前言

JsonCpp遇见中文的时候会将中文转义为\uXXXX类型，那么本文给出解决方法。

JsonCpp的处理方法

通过阅读代码，得知中文处理位于文件json_writer.cpp中valueToQuotedStringN函数。函数为如下代码

static String valueToQuotedStringN(const char* value, size_t length,
                                   bool emitUTF8 = false) {
  if (value == nullptr)
    return "";

  if (!doesAnyCharRequireEscaping(value, length))
    return String("\"") + value + "\"";
  // We have to walk value and escape any special characters.
  // Appending to String is not efficient, but this should be rare.
  // (Note: forward slashes are *not* rare, but I am not escaping them.)
  String::size_type maxsize = length * 2 + 3; // allescaped+quotes+NULL
  String result;
  result.reserve(maxsize); // to avoid lots of mallocs
  result += "\"";
  char const* end = value + length;
  for (const char* c = value; c != end; ++c) {
    switch (*c) {
    case '\"':
      result += "\\\"";
      break;
    case '\\':
      result += "\\\\";
      break;
    case '\b':
      result += "\\b";
      break;
    case '\f':
      result += "\\f";
      break;
    case '\n':
      result += "\\n";
      break;
    case '\r':
      result += "\\r";
      break;
    case '\t':
      result += "\\t";
      break;
    // case '/':
    // Even though \/ is considered a legal escape in JSON, a bare
    // slash is also legal, so I see no reason to escape it.
    // (I hope I am not misunderstanding something.)
    // blep notes: actually escaping \/ may be useful in javascript to avoid </
    // sequence.
    // Should add a flag to allow this compatibility mode and prevent this
    // sequence from occurring.
    default: {
      if (emitUTF8) {
        unsigned codepoint = static_cast<unsigned char>(*c);
        if (codepoint < 0x20) {
          appendHex(result, codepoint);
        } else {
          appendRaw(result, codepoint);
        }
      } else {
        unsigned codepoint = utf8ToCodepoint(c, end); // modifies `c`
        if (codepoint < 0x20) {
          appendHex(result, codepoint);
        } else if (codepoint < 0x80) {
          appendRaw(result, codepoint);
        } else if (codepoint < 0x10000) {
          // Basic Multilingual Plane
          appendHex(result, codepoint);
        } else {
          // Extended Unicode. Encode 20 bits as a surrogate pair.
          codepoint -= 0x10000;
          appendHex(result, 0xd800 + ((codepoint >> 10) & 0x3ff));
          appendHex(result, 0xdc00 + (codepoint & 0x3ff));
        }
      }
    } break;
    }
  }
  result += "\"";
  return result;
}

通过阅读上述代码，我们得知当switch分支走到default的时候就是处理中文的时候，那么只需要将emitUTF8设置为true即可。
而JsonCpp的emitUTF8的属性值默认为false。

所以我们在构建StreamWriterBuilder的时候对emitUTF8设置为true就可以解决中文变成\uXXXX类型了。如下代码所示：

    Json::StreamWriterBuilder streamWriterBuilder;
    streamWriterBuilder["emitUTF8"] = true;
    auto strRes = Json::writeString(streamWriterBuilder, root);

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