std::string 字符串编码转换

可以使用iconv库将一个字符集编码转换成另一个字符集编码，其中iconv_open()函数用于初始化一个字符集编码转换描述符，iconv()函数用于进行实际的字符集编码转换，iconv_close()函数用于关闭字符集编码转换描述符。下面是一个示例代码：

#include <iconv.h>
#include <string.h>

int convert_encoding(const char* from_charset, const char* to_charset,
                     const char* inbuf, size_t inlen, char* outbuf, size_t outlen) {
  iconv_t cd = iconv_open(to_charset, from_charset);
  if (cd == (iconv_t)-1) {
    return -1;
  }

  char* inbuf_ptr = const_cast<char*>(inbuf);
  char* outbuf_ptr = outbuf;
  size_t outlen_left = outlen;
  size_t rc = iconv(cd, &inbuf_ptr, &inlen, &outbuf_ptr, &outlen_left);
  iconv_close(cd);

  if (rc == (size_t)-1) {
    return -1;
  }
  return outlen - outlen_left;
}

int main() {
  const char* utf8_str = "你好世界";
  char utf16_str[32];
  int rc = convert_encoding("UTF-8", "UTF-16", utf8_str, strlen(utf8_str), utf16_str, sizeof(utf16_str));
  if (rc == -1) {
    printf("convert failed\n");
    return -1;
  }
  utf16_str[rc] = '\0';
  printf("%d %s\n", rc, utf16_str);
  return 0;
}