c++结构体与json自动互转(nlohmann的使用)

说明

nlohmann实现了结构体与json自动互转。

下载

https://github.com/nlohmann/json.git

拷贝include/nlohmann/json.hpp到新建工程

例子

代码

#include <iostream>
#include "json.hpp"
#include <string>
using nlohmann::json;
using namespace std;
//普通结构体
struct tag1
{
    std::string name;
    int id;
    float value;
    NLOHMANN_DEFINE_TYPE_INTRUSIVE(tag1,name,id,value)
};
//有业务逻辑的结构体
struct tagLogical
{
    std::string name;
    int id;
    int type;
    int value1;
    int value2;
    int value3;
    //需要人工实现to_json();和from_json();
    //其中本例中c++基础数据类型也可以使用对象类型
    friend void to_json(nlohmann::json& j, const tagLogical& l)
    {
        j = nlohmann::json{
            {"name",l.name},
            {"id",l.id},
            {"type",l.type}
        };
        if (l.type == 0)
        {
            j["value1"] = l.value1;
        }
        else if (l.type == 1)
        {
            j["value2"] = l.value2;
        }
        else if (l.type == 2)
        {
            j["value3"] = l.value3;
        }
    }
    friend void from_json(const nlohmann::json& j, tagLogical& l)
    {
        if (j.contains("name")) j["name"].get_to(l.name);
        if (j.contains("id")) j["id"].get_to(l.id);
        if (j.contains("type")) j["type"].get_to(l.type);
        if (l.type==0)
        {
            l.value1=j["value1"];
        }
        else if (l.type == 1)
        {
            l.value2 = j["value2"];
        }
        else if(l.type == 2)
        {
            l.value3 = j["value3"];
        }
    }
};

int main()
{
    cout << "e.g.1" << endl;
    tag1 t1;
    t1.name = "aaa";
    t1.id = 1;
    t1.value = 1.1;
    json j;
    to_json(j,t1);
    cout << j.dump() << endl;
  
    std::string s = R"({"id":2,"name":"bbb","value":2.2})";
    json j2=json::parse(s);
    tag1 t2;
    from_json(j2, t2);
    cout << t2.name << " " << t2.id << " " << t2.value << endl;
    //
    {
        cout <<endl<< "e.g.2" << endl;
        tagLogical tagL;
        tagL.name = "aaa";
        tagL.id = 0;
        tagL.type = 0;
        tagL.value1 = 100;
        json j;
        to_json(j, tagL);
        cout << j.dump() << endl;

        std::string s = R"({"id":0,"name":"aaa","type":1,"value2":101})";
        json j2 = json::parse(s);
        tagLogical tagL2;
        from_json(j2, tagL2);
        cout << tagL2.name << " " << tagL2.id << " " << tagL2.type << " " << tagL2.value2 << endl;
    }
}

输出

e.g.1
{"id":1,"name":"aaa","value":1.100000023841858}
bbb 2 2.2

e.g.2
{"id":0,"name":"aaa","type":0,"value1":100}
aaa 0 1 101

总结

1.c++中常用map设计程序。map可以用json表示，又使用nlohmann轻松完成json字符串与c++对象内存互转。

那么把map的c++代码写到.json配置文件中，避免密密麻麻一片代码的场景，技术支持又可以自定义维护。

2.以后有时间调研nlohmann的使用对继承与多态支持程度，能否避免人工实现的to_json();from_json();。

3.以后有时间查看nlohmann源码实现原理。