golang：C++類到Go語言對象

《GO語言高級編程》設計中案例，僅作爲筆記進行收藏。CGO是C語⾔和Go語⾔之間的橋樑，原則上⽆法直接⽀持C++的類。CGO不⽀持C++語法的根本原因是C++⾄今爲⽌還沒有⼀個⼆進制接⼝規範(ABI)。⼀個C++類的構造函數在編譯爲⽬標⽂件時如何⽣成鏈接符號名稱、⽅法在不同平臺甚⾄是C++的不同版本之間都是不⼀樣的。但是C++是兼容C語⾔，所以我們可以通過增加⼀組C語⾔函數接⼝作爲C++類和CGO之間的橋樑，這樣就可以間接地實現C++和Go之間的互聯。當然，因爲CGO只⽀持C語⾔中值類型的數據類型，所以我們是⽆法直接使⽤C++的引⽤參數等特性的。

• 準備一個C++類

// my_buffer.h
#include <string>

struct MyBuffer {
	std::string* s_;

	MyBuffer(int size) {
		this->s_ = new std::string(size, char('\0'));
	}
	~MyBuffer() {
		delete this->s_;
	}

	int Size() const {
		return this->s_->size();
	}
	char* Data() {
		return (char*)this->s_->data();
	}
};

• 用純C函數接口封裝C++類

// my_buffer_capi.h
typedef struct MyBuffer_T MyBuffer_T;

MyBuffer_T* NewMyBuffer(int size);
void DeleteMyBuffer(MyBuffer_T* p);

char* MyBuffer_Data(MyBuffer_T* p);
int MyBuffer_Size(MyBuffer_T* p);

// my_buffer_capi.cc
#include "./my_buffer.h"

extern "C" {
	#include "./my_buffer_capi.h"
}

struct MyBuffer_T: MyBuffer {
	MyBuffer_T(int size): MyBuffer(size) {}
	~MyBuffer_T() {}
};

MyBuffer_T* NewMyBuffer(int size) {
	auto p = new MyBuffer_T(size);
	return p;
}
void DeleteMyBuffer(MyBuffer_T* p) {
	delete p;
}

char* MyBuffer_Data(MyBuffer_T* p) {
	return p->Data();
}
int MyBuffer_Size(MyBuffer_T* p) {
	return p->Size();
}

• 將純C接口函數轉爲Go函數

// my_buffer_capi.go
package main

/*
#cgo CXXFLAGS: -std=c++11

#include "my_buffer_capi.h"
*/
import "C"

type cgo_MyBuffer_T C.MyBuffer_T

func cgo_NewMyBuffer(size int) *cgo_MyBuffer_T {
	p := C.NewMyBuffer(C.int(size))
	return (*cgo_MyBuffer_T)(p)
}

func cgo_DeleteMyBuffer(p *cgo_MyBuffer_T) {
	C.DeleteMyBuffer((*C.MyBuffer_T)(p))
}

func cgo_MyBuffer_Data(p *cgo_MyBuffer_T) *C.char {
	return C.MyBuffer_Data((*C.MyBuffer_T)(p))
}

func cgo_MyBuffer_Size(p *cgo_MyBuffer_T) C.int {
	return C.MyBuffer_Size((*C.MyBuffer_T)(p))
}

• 包裝爲Go對象

// my_buffer.go
package main

import "unsafe"

type MyBuffer struct {
	cptr *cgo_MyBuffer_T
}

func NewMyBuffer(size int) *MyBuffer {
	return &MyBuffer{
		cptr: cgo_NewMyBuffer(size),
	}
}

func (p *MyBuffer) Delete() {
	cgo_DeleteMyBuffer(p.cptr)
}

func (p *MyBuffer) Data() []byte {
	data := cgo_MyBuffer_Data(p.cptr)
	size := cgo_MyBuffer_Size(p.cptr)
	return ((*[1 << 31]byte)(unsafe.Pointer(data)))[0:int(size):int(size)]
}

• main

// main
package main

//#include <stdio.h>
import "C"
import "unsafe"

func main() {
	buf := NewMyBuffer(1024)
	defer buf.Delete()

	copy(buf.Data(), []byte("hello\x00"))
	C.puts((*C.char)(unsafe.Pointer(&(buf.Data()[0]))))
}