通过JsonCpp读Json string的方法看这里：https://www.jianshu.com/p/0a631408f8a4
本文介绍一下写Json string的方法。

1 转换Json object为string

JsonCpp提供的老接口为：

class JSON_API FastWriter : public Writer {
public:
    JSONCPP_STRING write(const Value& root)；

使用此接口将Json数据转换为string的源代码为：

#include <iostream>
#include <string>
#include <jsoncpp/json/json.h>

int main()
{
    Json::Value root;

    root["Name"] = Json::Value("Tom");
    root["Age"] = Json::Value(20);
    root["Children"].append("Peter");
    root["Children"].append("John");

    Json::FastWriter writer;
    std::string str = writer.write(root);

    std::cout << str << std::endl;
    return 0;
}

编译运行：

$ g++ e.cpp -l jsoncpp && ./a.out
{"Age":20,"Children":["Peter","John"],"Name":"Tom"}

如果将FastWrite改为StyledWrite，可以得到带格式的Json字符串。

2 使用新接口

    Json::FastWriter writer;
    std::string str = writer.write(root);

上面是旧接口，可以完全用下面的新接口替换掉：

    Json::StreamWriterBuilder builder;
    std::string str = Json::writeString(builder, root);

看一下完整的代码：

#include <iostream>
#include <string>
#include <jsoncpp/json/json.h>

int main()
{
    Json::Value root;

    root["Name"] = Json::Value("Tom");
    root["Age"] = Json::Value(20);
    root["Children"].append("Peter");
    root["Children"].append("John");


//    Json::FastWriter writer;
//    std::string str = writer.write(root);

    Json::StreamWriterBuilder builder;
    std::string str = Json::writeString(builder, root);

    std::cout << str << std::endl;
    return 0;
}

编译运行：

$ g++ e1.cpp -l jsoncpp && ./a.out
{
    "Age" : 20,
    "Children" : 
    [
        "Peter",
        "John"
    ],
    "Name" : "Tom"
}

4 构造维基百科的Json数据

#include <iostream>
#include <string>
#include <jsoncpp/json/json.h>

int main()
{
    Json::Value root;

    root["firstName"] = Json::Value("John");
    root["lastName"] = Json::Value("Smith");
    root["sex"] = Json::Value("male");
    root["age"] = Json::Value(25);

    Json::Value address;

    address["streetAddress"] = Json::Value("21 2nd Street");
    address["city"] = Json::Value("New York");
    address["state"] = Json::Value("NY");
    address["postalCode"] = Json::Value("10021");

    root["address"] = Json::Value(address);

    Json::Value home_phone;
    Json::Value fax;

    home_phone["type"] = Json::Value("home");
    home_phone["number"] = Json::Value("212 555-1234");
    fax["type"] = Json::Value("fax");
    fax["number"] = Json::Value("646 555-4567");

    root["phoneNumber"].append(home_phone);
    root["phoneNumber"].append(fax);

    Json::StreamWriterBuilder builder;
    std::string str = Json::writeString(builder, root);

    std::cout << str << std::endl;
    return 0;
}

编译运行：

$ g++ h.cpp -l jsoncpp && ./a.out
{
    "address" : 
    {
        "city" : "New York",
        "postalCode" : "10021",
        "state" : "NY",
        "streetAddress" : "21 2nd Street"
    },
    "age" : 25,
    "firstName" : "John",
    "lastName" : "Smith",
    "phoneNumber" : 
    [
        {
            "number" : "646 555-4567",
            "type" : "fax"
        },
        {
            "number" : "212 555-1234",
            "type" : "home"
        }
    ],
    "sex" : "male"
}

5 参考

使用JsonCpp解析Json string https://www.jianshu.com/p/0a631408f8a4