JSON for Modern C++ 库的介绍与使用示例代码
文章目录
- JSON for Modern C++ 介绍
- 示例代码
-
- 简单使用
-
- 代码：
- 输出：
- 序列化与反序列化
-
- 代码：
- 输出：
- 像STL一样操作json对象
-
- 代码：
- 输出：
- Json Pointer 与 Patch
-
- 代码：
- 输出：
- 解析简单json文件
-
- 代码：
- 输出：
- 解析json文件为结构体以及将结构存放为json文件
-
- 代码：
- 输出：
- 优化后代码：
- 利用宏列化/反序列结构体
-
- 代码：[MacrosForSerialization](https://github.com/lichangke/Demos/tree/master/JsonDemo/MacrosForSerialization)
- 输出：
JSON for Modern C++ 介绍
相关链接
GitHub：https://github.com/nlohmann/json
api文档：https://nlohmann.github.io/json/api/basic_json/
头文件添加
// json.hpp 唯一需要引入的文件
#include CMakeLists 添加
find_package(nlohmann_json 3.2.0 REQUIRED) # 找包
target_link_libraries(${PROJECT_NAME}
PRIVATE
nlohmann_json::nlohmann_json) # link 库
示例代码
| 示例 | 说明 |
|---|---|
| 简单使用 | json对象的创建 |
| 序列化与反序列化 | 1、字符串的序列化与反序列化 2、流的序列化与反序列化（输入输出流+文件流） |
| 像STL一样操作json对象 | json对象形如STL操作的方法使用 |
| Json Pointer 与 Patch | 1、_json_pointer 的使用 2、patch与merge_patch的补丁操作 |
| 解析简单json文件 | 读取简单json文件，并将数据存放到不容变量中 |
| 解析json文件为结构体以及将结构存放为json文件 | 1、读取json文件多级数据并存放为结构体，int 、string、数组 2、将结构体数据循环处理后存放入json文件 |
简单使用
demo： SimpleUseTest
代码：
GitHub SimpleUseTest
#include 输出：
json j1 = {"answer":{"everything":42},"happy":true,"list":[1,0,2],"name":"Niels","nothing":null,"object":{"currency":"USD","value":42.99},"pi":3.141}
json j2 = {"answer":{"everything":42},"happy":true,"list":[1,0,2],"name":"Niels","nothing":null,"object":{"currency":"USD","value":42.99},"pi":3.141}
json empty_array_explicit = []
json empty_object_implicit = {}
json empty_object_explicit = {}
json array_not_object = [["currency","USD"],["value",42.99]]
json array_not_object[?][?] = ["value",42.99]
Process finished with exit code 0
序列化与反序列化
demo：SerializationAndDeserialization
代码：
GitHub SerializationAndDeserialization
#include ()
auto cpp_string = j_string.get<std::string>();
std::cout << "j_string.get() cpp_string = " << cpp_string << std::endl;
// 获取字符串值 并存到 变量
std::string cpp_string2;
j_string.get_to(cpp_string2);
// 获取序列化的值(显式JSON序列化)
std::string serialized_string = j_string.dump();
// 输出原始字符串
std::cout << "original string: " << cpp_string << " == " << cpp_string2 << " == " << j_string.get<std::string>() << '\n';
// 出书序列化值
std::cout << "serialized value: " << j_string << " == " << serialized_string << std::endl;
/// 从标准输入输出流中 序列化与反序列化
// 从标准输入反序列化
json j4;
std::cout << "please input:" << std::endl;
std::cin >> j4;
// 序列化到标准输出
std::cout << j4 << std::endl;
// 格式化输出
std::cout << std::setw(4) << j4 << std::endl;
/// 从文件流中 序列化与反序列化
// 读取一个json文件
std::ifstream i("file.json");
json j5;
i >> j5;
std::cout << "json j5 = " << j5 << std::endl;
// 将美化的JSON写入另一个文件
std::ofstream o("pretty.json");
o << std::setw(4) << j5 << std::endl; // std::setw(4) 设置格式
return 0;
}
输出：
json j1{"happy":true,"pi":3.141}
json j2{"happy":true,"pi":3.141}
json j3{"happy":true,"pi":3.141}
string s j3.dump(){"happy":true,"pi":3.141}
string s j3.dump(4){
"happy": true,
"pi": 3.141
}
json j_string = "this is a string"
j_string.get() cpp_string = this is a string
original string: this is a string == this is a string == this is a string
serialized value: "this is a string" == "this is a string"
please input:
{"happy":true,"pi":3.141}
{"happy":true,"pi":3.141}
{
"happy": true,
"pi": 3.141
}
json j5 = {"output":{"crf":31,"frameRate":20,"height":1080,"width":1000},"tracks":[{"name":"t1","pieces":[{"endTime":6,"file":"x.mp4","startTime":2},{"endTime":13,"file":"y.mp4","startTime":9}]},{"name":"t2","pieces":[{"endTime":10,"file":"z.mp4","startTime":0}]}]}
Process finished with exit code 0
像STL一样操作json对象
demo：Access_STL_like
代码：
GitHub Access_STL_like
#include 输出：
json j = ["foo",1,true]
json j = ["foo",1,true,1.78]
"foo"
1
true
1.78
==============
"foo"
1
true
1.78
==============
json j = ["foo",1,true,1.78]
json j = ["foo",42,true,1.78]
j.at(2) = 1
==============
j == "[\"foo\", 42, true]"_json : 0
j.size() : 4
j.empty() : 0
j.type() : array
json j = []
==============
json o = {"bar":false,"baz":3.141,"foo":23,"weather":"sunny"}
bar : false
baz : 3.141
foo : 23
weather : "sunny"
==============
bar : false
baz : 3.141
foo : 23
weather : "sunny"
==============
bar : false
baz : 3.141
foo : 23
weather : "sunny"
==============
o.contains("foo")
o.find("foo")
json o = {"bar":false,"baz":3.141,"weather":"sunny"}
Process finished with exit code 0
Json Pointer 与 Patch
demo： JsonPointerAndPatch
代码：
GitHub JsonPointerAndPatch
#include 输出：
json j_original = {"baz":["one","two","three"],"foo":"bar"}
var1 = "two"
var2 = "three"
var3= null
var4 = null
var5 = null
=============
json j_original = {"baz":["one","two","three",null,null,null],"foo":"bar"}
json j_result = {"baz":"boo","hello":["world"]}
op_patch = [{"op":"replace","path":"/baz","value":["one","two","three",null,null,null]},{"op":"remove","path":"/hello"},{"op":"add","path":"/foo","value":"bar"}]
=============
json j_document = {"a":"b","c":{"d":"e","f":"g"}}
json j_patch2 = {"a":"z","c":{"f":null}}
json j_document = [{"op":"replace","path":"/baz","value":"boo"},{"op":"add","path":"/hello","value":["world"]},{"op":"remove","path":"/foo"}]
=============
Process finished with exit code 0
解析简单json文件
demo： ParseJsonFileSimple
代码：
GitHub ParseJsonFileSimple
#include 输出：
json j = {"height":20.123,"name":"test","ok":true,"width":1000}
ok = 1
height = 20.123
width = 1000
name = test
Process finished with exit code 0
解析json文件为结构体以及将结构存放为json文件
demo：ParseJsonFileComplex
代码：
GitHub ParseJsonFileComplex
#include 输出：
Hello, World!
json j = {"output":{"crf":31,"frameRate":20,"height":1080,"width":720},"tracks":[{"name":"t1","pieces":[{"endTime":6,"file":"x.mp4","startTime":2},{"endTime":13,"file":"y.mp4","startTime":9}]},{"name":"t2","pieces":[{"endTime":10,"file":"z.mp4","startTime":0}]}]}
json j2 = {"output":{"crf":31,"frameRate":20,"height":1080,"width":720},"tracks":[{"name":"t1","pieces":[{"endTime":6,"file":"x.mp4","startTime":2},{"endTime":13,"file":"y.mp4","startTime":9}]},{"name":"t2","pieces":[{"endTime":10,"file":"z.mp4","startTime":0}]}]}
Process finished with exit code 0
优化后代码：
#include 利用宏列化/反序列结构体
如果您只想序列化/反序列化某些结构，则to_json / from_json函数可能是很多样板，可以利用两个宏来简化你的编码。
NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(name, member1, member2, ...)将在要为其创建代码的类/结构的命名空间内定义。
NLOHMANN_DEFINE_TYPE_INTRUSIVE(name, member1, member2, ...)将在要为其创建代码的类/结构中定义。 该宏还可以访问私有成员。
两个宏定义如下，其用到了可变参数宏的展开，宏重载相关内容，参见不定参数宏展开与__VA_ARGS__
/*!
@brief macro
@def NLOHMANN_DEFINE_TYPE_INTRUSIVE
@since version 3.9.0
*/
#define NLOHMANN_DEFINE_TYPE_INTRUSIVE(Type, ...) \
friend void to_json(nlohmann::json& nlohmann_json_j, const Type& nlohmann_json_t) { NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__)) } \
friend void from_json(const nlohmann::json& nlohmann_json_j, Type& nlohmann_json_t) { NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_FROM, __VA_ARGS__)) }
/*!
@brief macro
@def NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE
@since version 3.9.0
*/
#define NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(Type, ...) \
inline void to_json(nlohmann::json& nlohmann_json_j, const Type& nlohmann_json_t) { NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__)) } \
inline void from_json(const nlohmann::json& nlohmann_json_j, Type& nlohmann_json_t) { NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_FROM, __VA_ARGS__)) }
代码：MacrosForSerialization
#include 输出：
{
"housenumber": 100,
"postcode": 4096,
"street": "this is a address"
}
postcode = 4096
housenumber = 100
street = this is a address
{
"age": 18,
"name": "张三",
"num": 1
}
st2.name = 张三
st2.num = 1
st2.age = 18
Process finished with exit code 0
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