golang：C++类到Go语言对象

《GO语言高级编程》设计中案例，仅作为笔记进行收藏。CGO是C语⾔和Go语⾔之间的桥梁，原则上⽆法直接⽀持C++的类。CGO不⽀持C++语法的根本原因是C++⾄今为⽌还没有⼀个⼆进制接⼝规范(ABI)。⼀个C++类的构造函数在编译为⽬标⽂件时如何⽣成链接符号名称、⽅法在不同平台甚⾄是C++的不同版本之间都是不⼀样的。但是C++是兼容C语⾔，所以我们可以通过增加⼀组C语⾔函数接⼝作为C++类和CGO之间的桥梁，这样就可以间接地实现C++和Go之间的互联。当然，因为CGO只⽀持C语⾔中值类型的数据类型，所以我们是⽆法直接使⽤C++的引⽤参数等特性的。

• 准备一个C++类

// 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();
	}
};

• 用纯C函数接口封装C++类

// 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();
}

• 将纯C接口函数转为Go函数

// 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))
}

• 包装为Go对象

// 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]))))
}