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Install

  1. Build From Source
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    mkdir build
    cd build
    cmake -DCMAKE_INSTALL_PREFIX=/opt/cpplibs ..
    make
    make install
  2. 使用包管理工具 缺点不能灵活控制版本
    apt-get install libjsoncpp-dev

Examples

json_v1.cpp
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#include <iostream>
#include <json/json.h>
#include <string>


using namespace std;

int main(int argc, char* argv[])
{
    string jsonStr = "{\"name\": \"xxx\", \"array\": [\"a\", \"b\", \"c\"], \"obj\": {\"k\" : \"v\"}}";
    Json::Reader reader;
    Json::Value value;
    if (reader.parse(jsonStr, value)) {
        // read str field
        cout<<value["name"].asString()<<endl;
        // read array field
        for(int i = 0; i < value["array"].size(); i++){
            cout<<i<<":"<<value["array"][i].asString()<<endl;
        }
        // read obj field
        cout<<value["obj"]["k"].asString()<<endl;
    }
    return 0;
}

compile:
g++ -I/opt/cpplibs/include/ -o json_v1 json_v1.cpp /opt/cpplibs/lib/libjsoncpp.a && ./json_v1

output

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json_v1.cpp: In function ‘int main(int, char**)’:
json_v1.cpp:20:18: warning: ‘Reader’ is deprecated: Use CharReader and CharReaderBuilder instead [-Wdeprecated-declarations]
     Json::Reader reader;
                  ^~~~~~
In file included from /data/cpplibs/include/jsoncpp/json/json.h:11:0,
                 from json_v1.cpp:10:
/data/cpplibs/include/jsoncpp/json/reader.h:35:83: note: declared here
 class JSONCPP_DEPRECATED("Use CharReader and CharReaderBuilder instead") JSON_API Reader {
                                                                                   ^~~~~~
xxx
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1:b
2:c
v

注意这里使用老版本reader api会引起警告

Example New Api

json_v2.cpp
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#include <iostream>
#include <json/json.h>
#include <string>
#include <memory>


using namespace std;

int main(int argc, char* argv[])
{
    string jsonStr = "{\"name\": \"xxx\", \"array\": [\"a\", \"b\", \"c\"], \"obj\": {\"k\" : \"v\"}}";

    Json::Value value;

    // parse
    JSONCPP_STRING errs;
    Json::CharReaderBuilder readerBuilder;
    std::unique_ptr<Json::CharReader> const reader(readerBuilder.newCharReader());
    bool res = reader->parse(jsonStr.c_str(), jsonStr.c_str() + jsonStr.length(), &value, &errs);

    if (res && errs.empty())
    {
        // read str field
        cout<<value["name"].asString()<<endl;
        // read array field
        for(int i = 0; i < value["array"].size(); i++){
            cout<<i<<":"<<value["array"][i].asString()<<endl;
        }
        // read obj field
        cout<<value["obj"]["k"].asString()<<endl;
    }else{
        cout<<"json parse fail:"<<errs<<endl;
    }
    return 0;
}

源码方式
g++ -I/opt/cpplibs/include/ -o json_v2 json_v2.cpp /opt/cpplibs/lib/libjsoncpp.a && ./json_v2

如果使用包管理
共享库方式编译
g++ -I/usr/include/jsoncpp -ljsoncpp -o json_v2 json_v2.cpp
静态库方式编译
g++ -I/usr/include/jsoncpp -ljsoncpp -o json_v2 json_v2.cpp /usr/lib/x86_64-linux-gnu/libjsoncpp.a

output 已经没有警告了

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xxx
0:a
1:b
2:c
v

Json::Value

  1. 判断类型 bool isXxx()
    • isNull
    • isBool
    • isInt
    • isInt64
    • isNumbeic
    • isString
    • isObject
    • isArray
  2. 转换原生类型 asXxx()
    • asInt
    • asInt64
    • asBool
    • asDouble
  3. 访问字段
    • [] 方式访问 val[“field”]
    • key是否存在 isMember(“field”)
  4. 转成格式化字符串
    • String toStyledString()

流操作

  • 读取: Json::parseFromStream
  • 输出: writeString
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    Json::StreamWriterBuilder wbuilder; 
    wbuilder["indentation"] = "\t";
    std::string document = Json::writeString(wbuilder, root);
    Json::CharReaderBuilder rbuilder;
    rbuilder["collectComments"] = false;
    std::string errs;
    bool ok = Json::parseFromStream(rbuilder, std::cin, &root, &errs);