Using Go in C++ - c++

Question

linux Debian Buster
go version go1.11.6 linux/amd64
gcc version 8.3.0 (Debian 8.3.0-6)
libmylib.go
package main
import "C"
import (
"fmt"
)
func say(text string) {
fmt.Println(text)
}
func main(){}
mylib.h
#ifndef MY_LIB_H
#define MY_LIB_H
#include <string>
void say(std::string text);
#endif
main.cpp
#include <string>
#include "mylib.h"
using namespace std;
int main() {
string text = "Hello, world!";
say(text);
return 0;
}
CGO_ENABLED=1 go build -o libmylib.so -buildmode=c-shared libmylib.go
g++ -L/path/to/lib/ -lmylib main.cpp -o my-test-program
/usr/bin/ld: /tmp/ccu4fXFB.o: in function 'main':
main.cpp:(.text+0x53): undefined reference to `say(std::__cxx11::basic_string<char, std::char_traits, std::allocator >)' collect2: error: ld returned 1 exit status
with change: package main -> package mylib
CGO_ENABLED=1 go build -o libmylib.so -buildmode=c-shared libmylib.go
-buildmode=c-shared requires exactly one main package

Answer 1

You have to use GoString rather than std::string in the C++ version. The error message you are getting is because of the type mismatch, manifesting as a link-time error.
See the cgo reference.
Here's a working example. There's a few differences from yours. The //export directive is needed to include the function in the generated header file, the argument is *C.char rather than string or GoString. The C++ code uses the header generated by cgo, and there has to be a const-removing cast from the static string (because go doesn't have C-like const).
libmylib.go
package main
import "C"
import (
"fmt"
)
//export say
func say(text *C.char) {
fmt.Println(C.GoString(text))
}
func main() {}
main.cpp
#include "libmylib.h"
int main(void) {
say(const_cast<char*>("hello world"));
return 0;
}
commands
This compiles to go file, generating libmylib.so and libmylib.h in the current directory.
go build -o libmylib.so -buildmode=c-shared libmylib.go
The compiles the C++ program, linking it to the shared library above:
g++ -L. main.cpp -lmylib -o hello_program
To run the program, LD_LIBRARY_PATH needs to be set to the current directory. That would be different if program was installed and the shared library put in a sensible place.
LD_LIBRARY_PATH=. ./hello_program

Answer 2

g++ -L/path/to/lib/ -lmylib main.cpp -o test
is probably wrong. Read the invoking GCC chapter of the GCC documentation. Order of arguments to g++ matters a lot.
Also, test(1) could be some existing executable. I recommend to use some other name.
So consider compiling with
g++ -Wall -g -O main.cpp -L/path/to/lib/ -lmylib -o my-test-program
You probably want debugging information (-g), warnings (-Wall) and some optimization (-O)
You did comment
I need to use some functions from a Go file in my C ++ project.
This is curious. I assume your operating system is some Linux. Then, can't you just use inter-process communication facilities between a process running a Go program and another process running your C++ program? Consider perhaps using JSONRPC or HTTP between them. There exist several open source libraries in Go and in C++ to help you.
PS. As I commented, calling C++ code from a Go program could be much simpler. Of course you do need to read the Go documentation and the blog about cgo and probably the C++ dlopen minihowto and some C++ reference.