new_item["Voltage"] = 8.622;
new_item["Current"] = 8.63456;
new_item["Energy"] = 8.234234;

std::string valueToString(double value, bool useSpecialFloats, unsigned int precision) {
  // Allocate a buffer that is more than large enough to store the 16 digits of
  // precision requested below.
  char buffer[32];
  int len = -1;

  char formatString[6];
  sprintf(formatString, "%%.%dg", precision);

  // Print into the buffer. We need not request the alternative representation
  // that always has a decimal point because JSON doesn't distingish the
  // concepts of reals and integers.
  if (isfinite(value)) {
    len = snprintf(buffer, sizeof(buffer), formatString, value);
  } else {
    // IEEE standard states that NaN values will not compare to themselves
    if (value != value) {
      len = snprintf(buffer, sizeof(buffer), useSpecialFloats ? "NaN" : "null");
    } else if (value < 0) {
      len = snprintf(buffer, sizeof(buffer), useSpecialFloats ? "-Infinity" : "-1e+9999");
    } else {
      len = snprintf(buffer, sizeof(buffer), useSpecialFloats ? "Infinity" : "1e+9999");
    }
    // For those, we do not need to call fixNumLoc, but it is fast.
  }
  assert(len >= 0);
  fixNumericLocale(buffer, buffer + len);
  return buffer;
}

"avg_abcdd              " : 1.1632640000000014,
"avg_pxczzczxczxd       " : 7.0999999999999996,
"avg_shczxcdize         " : 802000.0,
"deviccxz               " : "shebei25",
"sh323423fd             " : 1420,
"vcxzcasdasdadczco      " : 231

Json::Value res;?
?
std::string = entry_str;?
?
entry_str.append("zhangsan,123");?
entry_str.append("abc,2596");?
.......?
?
res["entry"] = entry_str;