jsoncpp笔记

Json

Json是一种轻量级数据交换格式，可以表示数字，字符串，布尔值，null，数组，键值对：

{
    "encoding" : "UTF-8",
    "plug-ins" : [
        "python",
        "c++",
        "ruby"
        ],
    "indent" : { "length" : 3, "use_space": true }
}

JsonCpp

JsonCpp是用C++编写的Json库，接口设计精巧易用：

# 安装步骤
wget https://github.com/open-source-parsers/jsoncpp/archive/1.8.4.tar.gz -O jsoncpp-1.8.4.tar.gz
tar -xzf jsoncpp-1.8.4.tar.gz
cd jsoncpp-1.8.4
mkdir build && cd build
cmake3 -DJSONCPP_WITH_TESTS=Off -DJSONCPP_WITH_PKGCONFIG_SUPPORT=Off -DBUILD_STATIC_LIBS=ON ..
sudo make install

使用案例

从字符串解析出Json对象

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

int main(int argc, char const *argv[])
{
    (void)argc;
    (void)argv;

    std::string rawJson = R"({
        "encoding" : "UTF-8",
        "plug-ins" : [
            "python",
            "c++",
            "ruby"
            ],
        "indent" : { "length" : 3, "use_space": true }
    })";
    Json::Value root;   // 根json对象
    std::string err;
    Json::CharReaderBuilder builder;
    std::unique_ptr<Json::CharReader> reader(builder.newCharReader());
    if (!reader->parse(rawJson.c_str(), rawJson.c_str() + rawJson.size(), &root, &err)) {
        std::cout << "err:" << err << std::endl;
        return 1;
    }

    // 对于不存在的字段，返回null json对象
    if (root["nonexist"].isNull()) {
        std::cout << "root[nonexist] is null" << std::endl;
    }

    std::string encoding = root["encoding"].asString();
    std::cout << "root[encoding] is " << encoding << std::endl;

    // 对于获取数组第0个元素，需要写0u，以便和NULL区别开
    std::string plugin0 = root["plug-ins"][0u].asString();
    std::cout << "root[plug-ins][0u] is " << plugin0 << std::endl;

    // 类型不匹配抛出异常，但是对于可以转换的比如true可以转换为数字1，数字可以转换为字符串等等则进行自动转换
    int n = root["plug-ins"][0u].asInt();
    std::cout << n << std::endl;

    return 0;
}

从输入流解析出Json对象

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

int main(int argc, char const *argv[])
{
    (void)argc;
    (void)argv;

    Json::Value root;
    std::ifstream ifs("tmp.json");

    Json::CharReaderBuilder builder;
    builder["collectComments"] = true;
    std::string errs;
    if (!parseFromStream(builder, ifs, &root, &errs)) {
        std::cout << errs << std::endl;
        return 1;
    }
    std::cout << root << std::endl;

    return 0;
}

将json对象写入字符串

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

int main(int argc, char const *argv[])
{
    (void)argc;
    (void)argv;

    Json::Value root;
    Json::Value data;
    root["action"] = "run";
    data["number"] = 1;
    root["data"] = data;

    Json::StreamWriterBuilder builder;
    std::string jsonStr = Json::writeString(builder, root);
    std::cout << jsonStr << std::endl;

    return 0;
}

写入输出流

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

int main(int argc, char const *argv[])
{
    (void)argc;
    (void)argv;

    Json::Value root;
    Json::StreamWriterBuilder builder;
    std::unique_ptr<Json::StreamWriter> writer(builder.newStreamWriter());

    root["Name"] = "robin";
    root["Age"] = 20;
    writer->write(root, &std::cout);

    return 0;
}