array.md

Array's

Parsing arrays is common and it is a fundamental data structure in JSON. It can be both the root object or a member.

Simple Array of int

[
  1,
  2,
  3,
  4,
  5
]

The above JSON document contains an array of integers as the root object.

The C++ to parse this could look like

std::vector<int> vec = from_json_array<int>( json_str );

To see a working example of the following, refer to cookbook_array1_test.cpp The previous is a simple example, had the element type of the array been more complicated, such as a class, a data mapping would be required.

Array of a class

[
  {
    "a": "Hello World",
    "b": 1234,
    "c": 1.23,
    "d": false
  },
  {
    "a": "Goodbye World",
    "b": 4321,
    "c": 123,
    "d": true
  }
]

Here we hae a JSON array containing a class with members of type string, unsigned, float, and boolean.

The C++ data structures and the mapping could look like the following To see a working example using this code, refer to cookbook_array2_test.cpp

struct MyClass4 {
  std::string a;
  unsigned b;
  float c;
  bool d;
};

namespace daw::json {
  template<>
  struct json_data_contract<MyClass4> {
  using type = json_member_list<
    json_string<"a">, 
    json_number<"b", unsigned>,
    json_number<"c", float>, 
    json_bool<"d">
  >;

  static inline auto to_json_data( MyClass4 const &value ) {
    return std::forward_as_tuple( 
      value.a, 
      value.b, 
      value.c, 
      value.d );
    }
  };
} 

std::vector<MyClass4> v = from_json_array<MyClass4>( str );

The above would construct MyClass4 with arguments of types std::string, unsigned, float, bool

Array's as members

Use the json_array member type in the member list to describe a member that is an array type.

To see a working example using this code, refer to cookbook_array3_test.cpp

{
  "member0": 5,
  "member1": [
    1,
    2,
    3,
    4,
    5
  ],
  "member2": [
    "hello",
    "world"
  ]
}

The above JSON document, has an object root, with int, array of int, and an array of string members

The C++ data structures could look like the following

struct MyArrayClass1 {
  int member0;
  std::vector<int> member1;
  std::vector<std::string> member2;
};

The json_data_contract specialization as follows

namespace daw::json {
  template<>
  struct json_data_contract<MyArrayClass1> {
    using type = json_member_list<
      json_number<"member0", int>,
      json_array<"member1", int>,
      json_array<"member2", std::string>>;

    static inline auto
    to_json_data( MyArrayClass1 const &value ) {
      return std::forward_as_tuple( 
        value.member0, 
        value.member1,
        value.member2 );
    }
  };
}

Pointer like arrays

For dealing with pointer like arrays(T *, has element_type type alias) see int_ptr_test