c++ gbk utf8 converting

```c++
#include <iconv.h> //for gbk/big5/utf8
int code_convert(char *from_charset,char *to_charset,char *inbuf,size_t inlen,char *outbuf,size_t outlen)
{
	iconv_t cd;
	int rc;
	char **pin = &inbuf;
	char **pout = &outbuf;
	cd = iconv_open(to_charset,from_charset);
	if (cd==0)
		return -1;
	memset(outbuf,0,outlen);
	if (iconv(cd,pin,&inlen,pout,&outlen) == -1)
		return -1;
	iconv_close(cd);
	return 0;
}
std::string any2utf8(std::string in,std::string fromEncode,std::string toEncode)
{
	char* inbuf=(char*) in.c_str();
	int inlen=strlen(inbuf);
	int outlen=inlen*3;//in case unicode 3 times than ascii
	char outbuf[outlen]={0};
	int rst=code_convert((char*)fromEncode.c_str(),(char*)toEncode.c_str(),inbuf,inlen,outbuf,outlen);
	if(rst==0){
		return std::string(outbuf);
	}else{
		return in;
	}
}
std::string gbk2utf8(const char* in)
{
	return any2utf8(std::string(in),std::string("gbk"),std::string("utf-8"));
}
```