Go / C++ bindings example

README.md

Go / C++ bindings example

This is an example of Go code calling to a C++ library with a C wrapper.

Build

go build  # this only ensures it compiles

Test

go test  # actually runs tests

num.cpp

// num.cpp
#include "nummer.hpp"
#include "num.h"

Num NumInit() {
    cxxNum * ret = new cxxNum(1);
    return (void*)ret;
}

void NumFree(Num n) {
    cxxNum * num = (cxxNum*)n;
    delete num;
}

void NumIncrement(Num n) {
    cxxNum * num = (cxxNum*)n;
    num->Increment();
}

int NumGetValue(Num n) {
    cxxNum * num = (cxxNum*)n;
    return num->GetValue();
}

num.go

package num

// #cgo LDFLAGS: -L. -lstdc++
// #cgo CXXFLAGS: -std=c++14 -I.
// #include "num.h"
import "C"
import "unsafe"

type GoNum struct {
	num C.Num
}

func New() GoNum {
	var ret GoNum
	ret.num = C.NumInit()
	return ret
}
func (n GoNum) Free() {
	C.NumFree((C.Num)(unsafe.Pointer(n.num)))
}
func (n GoNum) Inc() {
	C.NumIncrement((C.Num)(unsafe.Pointer(n.num)))
}
func (n GoNum) GetValue() int {
	return int(C.NumGetValue((C.Num)(unsafe.Pointer(n.num))))
}

num.h

// num.h
#ifdef __cplusplus
extern "C" {
#endif

typedef void* Num;
Num NumInit(void);
void NumFree(Num);
void NumIncrement(Num);
int NumGetValue(Num);

#ifdef __cplusplus
}
#endif

num_test.go

package num

import (
	"reflect"
	"testing"
)

func TestNum(t *testing.T) {
	num := New()
	num.Inc()
	if num.GetValue() != 2 {
		t.Error("unexpected value received")
	}
	num.Inc()
	num.Inc()
	num.Inc()
	if num.GetValue() != 5 {
		t.Error("unexpected value received")
	}
	value := num.GetValue()
	num.Free()

	typ := reflect.TypeOf(value)
	if typ.Name() != "int" {
		t.Error("got unexpected type")
	}
}

nummer.cpp

#include <iostream>
#include "nummer.hpp"

void cxxNum::Increment(void) {
    this->value++;
}

int cxxNum::GetValue(void) {
    return this->value;
}

nummer.hpp

class cxxNum {
private:
    int value;

public:
    cxxNum(int _num):value(_num){};
    ~cxxNum(){};
    void Increment(void);
    int GetValue(void);
};

How can I make it work if I put the nummer.hpp and nummer.cpp file to a sub folder?

├── go.mod
└── num
    ├── num.cpp
    ├── num.go
    ├── num.h
    ├── nummer
    │   ├── nummer.cpp
    │   └── nummer.hpp
    └── num_test.go

The num.cpp is like:

#include "nummer/nummer.hpp"
#include "num.h"

// ......

I get some error:

/usr/lib/go-1.20/pkg/tool/linux_amd64/link: running g++ failed: exit status 1
/usr/bin/ld: /tmp/go-link-1235907040/000002.o: in function `NumIncrement':
/Go/cpp/num/num.cpp:16: undefined reference to `cxxNum::Increment()'
/usr/bin/ld: /tmp/go-link-1235907040/000002.o: in function `NumGetValue':
/Go/cpp/num/num.cpp:21: undefined reference to `cxxNum::GetValue()'
collect2: error: ld returned 1 exit status