JsonCpp中double的问题解决

json文件里的值

程序代码

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

程序运行结果

Jsoncpp的json_write.cpp中

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;
}

这里sprintf(formatString, “%%.%dg”, precision);的结果是“%.17g”。是输出17位的有效数字。不足的补足17位。

ps：JsonCpp的小数精度问题和插入输出顺序问题

直接说吧，这两个问题无法解决，如下：

官方不支持指定小数位数，double默认位宽为17位，如："value" : 7.0999999999999996,
官方不支持按插入顺序输出，而是按照key的字母排序输出的,不管你什么顺序插入，下面的都是这样的顺序输出的：

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

个人应急想法 

数字精度问题，可以考虑在C++中转为自己需求的精度，然后再当作字符串放到json中，至于之后的解析，读字符串再转数字即可；

顺序问题，两个想法：

1)不要用key，采用append的形式，也就是将每个条目放在一个容器中

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

2）那就按名字命令咯，顺应规则，2333333