文章目录
一、介绍
1.1 json 介绍
二、C/C++ json 库选型
2.1 选型范围
- 资料
- 开源库
- Rapidjson、Rapidjson_FullPrec、Rapidjson_AutoUTF
- nlohmann / json
- jsoncpp
- 结论:
- 注重最佳性能,选 Rapidjson (cereal序列化库使用)
- 注重易用性,选 jsoncpp (ros 使用)、nlohmann / json
2.2 jsoncpp
- 精度控制:precision
- 15,16,17:原值会变
- 0-14:原值不变的情况下,四舍五入
2.2.2 jsoncpp 编译和交叉编译
编译
mkdir build; cd build cmake -DCMAKE_BUILD_TYPE=Release \ -DBUILD_SHARED_LIBS=ON \ -DCMAKE_INSTALL_PREFIX=`pwd`/result \ -DJSONCPP_WITH_TESTS=OFF \ .. make install -j4
交叉编译
mkdir build; cd build cmake -DCMAKE_BUILD_TYPE=Release \ -DBUILD_SHARED_LIBS=ON \ -DCMAKE_INSTALL_PREFIX=`pwd`/result \ -DJSONCPP_WITH_TESTS=OFF \ .. -DCMAKE_TOOLCHAIN_FILE=toolchain.cmake \ .. make install -j4 # toolchain.cmake 定义交叉编译环境变量 SET(CMAKE_SYSROOT "/opt/fslc-x11/2.4.4/sysroots/armv7at2hf-neon-fslc-linux-gnueabi") set(CMAKE_SYSTEM_NAME Linux) set(CMAKE_SYSTEM_PROCESSOR arm) set(TOOLS /opt/fslc-x11/2.4.4/sysroots/x86_64-fslcsdk-linux/usr/bin/arm-fslc-linux-gnueabi) set(CMAKE_C_COMPILER "${TOOLS}/arm-fslc-linux-gnueabi-gcc") set(CMAKE_CXX_COMPILER "${TOOLS}/arm-fslc-linux-gnueabi-g++") set(CMAKE_AR "${TOOLS}/arm-fslc-linux-gnueabi-ar")
include/json
lib/cmake/xxx.cmake
lib/pkgconfig/jsoncpp.pc
pkgconfig
mayue@PC-MAYUE:/mnt/d/hik/opensource/jsoncpp-1.9.5/build/result/lib$ cat pkgconfig/jsoncpp.pc prefix=/mnt/d/hik/opensource/jsoncpp-1.9.5/build/result exec_prefix=/mnt/d/hik/opensource/jsoncpp-1.9.5/build/result libdir=${exec_prefix}/lib includedir=${prefix}/include Name: jsoncpp Description: A C++ library for interacting with JSON Version: 1.9.5 URL: https://github.com/open-source-parsers/jsoncpp Libs: -L${libdir} -ljsoncpp Cflags: -I${includedir}
指定连接静态库
g++ jsoncpp-test.cpp -I./include -L ./lib -l:libjsoncpp.a
2.3 rapidjson
2.4 nlohmann/json
2.5 sonic-cpp
五、常见问题
5.1 jsoncpp 中关于浮点数的控制和中文显示问题
5.2 jsoncpp序列化double类型时精度损失问题的解决办法
解决办法1:此法不需要改源码,使用StreamWriterBuilder进行序列化
#include <json/json.h>
#include <json/writer.h>
#include <iostream>
#include <string>
void test_precision(int precision)
{
Json::Value root;
root["pi"] = 3.1415926;
root["count"] = 43.32558674566;
Json::StreamWriterBuilder builder;
//设置精度 注意这个默认设置的是数字总长度
//如果想设置小数点后的位数需设置precisionType为decimal
builder.settings_["precision"] = precision;
//设置精度类型 只可设置2种字符串 significant精度位数为数字总长度(jsoncpp默认为此类型) decimal精度位数为小数点后的长度
builder.settings_["precisionType"] = "decimal";
// 设置输出为紧凑格式,不带换行和空格
builder["commentStyle"] = "None"; // 防止输出注释,默认就是none
builder["indentation"] = ""; // 空字符串表示不缩进
std::unique_ptr<Json::StreamWriter> writer(builder.newStreamWriter());
std::ostringstream oss;
writer->write(root, &oss);
std::string jsonText = oss.str();
// 输出 JSON 字符串
std::cout << "specified precision: " << precision << ", content:" << jsonText << std::endl;
}
int main() {
for(int i=17; i>=0; i--)
{
test_precision(i);
}
return 0;
}
specified precision: 17, content:{"count":43.32558674565999723,"pi":3.14159260000000007}
specified precision: 16, content:{"count":43.3255867456599972,"pi":3.1415926000000001}
specified precision: 15, content:{"count":43.325586745659997,"pi":3.1415926}
specified precision: 14, content:{"count":43.32558674566,"pi":3.1415926}
specified precision: 13, content:{"count":43.32558674566,"pi":3.1415926}
specified precision: 12, content:{"count":43.32558674566,"pi":3.1415926}
specified precision: 11, content:{"count":43.32558674566,"pi":3.1415926}
specified precision: 10, content:{"count":43.3255867457,"pi":3.1415926}
specified precision: 9, content:{"count":43.325586746,"pi":3.1415926}
specified precision: 8, content:{"count":43.32558675,"pi":3.1415926}
specified precision: 7, content:{"count":43.3255867,"pi":3.1415926}
specified precision: 6, content:{"count":43.325587,"pi":3.141593}
specified precision: 5, content:{"count":43.32559,"pi":3.14159}
specified precision: 4, content:{"count":43.3256,"pi":3.1416}
specified precision: 3, content:{"count":43.326,"pi":3.142}
specified precision: 2, content:{"count":43.33,"pi":3.14}
specified precision: 1, content:{"count":43.3,"pi":3.1}
specified precision: 0, content:{"count":43,"pi":3}
精度控制:precision
- 15,16,17:原值会变、四舍五入
- 0-14:原值不变的情况下,四舍五入
特别注意:精度设置一定要大于你需求的精度位数,比如需要三位可以设置4位或5位,因为最后一位可能会不准(做了四舍五入)
不足之处:
StreamWriterBuilder序列化的字符串是可读形式的,就像上面的输出,是有换行和缩进的(转换效率会比FastWrite低),我的服务端代码里其实不需要转换json为可读的,更需要的是效率,所以还有下面一种方法改FasetWrite源码
解决办法2:此法需要改源码,使用FastWriter进行序列化
注意:需升级jsoncpp到最新版本1.9.5版本
修改源码(writer.h):FastWriter类新增2个成员变量(precision_和precisionType_)和成员函数(set_precision和set_precisionType)
#if defined(_MSC_VER) #pragma warning(push) #pragma warning(disable : 4996) // Deriving from deprecated class #endif class JSON_API FastWriter : public Writer { public: FastWriter(); ~FastWriter() override = default; void enableYAMLCompatibility(); /** \brief Drop the "null" string from the writer's output for nullValues. * Strictly speaking, this is not valid JSON. But when the output is being * fed to a browser's JavaScript, it makes for smaller output and the * browser can handle the output just fine. */ void dropNullPlaceholders(); void omitEndingLineFeed(); public: // overridden from Writer String write(const Value& root) override; //设置精度位数 void set_precision(unsigned int precision) { precision_ = (precision > 17)?17:precision; }; //设置精度类型 默认为数字总长 //入参:isDecimal true表示类型为小数点后长度 false表示类型为数字总长 void set_precisionType(bool isDecimal) { isDecimal ? (precisionType_ = PrecisionType::decimalPlaces) : (precisionType_ = PrecisionType::significantDigits); }; private: void writeValue(const Value& value); String document_; bool yamlCompatibilityEnabled_{false}; bool dropNullPlaceholders_{false}; bool omitEndingLineFeed_{false}; int precision_{ 17 };//精度位数 默认17位 PrecisionType precisionType_{ PrecisionType::significantDigits };//精度类型 默认为数字总长 }; #if defined(_MSC_VER) #pragma warning(pop) #endif
修改源码(json_writer.cpp):只修改了1行代码,FastWriter::writeValue函数中case realValue的处理中调用的valueToString新增了2个参数传递(源码中没有传递用的函数默认值,现在传了并且可以通过新增的2个成员函数进行设置)
void FastWriter::writeValue(const Value& value) { switch (value.type()) { case nullValue: if (!dropNullPlaceholders_) document_ += "null"; break; case intValue: document_ += valueToString(value.asLargestInt()); break; case uintValue: document_ += valueToString(value.asLargestUInt()); break; case realValue: //这里原先是document_ += valueToString(value.asDouble()); //因为后2个参数没传,所以用的函数默认值即精度位数=17,精度类型=PrecisionType::significantDigits //修改后 现在会传这2个参数,具体值可以通过新增加的2个成员函数设置 document_ += valueToString(value.asDouble(), precision_, precisionType_); break; case stringValue: { // Is NULL possible for value.string_? No. char const* str; char const* end; bool ok = value.getString(&str, &end); if (ok) document_ += valueToQuotedStringN(str, static_cast<size_t>(end - str)); break; } case booleanValue: document_ += valueToString(value.asBool()); break; case arrayValue: { document_ += '['; ArrayIndex size = value.size(); for (ArrayIndex index = 0; index < size; ++index) { if (index > 0) document_ += ','; writeValue(value[index]); } document_ += ']'; } break; case objectValue: { Value::Members members(value.getMemberNames()); document_ += '{'; for (auto it = members.begin(); it != members.end(); ++it) { const String& name = *it; if (it != members.begin()) document_ += ','; document_ += valueToQuotedStringN(name.data(), name.length()); document_ += yamlCompatibilityEnabled_ ? ": " : ":"; writeValue(value[name]); } document_ += '}'; } break; } }
需要重新编译库,然后demo验证,使用的代码:
#include <json/json.h> #include <json/writer.h> #include <iostream> #include <string> void test_precision(int precision) { Json::Value obj; Json::FastWriter write; write.set_precision(precision); write.set_precisionType(true); obj["d"] = 2.1; obj["d2"] = 9.111; obj["d3"] = 9.123456789; auto rtn = write.write(obj); std::cout << "specified precision: " << precision << ", rtn:" << rtn << std::endl; } int main() { for(int i=17; i>=0; i--) { test_precision(i); } return 0; }
specified precision: 17, rtn:{"d":2.10000000000000009,"d2":9.11100000000000065,"d3":9.12345678900000046} specified precision: 16, rtn:{"d":2.1000000000000001,"d2":9.1110000000000007,"d3":9.1234567890000005} specified precision: 15, rtn:{"d":2.1,"d2":9.111000000000001,"d3":9.123456789} specified precision: 14, rtn:{"d":2.1,"d2":9.111,"d3":9.123456789} specified precision: 13, rtn:{"d":2.1,"d2":9.111,"d3":9.123456789} specified precision: 12, rtn:{"d":2.1,"d2":9.111,"d3":9.123456789} specified precision: 11, rtn:{"d":2.1,"d2":9.111,"d3":9.123456789} specified precision: 10, rtn:{"d":2.1,"d2":9.111,"d3":9.123456789} specified precision: 9, rtn:{"d":2.1,"d2":9.111,"d3":9.123456789} specified precision: 8, rtn:{"d":2.1,"d2":9.111,"d3":9.12345679} specified precision: 7, rtn:{"d":2.1,"d2":9.111,"d3":9.1234568} specified precision: 6, rtn:{"d":2.1,"d2":9.111,"d3":9.123457} specified precision: 5, rtn:{"d":2.1,"d2":9.111,"d3":9.12346} specified precision: 4, rtn:{"d":2.1,"d2":9.111,"d3":9.1235} specified precision: 3, rtn:{"d":2.1,"d2":9.111,"d3":9.123} specified precision: 2, rtn:{"d":2.1,"d2":9.11,"d3":9.12} specified precision: 1, rtn:{"d":2.1,"d2":9.1,"d3":9.1} specified precision: 0, rtn:{"d":2,"d2":9,"d3":9}
C++ json序列化库有哪些,哪个性能最好
C++中有多种JSON序列化库可供选择,包括但不限于以下几种:
Rapidjson:这是一个非常流行的C++ JSON库,以其高性能著称,由腾讯团队开发 。
nlohmann/json:这是一个现代的、基于C++11的JSON库,以其易用性和直观的接口而受到许多C++程序员的青睐 。
sonic-cpp:由字节跳动STE团队和服务框架团队共同研发的高效JSON库,它利用CPU硬件特性和向量化编程,大幅提高了序列化和反序列化的性能。据报道,其解析性能是rapidjson的2.5倍 。
JsonCpp:这是一个成熟的库,提供了丰富的功能来处理JSON数据。
simdjson:这是一个使用SIMD指令集来加速解析的库,它提供了快速的解析性能,但不支持修改解析后的JSON结构 。
yyjson:这是一个追求解析性能的库,使用链表结构,但在查找数据时性能较差 。
在这些库中,sonic-cpp 被报道为性能最好的库,它不仅提供了高效的解析性能,还解决了其他一些库的缺点,如simdjson和yyjson的问题,并支持高效的增删改查操作 。此外,sonic-cpp已经在字节跳动的多个核心业务中大规模使用,并通过了工程化的考验 。
如果您对性能有极高的要求,sonic-cpp可能是一个不错的选择。然而,选择哪个库还应考虑其他因素,如易用性、社区支持、库的活跃度和维护情况。