After spending some time trying to find a good JSON library for C++, I realized that all the libraries out there are too heavy to use. Some of them look very good but their usage looks heavy. So I decided to write my own. My library is compliant with RFC 4627 except that it doesn't support unicode and numbers in exponential format .
Seriously, this library is really easy to use and has no dependencies (other than STL). I cannot find another C++ json library that is that simple to use.
Usage examples
The library exposes one object that is used to do everything you need. the "JSON" object. So there is no need to include a whole bunch of header files and use a whole bunch of class. You only need the JSON object to do everything you need. The object exposes these methods:
| Method | Description |
|---|---|
| JSON& operator[](int i); | Access a list item |
| JSON& operator[](std::string str); | Access an object member |
| std::string str(); | Get value of item |
| void parse(std::string json); | Parse a JSON document |
| std::string stringify(bool formatted=false); | serialize JSON object |
| JSON& addObject(const std::string& name=""); | Add object |
| JSON& addList(const std::string& name=""); | Add List |
| JSON& addValue(const std::string& val,const std::string& name=""); | Add string value |
| JSON& addValue(int val, const std::string& name=""); | Add integer value |
| JSON& addValue(double val, const std::string& name=""); | Add double precision FP value |
Reading a JSON document
The library is very simple to use. Just compile it and it will output a "test" executable and a .a that you can link against. Then let's say you have the following JSON document:
{
"obj1":{
"member1":[
"val5",
"val4"
],
"member2":"val3"
},
"list1":[
"listItem1",
"listeItem2",
{
"listObject1":"val2"
}
],
"value1":"val1"
}
The following code is an example on how to use the library:
JSON json;
std::string val;
std::string str = someFunctionThatReadsAJSONDocumentFromFileOrNetworkOrWhatever();
json.parse(str);
val= json["obj1"]["member1"][0].str(); // would give "val5"
val= json["list1"][1]["listObject1"].str(); // would give "val2"
Each access to a member will return a JSON object. So you only have 1 class to use at all time. So you can create a new variable each time or you can access all members by chaining the function calls.
val= json["obj1"]["member1"][0].str(); // would give "val5"
JSON& j1 = json["obj1"];
JSON& j2 = j1["member1"];
val = j2[0].str(); // would give "val5"
val = j2.str(); // would give "{list}"
val = j1.str(); // would give "{object}"
Invalid paths
The nice thing about this is that you don't need to worry about null objects. If you try to access an invalid member, you will get an invalid JSON object. But if you try to access another member from an invalid JSON object, you will also get an invalid JSON object. You will never get a NULL object that could crash your application.
val= json["obj1"][1].str(); // would give "{invalid}" because obj1 is an object, not a list
val= json["obj2"].str(); // would give "{invalid}"
val= json["obj2"]["member2"].str(); // would also give "{invalid}"
val= json["list"][100].str(); // would give "{invalid}"
val= json["list"][100]["something"].str(); // would also give "{invalid}"
Writing a JSON document
There are 3 methods provided to add items in the JSON document:
- addObject(const std::string& keyName="")
- addList(const std::string& keyName="")
- addValue(const std::string& val, const std::string& keyName="")
All 3 functions have an optional keyName parameter. That is because if you add an item to an Object, you need to specify the key name that will be used in the parent. Again, I wanted to have a simple interface without having to force the programmer to use different classes if using a list or an object. So this here is the behavior of those function calls if you provide the key name or not.
| Action | Result |
|---|---|
| addXXX on Object and provide key name | item added and parent uses keyName |
| addXXX on Object and don't provide key name | item added and a key name is auto generated |
| addXXX on List and provide key name | item added and key name ignored |
| addXXX on List and don't provide key name | item added |
| addXXX on Value and provide key name | operation is ignored |
| addXXX on Value and don'tprovide key name | operation is ignored |
After adding items in the json object, you can then serialize it with stringing().
JSON json;
json.addObject("obj1");
json.addList("list1");
json.addValue("val1"); // key autogenerated because key name not provided
json["list1"].addValue("item1");
json["list1"].addValue("item2");
std::string val = json.stringify(true);
Would output:
{
"obj1":{
},
"list1":[
"item1",
"item2"
],
"key2":"val1"
}