

import strformat
import strutils
import sequtils

echo "clib initialized"
echo GC_getStatistics()

proc hello_echo*(message:cstring):cstring {.exportc.} =
  ## Echo a message back
  echo GC_getStatistics()
  var s_message:string = $message
  result = &"{s_message} echo"
  echo &"Nim echo result: {result}"

Makefile

NIMBASE = ~/lib/Nim/lib/nimbase.h

.PHONY: run

run: a.out
	./a.out

a.out: test.cpp csrc/clib.cpp csrc/nimbase.h
	g++ -Icsrc test.cpp

csrc/clib.cpp: clib.nim
	nim cpp --checks:on -d:traceGC --memTracker:on -d:memProfiler -d:lineDir -d:leakDetector -d:useSysAssert -d:useGcAssert --header --nimcache:csrc --compileOnly ./clib.nim

csrc/nimbase.h: $(NIMBASE) csrc/clib.cpp
	cp $(NIMBASE) csrc/nimbase.h

test.cpp

#include <iostream>
#include "csrc/clib.h"
using namespace std;

N_CDECL(void, NimMain)(void);

std::string echo(std::string arg) {
    const char* message = arg.c_str();
    cout << "C++ passing message: " << message << "\n";
    cout << "C++ message addr: " << (void*)message << "\n";
    char* retval = hello_echo(
        (char *)(message)
    );
    cout << "C++ retval addr : " << (void*)retval << "\n";
    return std::string(retval);
}

int main(void) {
    NimMain();
    cout << "Hello world\n";
    int i = 0;
    for (i = 0; i < 10; i++) {
        std::string msg = std::string("Hello");
        std::string ret = echo(msg);
        cout << "Got reply: " << ret << "\n";
    }
    return 0;
}